Is entropy $S$ a fundamental quantity like Temperature?

Question: What I'm trying to say is that $$S=\int\limits_{T_1}^{T_2}\frac{\mathrm dQ}{T}\tag{1}$$ depends only on the initial and final states. Why is that so? Is it like a "law" (like Newton's law of gravity or any other law instated by Nature) or somehow can it be deducted that even though temperature ($T$) and heat supplied ($\mathrm dQ$) vary depending on the path taken, $S=\int\limits_{T_1}^{T_2}\frac{\mathrm dQ}{T}$ will be independent of the path?
Comment: The integral of dQ/T to get $\Delta S$ is valid only for reversible paths.  So it is not independent of path, since it cannot be used for irreversible paths.  And, for reversible paths, the integral is the same for all of them, irrespective of the reversible path chosen.
Comment: That $\oint \frac{dQ_{rev}}{T}  = 0$ is part of Clausius Theorem, which you should be able to find a proof of in any thermodynamics textbook (unfortunately I don't know a good reference online). From this it follows that entropy is a function of state. Also note that it is *reversible* heat transfer that appears in the definition of entropy and only along these paths that the integral is path independent.
Comment: Entropy is a physical property of the materials comprising the system, rather than a characteristic of any process that the system is subjected to.  For an irreversible process, there are only two ways that the entropy of a system can change:  1.  Entropy flow across the boundary of the system with its surroundings as a result of heat transfer at the boundary at the boundary temperature.  2.  Entropy generation within the confines of the system as a result of finite viscous heat generation and heat conduction at finite temperature gradients within the system.  .
Comment: @ChetMiller Can we comment anything on the Entropy associated with Irreverisible process?
Comment: Additional remark regarding @ChetMiller's comment: His remarks are valid as long as the system is closed. For an open system obviously there might be an additional entropy transport due to mass transfer. Have a look at the entropy balance equation ([Wikipedia](https://en.wikipedia.org/wiki/Entropy#Entropy_balance_equation_for_open_systems) or [here](https://www.ecourses.ou.edu/cgi-bin/ebook.cgi?doc=&topic=th&chap_sec=06.6&page=theory)) if you are interested in that.
Answer: For the integral
$$ S_T = \int_{T=T_1}^{T_2} \frac{\delta Q_{rev}}{T} $$
to be valid you have to assume that a process, including the heat transfer, is completely reversible. The system is in thermodynamic equilibrium throughout the entire process. For this reason also the index $rev$ is used. This is generally not the case as friction is involved but for modelling some processes you assume such an ideal "comparison" process where the direction of every heat transfer is assumed to be arbitrary as well (continuous quasi-equilibrium), e.g. reversible cycles in thermodynamics where the starting point of your cycle is equal to the endpoint. In this case the thermodynamic entropy is a state variable.
I have already taken some notes on it a while back and I will simply copy and paste them over here. The following section explains what qualifies a state variable and derives why this is the case for the entropy. The basic idea is that for a state variable the order of derivation with respect to the underlying variables does not matter. It is shown that for a perfect gas and a reversible paths the specific heat is no state but instead a process variable but introducing an integrating factor $\frac{1}{T}$ a state variable, the specific thermodynamic entropy $s_T$, can be generated.

State and process variables
In thermodynamics one distinguishes between two main types of quantities: State variables depend only on the current equilibrium state of a system and do not depend on the path it has taken to reach the corresponding condition whereas process variables are an adequate measure for the path taken to reach the corresponding state. In order to differentiate these two types of quantities the latter is generally denoted by small Greek delta $\delta$ instead of exact differentials $d$.
For a state variable $f(u,v)$ (a function of two arbitrary variables $u$ and $v$) the order of derivatives does not matter, the symmetry of second derivatives, the Schwarz's theorem
$$ \frac{\partial}{\partial u} \left( \frac{\partial f}{\partial v} \right) = \frac{\partial}{\partial v} \left( \frac{\partial f}{\partial u} \right), \tag{1}\label{1}$$
must hold: The order of the derivatives has no impact on the change of the state variable.
State variables can be further classified: Extensive variables such as energy $E$ or volume $V$ depend on the size of the system whereas intensive variables such as pressure $p$ or temperature $T$ do not. Dividing an extensive variable by the mass of the corresponding system yields so called specific variables. In the case of the volume this leads to the specific volume $v$ which is the reciprocal of the density $\rho$
$$ v := \frac{V}{m} = \frac{1}{\rho}. $$
Internal energy
One main measure of the state of a thermodynamic system is the so called internal energy, an abstraction for a variety of different forms of energy such as kinetic and rotational energy of the corresponding molecules that reflect the internal state of a fluid regardless of the macroscopic fluid flow. The change of the internal energy can be described using temperature and volume as
$$ dE_{in} = \left( \frac{\partial E_{in}}{\partial T} \right)_V dT+ \left( \frac{\partial E_{in}}{\partial V} \right)_T dV. \tag{2}\label{2}$$
The first law of thermodynamics for a closed system
The change of the internal energy may be described using a basic conservation of energy given by the first law of thermodynamics. A closed system is able to store energy either in macroscopic external energy $E_{ex}$ such as potential or kinetic energy of the fluid flow or the aforementioned internal energy $E_{in}$. These quantities are state variables and are therefore denoted with exact differentials.

For a closed system (no mass transport) the energy can only be changed by the work $W$ and heat $Q$ transferred to the system, which generally depend on the exact process path and therefore are denoted by partial differentials. The most common type of work is the expansion of the corresponding control volume against the surrounding pressure $p$ that can be calculated to $\delta W_V = - p \, dV$. Therefore for a closed system the following energy budget must hold:
$$ \delta Q + \delta W = \underbrace{dE_{in} + dE_{ex}}_{dE} \tag{3}\label{3}$$
Ideal gas
So far no assumption regarding the equation of state was made. However, the equations are simplified significantly when assuming gases characterized by simple interactions. One such simplified model is the ideal gas, an idealised model of a real gas where particles of infinitesimal size interact with each other only in elastic collisions. This allows for a simple description including a very simple equation of state given by
$$ p V = R_m T. \tag{4}\label{4}$$
Additionally it can be found experimentally (Joule expansion) that the internal energy of such an ideal gas is no function of the volume the gas occupies
$$ \left( \frac{\partial E_{in}}{\partial V} \right)_T dV \approx 0. $$
but rather only a function of the temperature
$$ dE_{in} \approx \left( \frac{\partial E_{in}}{\partial T} \right)_V dT. $$
Perfect gas
An even more basic behaviour can be achieved by neglecting the intermolecular forces resulting in constant heat capacities. Such a model gas is referred to as a calorically perfect gas.
Enthalpy and heat capacities
Combining the first law of thermodynamics for a close system (equation \eqref{3}) with the differential of the internal energy (equation \eqref{2}), neglecting changes due to the external energy (see the figure above), yields
$$ \delta Q \underbrace{- p \, dV}_{\delta W_V} = \left( \frac{\partial E_{in}}{\partial T} \right)_V dT+ \left( \frac{\partial E_{in}}{\partial V} \right)_T dV. \tag{5}\label{5}$$
Considering an isochor system, meaning the terms including $dV$ vanish in equation \eqref{5}, and dividing the equation by the mass $m$, results in an equation of the specific heat
$$ \partial q = \left( \frac{\partial e_{in}}{\partial T} \right)_v dT $$
where the term
$$ c_v := \left( \frac{\partial e_{in}}{\partial T} \right)_v $$
is referred to as the heat capacity at constant volume.
Analogously the enthalpy $H$ is introduced as
$$ H := E_{in} + p V $$
which takes the place of the internal energy for isobar processes and leads to the heat capacity at constant pressure
$$ c_p := \left( \frac{\partial h}{\partial T} \right)_p. $$
One can find a correlation between the two heat capacities dividing the right-hand side of equation \eqref{5} by the mass $m$
$$ de_{in} = c_v dT + \left( \frac{\partial e_{in}}{\partial v} \right)_T dv $$
which can be rewritten using the first law of thermodynamics (equation \eqref{3}) to
$$ \partial q - c_v dT = \left[ \left( \frac{\partial e_{in}}{\partial v} \right)_T + p \right] dv $$
Dividing by the differential of the temperature $d T$ we yield
$$ \left(\frac{\delta q}{d T} \right)_p - c_v = \underbrace{ \left[ \underbrace{ \left( \frac{\partial e_{in}}{\partial v} \right)_T }_{\approx 0} + p \right] \left(\frac{\partial v}{\partial T} \right)_p }_{R_m} = c_p - c_v. $$
As for ideal gases the internal energy is only a function of temperature but not of the specific volume with equation \eqref{4} the expression above leads to
$$ c_p - c_v = R_m. $$
Additionally the heat capacity ratio is defined as the ratio between the two specific heats
$$ \gamma := \frac{c_p}{c_v}. $$
Entropy on the continuum level
In classical mechanics all laws are reversible: The laws are deterministic in the past as well as in the future, they are essentially symmetric in time - Information about prior states is not lost but instead conserved. This contradicts though our perception of the world as heat always flows from points of higher temperature to points of lower and thus gives the physical laws direction. Nonetheless using the findings from the section above as well as assuming a fictive reversible process, where also the heat flux may be arbitrarily reversed, we are able to formulate the specific reversible heat for an ideal gas as (dividing equation \eqref{3} by the mass $m$ and further considering an ideal gas $ \left( \frac{\partial e_{in}}{\partial \hat v} \right)_T d \hat v \approx 0$)
$$ \delta q_{rev} := de_{in} + p d \hat{v} = c_{\hat v} dT + p d \hat{v}. \tag{6}\label{6}$$
If the heat was a state variable it would have to fulfil the symmetry of second order derivatives. This is though not the case due for a perfect gas as (right-hand side of equation \eqref{6} and the ideal gas law \eqref{4})
$$ \left( \frac{\partial c_{\hat v}}{\partial \hat{v}} \right)_T \neq - \left( \frac{\partial \left( \frac{R_m T}{\hat{v}} \right)}{\partial T} \right)_{\hat{v}} = - \left( \frac{\partial p}{\partial T} \right)_{\hat{v}}. $$
As $c_v$ is by definition independent of the specific volume the left side is equal to zero whereas the right side in the case of an ideal gas yields $-\frac{R_m}{\hat{v}}$. If the temperature on the right side would not appear, the Schwartz's theorem would be fulfilled and we would have found a state variable.
This can be achieved introducing the thermodynamic Clausius entropy $s_T$, using the integrating factor $\frac{1}{T}$, which yields
$$ ds_T := \frac{\delta q_{rev}}{T} = \frac{c_{\hat v}}{T} dT + \frac{p}{T} d \hat{v}. \tag{7}\label{7}$$
This is sort of an obscure state variable as its meaning is not directly accessible at first. For reversible processes, where the heat flux may be assumed as reversible, this entity vanishes whereas for every irreversible process, where losses due to dissipation happen, which is the norm for common processes found in nature, but no energy may be discharged (closed system), it steadily increases (second law of thermodynamics). Entropy is responsible for the asymmetry of physical laws in time, it gives processes direction: Differences in temperature drive every process and are the source of this irreversibility.
Integrating the differential definition (equation \eqref{7}) assuming a perfect gas yields
$$ \frac{p}{p_1} = \left( \frac{\hat{v}_1}{\hat{v}} \right)^{\gamma} e ^{\frac{s-s_1}{c_v}} = \left( \frac{\rho}{\rho_1} \right)^{\gamma} e ^{\frac{s-s_1}{c_v}} = \left( \frac{T}{T_1} \right)^{\frac{\gamma}{\gamma -1}} e ^{-\frac{s-s_1}{R_m}}, $$
which can be rewritten assuming isentropy $s-s_1 = 0$ to
$$ \frac{p}{\rho^{\gamma}} = const. $$
Answer:
What I'm trying to say is that
  $$S=\int\limits_{T_1}^{T_2}\frac{\mathrm dQ}{T}\tag{1}$$ depends only
  on the initial and final states.

First, regarding the equation, it should be
$$dS=\frac{\delta Q_{rev}}{T}$$
$$\Delta S=S_{2}-S_{1}=\int_1^2\frac{\delta Q_{rev}}{T}$$
The integral is not defined between two temperatures $T_2$ and $T_1$, but between two equilibrium states 2 and 1. You can have a change in entropy with no change in temperature, that is, a change in entropy where $T_{2}=T_{1}$. An example is an isothermal process (as discussed below)..

Why is that so? Is it like a "law" (like Newton's law of gravity or
  any other law instated by Nature)

As a matter of fact it is. Entropy is a property of a system that depends only on the initial and final states. It's in part a consequence of a law of nature that heat transfer has never been observed to occur spontaneously from a low temperature body to a high temperature body. The Clausius statement of the second law is
"It is not possible to construct a device that operates in a cycle and whose sole effect is to transfer heat from a colder body to a hotter body"
Since the spontaneous transfer of heat from a cold to hot body did not violate the first law of thermodynamics (conservation of energy), the second law was developed. The second law had to recognize the fact that heat can only naturally go from a hotter body to a colder body. The law necessitated a new property that not only takes into account conservation of energy, like the first law, but also the direction in which natural processes are possible, including the temperatures that dictate the direction of heat transfer.
The second law says the total change in entropy of the system + surroundings is equal to or greater than zero. This is sometimes referred to as the principle of entropy increase. Therefore, according to the second law:
$$\Delta S_{total}=\Delta S_{system}+\Delta S_{surroundings}≥0$$
where the equality applies to an ideal reversible process and the inequality applies to all irreversible (real) processes.
To illustrate how the second law precludes the possibility of spontaneous heat transfer from a cold body to a hot body, consider heat transfer between two bodies A and B that are thermal reservoirs (constant temperature heat source and heat sink). Either body can be considered the system or surroundings.
Let the temperature of body A be $T_A$ and the temperature of body B be $T_B$. Let a quantity of heat, $\delta Q$, transfer isothermally out of A and into B. Since the heat out of A equals the heat into B, and no work is involved, the first law (conservation of energy) is satisfied. Since the temperatures of the two bodies are constant they come out of the integral for determining the changes in entropy of each body and the total entropy change. Those changes become:
$$\Delta S_{A}=-\frac{\delta Q}{T_{A}}$$
$$\Delta S_{B}=+\frac{\delta Q}{T_{B}}$$
Applying the second law for the total change in entropy
$$\Delta S_{tot}=\Delta S_{A}+\Delta S_{B}=-\frac{\delta Q}{T_{A}}+\frac{\delta Q}{T_{B}}$$
In the limit, when the temperature difference approaches zero ($T_{A}=T_{B}$), $\Delta S$ approaches zero and the process is said to be reversible. However, since a finite temperature difference ($T_{A}>T_{B}$) is always necessary for heat transfer to occur, $\Delta S>0$ and all real processes are necessarily irreversible.
Now let's instead consider the possibility that heat transfer occurs in the reverse direction, that is from the colder body B to the hotter body A. Applying the second law for total entropy change we have
$$\Delta S_{tot}=\frac{+\delta Q}{T_A}+\frac{-\delta Q}{T_B}$$
Which gives us, for all $T_{A}>T_{B}$, $\Delta S_{tot}<0$, in violation of the principle of entropy increase and the second law.
Hope this helps.


Term for "letter obsession"

Question: Is there a term for a person with an obsession on a single character or letter? For example: A person that is obsessed with the letter "X".
Comment: Obsessed with X? Yes, they're called mathematicians. :)
Comment: Someone this word applies to might enjoy [univocalics](https://en.wikipedia.org/wiki/Univocalic) - pieces of writing using only one vowel. One famous example is the book [Eunoia](https://en.wikipedia.org/wiki/Eunoia_(book)) where each of its five chapters is a short story using only one vowel.
Comment: Well, if you would coin a word, it would be *grammamania* (or *gramamania*) because the Greek word for letter/character is γράμμα (grámma).
Answer: .A. If the obsession embraces all the letters in turn, then they are Abecedarians. People who like alphabets.
.B. Someone who puts a chosen letter at the beginning of a cluster of words is practising Alliteration. Someone who alliterates is an 'alliterator.' Not usually monogamously loyal to one letter.
There is a famous alliterative poem by AAWatts, which starts with…

AN Austrian army, awfully arrayed,
  Boldly by battery besieged Belgrade.
  Cossack commanders cannonading come,
  Dealing destruction's devastating doom. to...XYZ&A

Only a couple of single-letter infatuations have a name:
.C. iotacism Merriam-Webster Medical:

a speech defect marked by use of the sound \ē\ in place of other vowel
  sounds.

In language studies iotacism refers to the vowel shift e.g. in modern Greek where ei, oi, i, and ee all sound like ee, or i.
.D. lisp (Oxf Dictionaries)

in which s is pronounced like th in thick and z is pronounced like
  th in this:

The bigger editions say this is sometimes an affectation, simply a stylish preference for th, v, ff; for sibilants. Also some choirs are taught to avoid final,whistling esses with a lisp.
.E. Chiastic ought to mean someone who likes Chi or X but it is only used for crossover patterns. Lambdaism shows a preference for arrow shapes or Lambdas Λ, but not usually LL sounds or letters.
Answer: Could they be an Xophile? Why do you need such a word anyway?
Comment: Call it Xiophile, it make it easier to say, and the Greek Xi  ξ adds class.
Comment: What is the definition of a Xophile? I can't seem it's definition even on google. Well, I am obsess/fixated with the letter "F" and I just want to know if there is a term for this, medical or general.
Comment: @xitnesscomplex  Xophile or even Xiophile are neologisms, They don't yet appear in any dictionary. You can introduce a new word (especially if you are Shakespeare or Milton) so long as you make the meaning clear by context or by definition. So, if I say "Clee Hill ends in steep escarpment of 599 feet," you guessed it was a sort of cliff/slope/edge.
Answer: The term for the person is 'monomaniac'. The term for the type of obsession is 'monomania':

mon·o·ma·ni·a (mon'ō-mā'nē-ă),
  An obsession or abnormally extreme enthusiasm for a single idea or subject ....

[monomania. (n.d.) Farlex Partner Medical Dictionary. (2012). Retrieved December 8 2015 from http://medical-dictionary.thefreedictionary.com/monomania .]
No more specific term than 'monomaniac' exists for somebody obsessed with single letters, nor for somebody obsessed with the individual single letters. If you have the desire (need?), such terms could, potentially, be formed.
Unfortunately, for the case of an obsession with single letters (without regard for which individual single letters), the term 'graphomania' (and, to a lesser extent, 'graphophilia') has a pre-existing more general meaning:

graphomania
  an obsession with writing.

[graphomania. (n.d.) -Ologies & -Isms. (2008). Retrieved December 8 2015 from http://www.thefreedictionary.com/graphomania .]
'Glyph' might offer a suitable combining form, as in 'glyphomaniac' or 'glyphophiliac':

glyph n.
  3. A symbol, such as a stylized figure or arrow on a public sign, that imparts information nonverbally.

[glyph. (n.d.) American Heritage® Dictionary of the English Language, Fifth Edition. (2011). Retrieved December 8 2015 from http://www.thefreedictionary.com/glyph .]
To form words expressing the idea of 'an obsession or abnormally extreme enthusiasm' for individual letters, you'd have to look to the names of the letters, then combine them with '-maniac' or '-philiac' to designate somebody obsessed with the particular letter.
Comment: I think monomania is the closest to what I am looking for.

Term for "letter obsession"


Is there a visually optimal area to view a 3-D movie at the theatre?

Question: 3-D films are becoming more and more common. While I find the experience interesting, I have yet to find "the best seat in the house". Opticaly, there ought to be a optimal viewing area where the visual experience is better than anywhere else.  Where, from the film editors point of view, is the optimal place to sit in order to get the best visual effects when viewing a 3-D film? Is there such an area?
Comment: Why are you specifically asking for `the film editors point of view`?
Comment: @Oliver_C I asked it that way because the film editor is tasked with presenting the movie in the best way possible. I figured the editor would be targeting the best place to watch the film from. I suppose asking just where the director sits while screening a 3-D film would have also have worked.
Answer: Major S, I would suggest that the focal point in any given shot is not determined by the editor, but by the director of photography and the director. In many instances the DP works in conjunction with the gaffer to light the scene so that the focal point (be it a character or object) is highlighted in accordance to the director's wishes.
This set-up carries through to 3D filming whereby specific elements in the frame can be enhanced or reduced in value in order to make the focal point 'pop'.
This is not carried out by the editor, but is a post-production process handled by the post houses, color timers, compositors et al.
As for an optimum position in the theater, in theory you are supposed to get a great 3D experience from any position in the house (except maybe in the first few rows, but then I know folks who love those seats...), but from personal experience it is always 'middle/middle' for the best in picture and especially sound which, at the end of the day, is 50% of the viewing experience any way.
For the record, any director I have ever known sits at the back of the auditorium during screenings and judges everyone else's reactions. By that point they are usually sick of watching their own film ;)


How do I reference dlls when "added as link" in project?

Question: I have two dll files in the project tree(not the references). They are added as link, they are assemblies of other project in solution. I'm trying to set their Build Action to Embedded Resource, so I can import them to .exe file. I can't write using statement, so I can't reference them in current project. How can that be done?
Comment: In case anyone trying to do same thing: I made it work in another way.
Take a look at this [question](http://stackoverflow.com/questions/7500877/how-do-i-import-referenced-projects-into-exe)
Answer: You need to add a hard reference to the assemblies and set their Copy Local to False, then extract the assemblies from your embedded resources to the application directory before they are invoked. You can't reference a linked (shortcut) like you want.
Key Points (in this example) and the Blog Article with Example Code

EmbeddedReferenceApplication hard references EmbeddedReference.dll
EmbeddedReference reference property Copy Local is set to False
Linked assembly (Add as Link) is set as Embedded Resource

Here is a working example. (EmbeddedReferenceApplication.exe | Console Application)
using System;
using System.Collections.Generic;
using System.Text;
using System.IO;
using System.Windows.Forms;
using System.Reflection;
using EmbeddedReference; // Hard reference with Copy Local = False

namespace EmbeddedReferenceApplication {
    class Program {
        static void Main(string[] args) {
            AppDomain.CurrentDomain.AssemblyResolve += AppDomain_AssemblyResolve;
            MyMain();
        }

        private static void MyMain() {
            EmbeddedReference.MessageHelper.ShowMessage();
        }

        private static Assembly AppDomain_AssemblyResolve(object sender, ResolveEventArgs args) {
            string manifestResourceName = "EmbeddedReferenceApplication.EmbeddedReference.dll"; // You can also do Assembly.GetExecutingAssembly().GetManifestResourceNames();
            string path = Path.Combine(Application.StartupPath, manifestResourceName.Replace("EmbeddedReferenceApplication.", ""));
            ExtractEmbeddedAssembly(manifestResourceName, path);
            Assembly resolvedAssembly = Assembly.LoadFile(path);
            return resolvedAssembly;
        }

        private static void ExtractEmbeddedAssembly(string manifestResourceName, string path) {
            Assembly assembly = Assembly.GetExecutingAssembly();
            using (Stream stream = assembly.GetManifestResourceStream(manifestResourceName)) {
                byte[] buffer = new byte[stream.Length];
                stream.Read(buffer, 0, buffer.Length);
                using (FileStream fstream = new FileStream(path, FileMode.Create)) {
                    fstream.Write(buffer, 0, buffer.Length);
                }
            }
        }
    }
}

In EmbeddedReference.dll
using System;
using System.Collections.Generic;
using System.Text;

namespace EmbeddedReference {
    public static class MessageHelper {
        public static void ShowMessage() {
            Console.WriteLine("Hello World!");
        }
    }
}

Comment: You add a hard reference to the actual DLL itself from the same location you added it as a link. You are just setting Copy Local to false so its not physically copied on disk. Your added as link copies set to embedded resources are what will ensure they get compiled into the output binary. Take a look at the blog if you havn't and download the example project, it is a fully functional example
Comment: I can't even run it when it's not copied. Can you provide an example? I need to get the dll dynamically somehow. It has to be build everytime.
Comment: You would have to extract the assemblies *before* any code calls on them, the best place to do this would be in your Program.cs class before any code is run. Otherwise when your program runs a line of code that makes a call that references that assembly and it cannot be resolved, your application will crash.
Comment: Can you provide an example please?
Comment: I will write a blog post on it with a Visual Studio solution for download and post the article back here when I'm done.
Comment: Updated my answer with a working solution. Blog post will come shortly.
Comment: Thanks a lot for your effort. Is that supposed to copy dll into its own path when executed?


Induced current in a circular loop

Question: In this question I tried using right hand thumb rule for long wire to get magnetic field into plane of paper which induces clockwise current in Loop,again by thumb rule. The problem arises when force is asked for which you cannot use thumb rule and for using right hand Flemings rule you need a certain direction of content not current looping.
Answer: Think this way: clockwise current in the loop creates flux into paper.
It would want to oppose the trend, which fixes the trend - flux change due to motion has to be out of paper, towards us.
That is achieved when motion is in eastern direction.
Hope that is helpful.
Comment: That is achieved when motion is in eastern direction. Please explain (which rule and how you use it)
Comment: So, we want to move the loop, so that flux change is toward us.
Comment: If you fix position, the flux from infinite wire is into paper.
Comment: If you decrease the magnitude, you will get desired action - flux change toward us. decreasing flux magnitude is achieved by moving loop farther from wire. Hope this helps.

Induced current in a circular loop


Divisibility problem.

Question: In line written squares of natural numbers from 1 to 2012. How many of these numbers have a remainder when divided by 17, which is divisible by 3?
Answer: Hint:  make a list of the squares of the numbers $0$ through $16$ and compute the remainder on dividing by $17$.  See how many and which ones they are.  There are $\lfloor \frac {2102}{17} \rfloor=118$ complete cycles and a bit.
Comment: I can't create rule to explain this. I have found two squares, which remainder on dividing by 17 is divisible by 3. It's 10 and 14, but i do not understand the next step.
Comment: @DanSh:  There are two others.
Comment: @DanSh:  The idea is that if $m^2$ has a remainder divisible by $3$ when divided by $17,$ so will $(m+17k)^2=m^2+34mk+289k^2$ because the last two terms are divisible by $17$
Comment: Still do not understand. what can i get from $(m+17k)^{2} = m^{2} + 34mk + 289k^{2}$? the last two terms are divisible, this is okey, but it is my start conditions, that $m^{2}$ must be divided by 17.
Comment: @DanSh:  It means that the cycle will repeat every $17$.  Just as $10^2 \equiv 15 \pmod {17}$, so are $(10+17)^2, (10+2\cdot 17)^2, \ldots$

Divisibility problem.


How to copy and paste in built-in dictionary

Question: I know how to look up words in the built-in dictionary in my iPhone, but is there some easy way to copy something from  the  dictionary or is there some 3rd party App that can do it?
Answer: There doesn't seem to be a way to copy from the built in dictionary, but there are workarounds.
As of recent versions of iOS, I simply take a screenshot and copy the text. Use the icon in the lower right and select text.

This works most of the time, but will not properly copy IPA.
You can also find most of the definitions (at least from NOAD and ODE) online by typing define word into Google/Bing. You can switch between them with these bookmarklets I made.
Switch to British English (ODE):
javascript:window.location=window.location.toString().includes('gl=us')?window.location.toString().replace(/gl=us/,'gl=gb'):window.location.toString()+'&gl=gb'

Switch to American English (NOAD):
javascript:window.location=window.location.toString().includes('gl=gb')?window.location.toString().replace(/gl=gb/,'gl=us'):window.location.toString()+'&gl=us'


How to View Windows Files on a Wubi install?

Question: I wanted to ask, how can i view all my windows files on Ubuntu. I found out how to get my D drive folders but have yet not figured out how to get my C drive files. Please give me an easy way to find my files. For more information, i have installed ubuntu via wubi. Please answer my question ASAP.
Answer: In a Wubi install your windows partition (your C: drive) is mounted in /host/
In nautilus, the file manager, click on the File System icon at the left bar. In the right panel there is view with some folders, all your C: drive contents will be inside of the one called host.
Comment: If C is the drive where windows is installed your contents should be accessible as I said.
Comment: Thnx but i want my C drive contents
Comment: Thank you so much.  Do you know if I can configure this to be readonly?
Comment: I don't think so. I don't really understand how /host works (it's mounted at boot time, but not in fstab), but the Ubuntu image is contained in it. It's likely that a read-only host means a read-only Ubuntu. But as I said I don't really understand how it works.
Answer: To explore your harddisks you can use Disk Utility. Open the Dash with the Windows-key and type in disk... and start the application.

On the left hand you find your Storage Devices. Choose the right one. Now you can mount the proper volume (if not yet done) on the right hand (Mount Volume) and open the folder by clicking the path shown under Mount Point.
Be careful with options like Format Drive, Edit Partition, and Delete Partition! You could lose data or damage your system.
You should be able to find your user data under (C)/Users/(Username)/... (Documents, Music, etc.)
Comment: As this is a wubi install, the volume should be already mounted. But anyway, this is a great idea, it appears there as mounted and the link works.
Answer: In Ubuntu, open Nautilus and navigate to the root of the file system( default open location should be /home/home, just navigate back to /)
After, open the directory called "host". It is C: directory from Windows.


Are letterhead and signatures necessary in the LOR?

Question: My manager (one of the recommenders) said that he is working from home for several months due to coronavirus. It will not be possible for him to give the recommendation letter on the letterhead.
Instead, he said that he would submit it via email.
My question is, will this letter count towards my application?
Comment: The letterhead probably exists in electronic form, and signatures can be added digitally.
Comment: You'd have to ask the university where you are applying. But I think it's the wrong question, if your manager knows how to use a web browser then he can upload it through the browser just like everyone else does.
Comment: @DanRomik, Respected Prof. Can you kindly tell me what are all the ways in which recommenders submit the recommendation. For example, email with reference number, form with PDF ...
Comment: It depends on the specific university, I can't say. But in the US most universities will expect recommenders to submit the letter by uploading a PDF of the letter to some web portal. There might be an option to mail in a paper copy for those who prefer to do things the old-fashioned way.
Comment: Is there any option to submit it via an email using the application reference number?
Answer: In this era of networked printers, few people ever actually load letterhead paper into a printer any more.  Instead, most organizations have electronic "identity" or "branding" collections that include templates for letterhead, business cards, etc.
As such, in my experience, most recipients (in the US and Europe at least) expect letters to arrive in PDF format. Many will not even allow a letter to be sent physically any more.
For most recommenders, then, using "letterhead" just means taking care to use the fancy formatting to make a good impression, just like one might wear formal clothing to make a good impression at a fancy event.  In dealing with academics, however, eccentricity is to be expected, and variation from norms is often tolerated.
Bottom line: your recommender should probably be able to use letterhead electronically. If for some strange reason they can't, it shouldn't be a big deal, as long as it's still easy to tell they are a real and respectable person (e.g., emailing from an institutional address).
Comment: @DanielHatton For certain subsets of the math/compsci community, sure. For most academic communities, that's just giving them extra work that they will not bother trying to figure out how to do.
Comment: @DeepakTatyajiAhire If I get a recommendation letter sent from "email@example.com", I have no idea who is writing to me.  If it comes from "firstname.lastname@example.com", then it's probably come from somebody named Mary Jones who works at Microsoft (or else a rather unusual phishing attack).  Not a particularly challenging test, but significant all the same.
Comment: 'as long as it's still easy to tell they are a real and respectable person (e.g., emailing from an institutional address).'  ... or making it a PGP/MIME signed e-mail with the public key distributed via the institution's https website.
Comment: Oh, you're quite right that most of the time the recipients don't know how to verify the PGP/MIME signature; but usually, having realized that they don't know how to verify the signature, they just accept the message is genuine without further question, which AIUI is the desired outcome here.
Comment: Also, take into account the coronavirus situations. Almost all employees are working from home and the work load is high.
Comment: Also, @jakebeal, can you elaborate on the authenticity if the recommendation is sent via organization/institutional email.
Answer: Yes, it will count. Since you write "manager" I assume it's someone in industry, not academia, and so your manager may not be familiar with academic letters of recommendation.
Probably the most important thing is that the letter should be a separate PDF document, and not just an email message.  If the organization has an electronic letterhead, your recommender should use it. If not, then the recommender should put all the information one would find in a letterhead at the top of the letter.  It should absolutely be mailed from the organization of the recommender, and not from firstname.lastname@example.com.
I have a facsimile letterhead and a scanned signature in blue that's set up as an Acrobat stamp.  I can make a PDF look like a scanned paper letter more easily than I can print and scan a paper letter!
Finally, if your recommender is not an academic, here is some advice I've written about writing letters of recommendation which the recommender may find helpful. http://ksuweb.kennesaw.edu/faculty/rbrow211/recommendations/recommend_advice.html
The advice about FERPA is specific to the United States, and if your recommender is not an academic at an institution where you've studied, it does not apply at all.  It probably doesn't apply for a letter of recommendation from an academic as there is a specific exemption in FERPA for such letterd.
Comment: That will be perfect.
Comment: My manager works at Microsoft and he will be submitting it via email from his organization email ID
Comment: Thank you @Bob Brown! This is a great relief.
Answer: Many universities have online systems for submitting recommendations in which the recommenders will get an email with a link to submit.  In many of these they can paste the recommendation into a form or else upload a PDF.  No signature needed. I usually do PDF with my letterhead template but it's not necessary.
Answer: I am not sure about this.  You ask if a letter send by email without letterhead and without signature will be ok.  If the system only accpects LOR via some annoying online portal, then the safe answer is no.  The letter might never even get into the system.
You need to check with the place you are applying.  My guess is that if they accept a LOR by email, then the letter you propose will probably be fine.


What are reasonable steps to ascertain or contact lost property's owner to avoid liability for theft by finding?

Question: Suppose one finds an item on the street that seems to have been lost by someone. What are examples of reasonable steps one could take to attempt to contact the item's rightful owner before concluding that it may in fact not be possible to do so? Once one has reasonably reached this conclusion, is one then legally entitled to keep the property for one's self?
Answer: Following the guidance of the police
In summary:

If the item is hazardous or dangerous you should report it to the police using the emergency number.
If the item is non-hazardous and found in a private place, hand it to the owner of the premises - it's their responsibility to attempt to find the owner and the property becomes theirs if they can't.
If the item is non-hazardous and found on public transport, hand it to the operator - they have their own by-laws about lost property.
If the item is non-hazardous and found in a public place,

if it has a serial number, hand it to the police - they may be able to trace the owner
if it is a government document, hand it to the issuer
if it is of low value, make reasonable efforts to find the owner "these could include asking people nearby or in offices or shops. You could also consider leaving a note with your details. If you can't find the owner there's nothing more we can do and you should dispose of the item."
if it is of high value, make reasonable efforts to find the owner and if you can't hand it to the police.
Comment: I love how there is specific guidance for "government documents" lost in a public place!
Answer: The Theft Act 1968 (see also the prosecution manual) says:

Theft is the dishonest appropriation of "property belonging to another with the intention of permanently depriving the other of it".
Appropriation is not regarded as dishonest if the appropriator "appropriates the property in the belief that the person to whom the property belongs cannot be discovered by taking reasonable steps".

This leaves two paths open to avoid a "theft by finding" charge.

If common-law property rules have made this abandonned property. Then it is not "property belonging to another". I know you're not asking about this path, but look up the law of finders: Parker v British Airways Board, [1982] 1 QB 1004; etc.
If the appropriator believes there's no way to discover the owner by taking reasonable steps. Sometimes this belief can only be reasonably formed after attempting and failing at various reasonable steps.

In assessing dishonesty, and as part of that, whether the "reasonable steps" exception applies, the judge (or jury) must answer:

What was the defendant's actual state of knowledge or belief as to the facts?
Was the defendant's conduct dishonest by the standards of ordinary decent people?

One example of a reasonable step is handing the property to the owner of the venue where the property was found and thereby placing the responsibility on them for attempting to return it. Many other examples are in the other answers to this question. The question of what is reasonable is determined by the objective standard of what ordinary and decent people should do.
Answer: Depending on the laws of your state and local city & county regulations, it otherwise boils down to being a matter of Civil Code and will vary depending where you live. But, for example Section 2080 of the California Civil Code more or less says you're not required to take responsibility for it (but by taking it, you are in some extent taking responsibility of it) whereas you could have just left it right where you found it so that if the person returned, they would find whatever it is they lost.

What are examples of reasonable steps one could take to attempt to
contact the item's rightful owner before concluding that it may in
fact not be possible to do so?

It depends on what was found, such as if it was something containing a unique serial number, IMEI # (in the case of a phone) or otherwise has a form of being identified as uniquely belonging to a particular person and registered to that person through a service provider or retail seller in the case of high end electronics as opposed to a general item like a Twenty dollar bill then "reasonable" steps would be very limited. But, in either case, "reasonable" is subjective and again, responsibility to find the owner only falls on you if you choose to take charge of it (pick it up and remove it from where it was found). But in either case, what you've done as reasonable may include

Posting a lost & found sign around the area it was found such as you might see with a lost & found pet sign, providing limited details for the person to know you have it and how they can contact you but will need to be vague enough so that the true owner can provide a specific detail to that only the owner would know about it, to prevent false claims of ownership
Posting about a found item online in the local community pages of places like Facebook Marketplace, craigslist etc. or you could try searching for posts of someone mentioning the lost item, and asking when they lost it to see if it coincides with around the time u found it or asking where they think might've lost it. It's hard to provide examples when the subject of what was found is so broad.
As reference, the police department, depending on the value, may takes steps such as trying to notify the owner or otherwise would even wait long enough to publish an item being found in the newspaper if the value of the item is high enough.

Seeing as how broadly stated the question is, if it's something of high value, it's better to check your city's & county's policy on the matter because most likely you'll be expected to turn it in.

Once  one has reasonably reached this conclusion, is one then legally
entitled to keep the property for one's self?

In the case of California, CIV Section 2080.1 says that if the item is worth $100 or more, you're expected to turn the item into the police department or sheriff's department of the county if outside of city limits and file an affidavit (make a statement) about where, when, and how you found the item.
If it's under $100, then there's no stipulation that says you couldn't keep the item per se but in terms of specifically "legally entitled to keep the property" when it's valued over $100 then the answer is, no, you're not "entitled" to keeping it based on your own conclusions but yes after the local authorities have taken a chance at trying to return the item.
What I mean is that you have to first turn it in to police or sheriff's department as aforementioned, then in terms of what happens next, if it's value is over $250 then it's only after the 90 days of a waiting period and 7 days of another waiting period after the published in the newspaper about it will then, only then will the finder have the opportunity to claim it (or be given title of the property) for themselves, assuming it wasn't found during the course of employment (where a customer left it at your work and didn't come back for it), and requires even the finder to pay the cost of publication back to the city before being able to get it back from the police.
Under that same provision, (CA CIV 2080.3),  it also mentions that if it's under $250, then the finder will have the opportunity to take title of the item after just the 90 days without having to wait for an additional 7 days for the newspaper print notices and since there's no newspaper publish, you don't have to pay for the city back for that cost before being able to claiming your title of it.
So legally speaking, you can't take ownership until you've turned it in to local authorities to give them a chance to try finding and contacting the owner and means you can't technically "just keep it" to be legally entitled to it
Notes

It's mostly a safer bet to turn it in and then wait the 90 days because then, if hypothetically you just kept it without turning it in, and that person somehow (depending on the item) discovers you have it, or reports the item as lost and you're somehow found with it later, although it's not a likely scenario, by finding you with the item and having not reported it, will make it appear as if you stole it and harder to claim it as being found.
The answer is based on California state laws and does not include consideration for county and city laws who have the ability of setting different limitations.
As noted for example, in the County of Orange the other stipulations not included part of California state Civil Code:

For Sections 2080.3(a) and 2080.3(b) to be applicable and for title to
be vested in the person who found or saved the property, he or she
must sign the affidavit on the back of the Found Property Report Form
when the deputy takes possession of the property. Refusal to sign the
affidavit at the time the property is turned over to the Sheriff's
Department shall be deemed the intention of the finder to abandon
title to the found property when such title should vest to the finder.
If no owner appears to prove his ownership of the property and the
finder fails to claim the property within 23 days after the 97 day
holding period (value $250 or over) or within 30 days after the 90 day
holding period (value under $250), or the finder refuses to pay all
reasonable charges, then title to the property shall not vest in the
finder and the property shall be processed as unclaimed property.
Answer: If you worry about this, the first step would be that you can ignore the item. It is entirely legal to completely ignore an item that you believe may be lost.
Obvious steps would be examining the item. For example, there is a debit card in my wallet. If you don't find me, then I'd hope that any court would convict you. You could have just called the bank (their number is on the card) and talked to them. Is that inconvenient? A bit, but you could have left my wallet where it is.
Another very obvious step would be to ask the police what to do.
The exact rules are different per country. For example, if you find my wallet on a train in Germany and don't find me, you are guilty of theft. That's because items on a train (or on anyone's property, like an airport, library, someone's home) is legally in the possession of the ones responsible for the property, like the train operator, and taking it away is theft.
Comment: Okay but the question is tagged as England and Wales.
Comment: I'm not sure it always was.
Comment: I think it was from the start.


How does AIC vs. LASSO work?

Question: I understand that LASSO and AIC are striking for a balance between model fit and size. However, how do they respectively measure the size/complexity of the model?
Does AIC measure the number of parameters and LASSO measures the coefficients?
Comment: See @FrankHarrell's answer here https://stats.stackexchange.com/questions/48200/lasso-vs-aic-for-feature-selection-with-the-cox-model?rq=1
Answer: Comparing AIC and LASSO is comparing apples and oranges.
LASSO is an estimator of the regression coefficients. It is the minimizer of a penalized/constrained least squares criterion. Which LASSO estimator to use (defined by the tuning parameter scaling the penalty or constraint set) is most commonly decided by cross-validation.
AIC, on the other hand, is an estimator of prediction error for a given estimator. Given a set of candidate estimators, one could compare these by comparing their respective AIC values. As you suggest, AIC does consider model complexity as it is partly a function of an estimator's degrees of freedom.
In principle, you could compare the set of LASSO estimators using a version of AIC rather than cross-validation (but this is potentially problematic, and is not advised in general). See Is it possible to calculate AIC and BIC for lasso regression models?.
Answer: So let's make an assumption of normality (so that MLE == OLS) and take a look at the AIC equation from wiki: AIC = 2k + n ln(RSS) here k is the number of parameters (variables), n is the sample size, and RSS is the residual sum of squares.  So for a given k and n we minimize the AIC by simply fitting for standard ols coefficeints.  In other words, it has nothing to do with our fitting procedure/coefficients. It is simply a cost function to compare across what we can change: the k.  So the AIC is used to compare the same model with differing features to essentially find the model with the most bang for your buck.  If k increases but your RSS does not decrease enough then your AIC will increase and you have a 'worse' model.  It really is about the balance of k and RSS.
Lasso on the other hand is a cost function that we are optimizing for, so it does have to do with our coefficients.  Specifically we choose coefficients which minimize: the sum of squared errors PLUS the sum of absolute value of our coefficients times a parameter lambda we select:
Basically, (assuming normality) the AIC will be minimized for a given k at OLS so the only real change we can make is changing k with different subsets.  For Lasso we make changes by changing our coefficients and naturally we can select a coefficient of 0 to drop it.

How does AIC vs. LASSO work?


Difference between Main Account and Account 1

Question: Is there a Difference between "Main Account" and "Account 1", because I lost my password for the Main Account, which has no Eth on it? Do I need the password for the main account in any way (technical or maybe in the future for updates etc)?
Thx.
Answer:
Is there a Difference between "Main Account" and "Account 1",

It depends what you mean by "Main Account".
The accounts are 0-indexed, meaning your "first" account is web3.eth.accounts[0], your "second" account is web3.eth.accounts[1], and so on.
Your coinbase account is the account to which any mined ether are sent. By default this will be account 0, but you can change it to point to any of your other accounts, e.g. web3.eth.coinbase = web3.eth.accounts[3].

Danger: Back-up your keystore before following the below part
If you want to remove any of your accounts, remove the associated folder from your ~/.ethereum/keystore/ directory. Removing your unused account[0] will cause any later accounts to shuffle up, meaning your old account[1] will become your new account[0].
Answer: They are separate accounts.
If there is no eth in "Main Account," and you don't plan on ever accessing that address, then you will not need its password for anything.
Answer: All accounts are equal! But if you are mining, make sure you have the keys to your coinbase account.


SharePoint workflow history SP2007

Question: How can I determine how many workflows were run in 2015 on my SharePoint 2007?
Thanks for any assistance!
Answer: First off I am going to offer some apologies as the solution I am going to provide may be a bit off as I have done very little custom dev with SP2007 so please bear with me and if something doesn't make sense then please ask.  My solution makes the assumption that all of your workflows write to the default Workflow History list and not a custom list.
You are going to want to create a console\batch app for this.  Every workflow that is created in a site has a GUID based ID.  So basically this solution is going to get all of the history for the year and then get the number of distinct entries for the field to get you that count.
For each site you are going to want to create an SPQuery object that looks something like this:
<Where>
  <Geq>
     <FieldRef Name='Created' />
     <Value IncludeTimeValue='TRUE' Type='DateTime'>2015-01-01T09:27:54Z</Value>
  </Geq>
</Where>

This will get you the list of items that were created in the year for that site.
Next run the code to get the data you need from the site's workflow history list:
using(SPSite spSite = new SPSite("site url"))
{
  using(SPWeb spWeb = spSite.RootWeb)
  {
    SPList wfHistList = spWeb.Lists["Workflow History"];

    //use the query above (created into an SPQuery object) to populate a datatable
    DataTable allItemsTable = wfHistList.GetItems(spQuery).GetDataTable();

    //create a new view against that table and move the items into the new table
    DataView distinctView = new DataView(allItemsTable);
    //It needs to be against the internal name of the Workflow History Parent Instance column
    String[] distinctColumn = {"WorkflowInstance"};

    //you can now create a datatable using the view as true indicates you only want the distinct items.
    DataTable distinctItemsTbl= distinctView.ToTable(true, distinctColumn);
  }
}

So you can now handle this as you will.  Add that to a master table or output per site, but either way, your distinctItemsTbl should only have a single row for each Workflow created in your Workflow History list.
I strongly suggest you do not run this during peak hours as it could definitely affect performance.
Hope this helps.
Comment: Thanks for the information. I do not have session access to the server(s)

SharePoint workflow history SP2007


Use 5V pH sensor with 3.3V NodeMCU

Question: I'm trying to use a pH probe with NodeMCU, the problem is that NodeMCU is 3.3V and the pH analog sensor requires 5V, at first I thought using the VIN pin but didn't work, then I used the VU pin from NodeMCU V3 (by Lolin) but it didn't give me the right values, like if it missed more power. Is there any other way? Or any esp8266 board which supports 5V into it's analog input?
Comment: Try and get a 5V ADC board that you can talk to over I2C and a 5v->3.3V logic level convertor for the I2C bus.
Comment: your divider was probably OK after all - some nodeMCU boards don't spit out 5v on VIN. There's not a ton of consistency on nodeMCU devboards but i've found the ones with CH340s to all be deficient in that regard, and all the CP2102s to be good, look for "amica" on the back, and avoid the ones that say "lolin". All the Wemos D1 minis i've tried (about 20 units in 3 variations) work and have 3.3ADCs and 5v VIN, as expected.
Answer: The simplest method is just a voltage divider - just as if you were interfacing a 5V logic signal output with a 3.3V input. However it may not work.
A voltage divider will only really be of use if the output of the pH sensor is low impedance. If it's not then you will need a very high impedance voltage divider, and the analog input of the ESP8266 isn't high enough impedance to support a very high impedance voltage divider - it will skew the results horribly.
So you may need to buffer the output of the pH sensor through an op-amp first.
If you use a unity gain non-inverting buffer you can then use a voltage divider to reduce the output voltage.

simulate this circuit – Schematic created using CircuitLab
Alternatively, as others have suggested, use an external 5V ADC chip and logic level conversion to communicate with the ESP8266.
Answer: Depending on your specific sensor and power supply setup you have following options:

Change supply voltage
Most resistive sensors don't have to power anything and will function just fine when supplied with less than 5V. Study the datasheet and make test measurements to find out whether this is suitable.
Use a voltage divider
Supply the sensor with 5V and use a simple voltage divider before    connecting to your MCU and therefore measure 0-3.3V.
Interface another IC
Get a small external ADC that can handle 5V readings and may be interfaced by any serial protocol supported by your esp8266 (namely UART and I2C).
Comment: I would also recommend using an external ADC for example: [link](https://www.adafruit.com/product/856)
Comment: The external ADC is imo the least favourable option as it needs more hardware work (maybe even different casing), MUCH more code and is generally less efficient.
Comment: @mistery Thanks for your answer! Well actually I already tried with a voltage divider, from 5V to 3.3V and worked but just in some cases; I'm using NodeMCU VU pin which supposedly gives 5V, but for example, when pH is lower, the sensor needs more power to give an accurate value, and when this happens NodeMCU stops getting those readings, let's say under 4.80 (in a scale of 0-14) it stops reading analog value changes, so I'm thinking probably NodeMCU can't really provide 5V.
My voltage divider is with 1K and 2K, I'm thinking to probably use a Wemos board, it has a 5V pin, what do you think?
Comment: I still don't know what exact board you have. my idea was to power the sensor from your power supply, not through the chip. You should really study the sensor's data sheet to get an idea of it's behaviour
Answer: Old topic, but in case it shows up in searches, like the one I was just doing ... I run a 5v ultrasonic SR04 off a Lolin nodemcu, and if you connect to "VU" on the Lolin nodemcu, you will get 5v.
Comment: Part of the problem was also reading 5v from sensor
Comment: Tested here with a voltmeter and indeed `VU` gives 5V. Good one!
Answer: Have you guys checked the specification of the AiThinker's ESP-12s? There's a piece of description in its Chinese edition document that says if you are hooking up other devices which work in 5v, you have to use the Level converter circuit in between.
Comment: the question is about powering, not about Serial comunication

Use 5V pH sensor with 3.3V NodeMCU


Require seatpost size of Dahon Vitesse D7

Question: I have a Dahon Vitesse D7.. Have lost the seat post. Can you let me know what is the size of the seatpost  will fit to the mentioned model
Comment: You need to measure the place where the post goes.  Or better, find a post that fits (even if the wrong style) and measure that.  There are too many different post sizes to give a number out of the air.
Answer: Go to a bike shop and get it measured.
They'll have some seatpost measuring tool like this:

You basically stick it into the seat tube and it tells you what size seatpost you need.
Alternatively, you could use a vernier caliper to measure the seat tube diameter on the inside (this is key!).
Note that the seat tube diameter is only one dimension; you also need to know what length tube you need. Most seatposts have the same clamp type as well, but you may want to get one with extra fore/aft adjustment or whatever. So, going to a shop and buying one isn't a bad idea if you don't know what you need.


Can an attacker sniff data in a URL over HTTPS?

Question: Can data included in a URL be considered secure if the connection is made over HTTPS?  For example, if a user clicks a link in an email that points to https://mysite.com?mysecretstring=1234 would it be possible for an attacker to grab "mysecretstring" from the URL?
Answer: The entire HTTP request (and response) is encrypted, including the URL.
But yes, there is a way an attacker could grab the full URL: through the Referer header. If there is any external file (Javscript, CSS, etc.) which is not over HTTPS, the full URL could be sniffed in the Referer header. Same if the user click on a link in the page that leads to an HTTP (no SSL) page.
Also, DNS requests are not encrypted, so an attacker could know the user is going to mysite.com.
Comment: so, load no external images, css, js. use/store the secret string, and redirect internally to get rid of secret string. after that can use external urls.
Comment: When you say "the full URL", does that include the parameters (e.g. mysecretstring=1234)?
Comment: On the Referer header, if parameters are in the URL it can be seen
Answer: No, they can see the connection ie mysite.com but not the ?mysecretstring=1234 the https is server to server
Comment: In fact they cannot even see which domain name you are connecting to but which IP address. Since SSL certificates only reasonably work on a 1:1 domain-name-to-IP-address relationship, this is most likely irrelevant. Also if the attacker can sniff your DNS traffic, this might be revealed. GET and POST parameters are as secure as the HTTPS traffic: If you are the client and the server certificate is valid an uncompromised, the data is secure against eavesdropping by third parties.
Answer: They would need to have the encryption key. Theoretically this is not possible but any good attack could. This is the whole purpose of SSL to encrypt all data sent to and from the server to prevent being able to sniff.
Answer: Keep your weblogs secure, or, don't even write them.  If you get a remote exploit where the logs can be read, any URL data will be visible in the logs.
Comment: Post data isn't in the logs.
Comment: all of this is very configuration dependent =)
Answer: Only if they are able to sniff the https auth through some kind of spoofing
Comment: Do you mean a man-in-the-middle attack? This is more than sniffing, the attacker need to impersonate the server.
Comment: well its MiM for the handshake, but after it is just sniffing, the specific tool is hamster and ferret that i saw demonstrated


Want to bottle and sell a marinade/sauce

Question: I have a marinade recipe I want to sell to local small stores but I don't know the logistics of how to make the product last with shelf life and I've read about oils causing botulism. I'm not sure where to find this information or how to go about it .
Answer: Check the colleges near you as they may have a food incubator program. There are several in Louisiana both affiliated with colleges and privately owned. They have programs to teach you the basics, professors to help answer questions, staff to help take your recipe from small batch to larger quantities, and commercial kitchens for rent.
You can also contact the Department of Health. they were quite helpful when I was starting my company, especially with label reviews.
If you are planning on making an acidified product, you should look into a Better Process Control course. These are offered at some universities (some have online options) and are a requirement of most food incubators. Hope this helps.
Answer: From what I understand it's all about how acid/salty, low water is the food
(Shelf stable acid preserved foods and Food Processing for Entrepreneurs Series)
It is also about how the food is packaged, you need to have a sterile environment (as sterile as possible); hot packaging, pasteurization,...
I suggest looking at local regulations for commercial kitchen.


Do quantum superposition occur at sound waves also like at Electromagnetic waves?

Question: Like at the electromagnetic waves we see that they interfers the way like two different wave with frequencies can exist in the same place.
Answer: Why quantum superposition? An electromagnetic wave (in vacuum of in a medium) can be thought of as a classical wave and so does a sound wave (through air or any other medium).
Any wave obeys superposition at the classical level! Since the wave equation is linear, a superposition of two propagating waves is also a solution of the wave equation. And superposition can give rise to a whole host of phenomena. For example, when two sound waves (or the electric fields of an electromagnetic wave) of same frequency and constant phase difference are superposed, it gives rise to the phenomenon of interference while the superposition of two sound waves of slightly different frequencies lead to beats.


get list of users of fans on my facebook page GRAPH API

Question: How can I get list of users of fans on my facebook page
Answer: Unfortunately it is currently not possible to get a list of fans for your Facebook page. It is not clear whether this is by design or is a bug.
This Facebook bug report suggests that this feature stopped working during October 2010 and is still an outstanding issue.
I hope that it is fixed soon!


Modal verbs usage

Question: Why do we use "have to have a visa to travel abroad", but not "must have a visa to travel abroad"? As far as I'm concerned, it's impossible to travel overseas without a visa, it's a law, so we should use "must" right?
Comment: You (in most cases) have to have a passport to travel abroad, but depending on what passport you have and what country you are travelling to, you might not need a visa. But that's a question for Travel Stackexchange, not here.
Comment: Can you give the complete sentence you're asking about, because "must" can be used depending on the situation. It might relate to tense or other grammatical constructions, but we can't read minds.
Answer: In everyday usage, "have to" and "must" are used interchangeably. In your example, either one can be used, with no change in meaning.
In very formal contexts, you might prefer to use "must".
In the past tense, you must use "had to"

Last year I had to visit the doctor 4 times.

not

(*) Last year I must visit the doctor 4 times.
Comment: Thanks, The Photon. I got it. The example was from a Cambridge's exercise book, using only have to, so I was a bit convinced that it's the only correct way to say. Now I know.

Modal verbs usage


Is there a sense in which one could expand $\frac{\sin (x/\epsilon )}{x} $ in powers of $\epsilon $?

Question: A standard representation of the $\delta $-distribution is
$$
\pi \delta (x) = \lim_{\epsilon \searrow 0} \frac{\sin (x/\epsilon )}{x}
$$
Is there a sense in which this could be seen as the leading term in an expansion of $\sin (x/\epsilon ) /x$ in positive powers of $\epsilon $, presumably with distributions as coefficients? If so, is it possible to give the expansion explicitly?
Comment: Your distribution is the Dirichlet kernel, so the discrepancy $f(0)-(D_{1/\epsilon}*f)(0) = \int_{|t|>1/\epsilon} \widehat{f}(t)\, dt$ goes to zero faster than any power of $\epsilon$, thanks to the rapid decay of $\widehat{f}$.
Comment: @ChristianRemling - ah yes, I see what you're saying, this provides some clarity. The behavior is manifestly determined by the behavior of the Fourier transform of the test function at infinity, and strictly speaking, that should decay faster than any power. Now, I might get physicist notions like applying this on less well-behaved spaces than bona-fide test functions - so the Fourier transform might only decay as a power, and then I'd get nonvanishing coefficients in the expansion in powers of $\epsilon $. I wonder how this would look in $x$-space, that's another way of putting the question.
Comment: Do you see the sign of the expression depends on the sign of $\epsilon$, so the direction of the limit is important? .
Comment: For what it worth, at $x\to0$ the function becomes $\frac1\epsilon$.
Comment: Of course, I had in mind the usual $\epsilon \searrow 0$. I've edited the question.
Comment: I've asked the same question on MSE but it went essentially unanswered. Here is the link https://math.stackexchange.com/questions/463724/sinc-to-delta-function-error-term
Answer: If you are interested in Laurent series, the expansion is
$$\frac{\sin (x/\epsilon)}x=\frac{1}{\epsilon }-\frac{x^2}{6 \epsilon ^3}+\frac{x^4}{120 \epsilon ^5}-\frac{x^6}{5040 \epsilon ^7}+\frac{x^8}{362880 \epsilon ^9}+O\left(\frac{1}{\epsilon^{11} }\right)=\sum_{n=0}^\infty \frac {(-1)^{2n}x^{2n}}{(2n+1)!\epsilon^{2n+1}}$$
Comment: Ah, I see I've again not been specific enough in the formulation of the question. No, by power series I meant positive powers, not the Laurent series, which is of course not very useful. I've once more edited the question.
Answer: The expansion around point $a$ in positive powers exists, but what is the general form of the term and whether it is convergent requires further research.
$$\frac{\sin (x/\epsilon)}x=\frac{\sin \left(\frac{x}{a}\right)}{x}-\frac{(\epsilon -a) \cos \left(\frac{x}{a}\right)}{a^2}+\frac{(\epsilon -a)^2 \left(2 a \cos \left(\frac{x}{a}\right)-x \sin \left(\frac{x}{a}\right)\right)}{2 a^4}$$
$$+\frac{(\epsilon -a)^3 \left(-6 a^2 \cos \left(\frac{x}{a}\right)+x^2 \cos \left(\frac{x}{a}\right)+6 a x \sin \left(\frac{x}{a}\right)\right)}{6 a^6}$$
$$+\frac{(\epsilon -a)^4 \left(24 a^3 \cos \left(\frac{x}{a}\right)-36 a^2 x \sin \left(\frac{x}{a}\right)+x^3 \sin \left(\frac{x}{a}\right)-12 a x^2 \cos \left(\frac{x}{a}\right)\right)}{24 a^8}$$
$$+\frac{(\epsilon -a)^5 \left(-120 a^4 \cos \left(\frac{x}{a}\right)+240 a^3 x \sin \left(\frac{x}{a}\right)+120 a^2 x^2 \cos \left(\frac{x}{a}\right)-x^4 \cos \left(\frac{x}{a}\right)-20 a x^3 \sin \left(\frac{x}{a}\right)\right)}{120 a^{10}}$$
$$+O\left((\epsilon -a)^6\right)$$
Comment: This of course works for $a\neq 0$. Can we make sense of this as $a\searrow 0$? Do the coefficients perhaps converge in a suitable (distribution?) sense to give a series convergent to $\sin(x/\epsilon)/x$?


Does one need cash in the UK?

Question: Are there any places in the UK where one still can't pay with a card / contactless? Restaurants, campsites, etc.? Is it a good idea to bring / withdraw a few GBP just to be on the safe side, or is it completely unnecessary? I've got an EUR bank account with a Mastercard debit card. My bank doesn't charge extra fees for non-EUR card transactions (other than cash withdrawals, which cost €5 flat).
Comment: @MartinBa It's perfectly standard for banks not to charge fees for card transactions – I don't know any banks here that do so for private cards. What they do often do is give you lousy exchange rates, which is essentially a hidden fee, but there's no explicit fee, unlike international cash withdrawals (which generally result in bad exchange rates _and_ a flat fee).
Comment: Homeless people.
Comment: "My bank doesn't charge extra fees for non-EUR card transactions" - wow. congrats on your contract. :-)
Comment: Your main problem will be buying a can of coke or a packet of crisps. A lot of places will only accept cash for that sort of transaction.
Comment: @MartinBa Of course there is an exchange rate penalty, but that can't be avoided anyway. I could of course withdraw some EUR at home, bring them to the UK, find a money changer with the best rate and convert them to GBP. This way, I might save roughly €2 on every €100 changed. IMO, absolutely not worth the hassle.
Comment: @Johnnyjanko check your card exchange rate or take out a credit card with a visa/mastercard exchange rate so you can pay in pounds vs taking out cash with your relevant travel money provider.
This is the for the UK but some of these cards you might be able too apply for
https://www.moneysavingexpert.com/credit-cards/travel-credit-cards/
Comment: Can you clarify you have any dependants (children) as having cash to spend might be easier than their own cards unless you create their own bank accounts.
See @Chris H - UK Comment below
Comment: @maskin Yes, I'll be travelling with kids, but since all answers already are "also carry some cash", I guess it doesn't really change anything.
Comment: Depends how far out in the sticks you go.
Comment: @Johnnyjanko There are some cards for children for example,
https://www.starlingbank.com/current-account/kite-debit-card-for-kids/
Starling Bank has euro and pound accounts.
Comment: @Johnnyjanko - well, both my Austrian banks *do* charge extra fees *per transaction* when dealing in non EUR. (On top of any possibly suboptimal exchange rate.)
Answer: I live in the UK, and don't use cash very often any more, but wouldn't be without it.
Twice in the last few weeks I've been to takeaway food places that were cash only. One was across the road from a cash machine/ATM but the other wasn't - and the former would have made for some very expensive chips, with a €5 fee for cash. In both cases, they were the last places open without a detour, while cycling, so just choosing somewhere else wasn't realistic (in one case it would have meant a hungry night in the middle of a 600km ride).
Similarly on long rides (that's mainly how I travel these days) I've often stopped at village hall cake sales - more interesting than a coffee shop, but cash only.
Some convenience stores and small shops still have a minimum card spend, which isn't handy if you only want a drink. This includes things like ice creams at campsite reception, when they otherwise like cards.
When going to the beach, cheap privately-run seasonal car parks are often cash only. They may lack enough signal or just not have invested in mobile payment terminals. This is often the sort of place with no ATM nearby. In general, if there are limited options to get dinner, and you really don't want to carry cash, you should probably book somewhere in advance to be sure of getting in at a place that takes cards (the vast majority of pubs and restaurants do, so this is about capacity and opening hours).
Contactless payment terminals are becoming much more realistic for small businesses, even market stalls, to the extent that a coffee stand that was cash-only before covid is now card-only.
As vending machines are still often coin-only (though newer ones take cards), it can be worth carrying coins if you think you might want to use them.
If you don't have a backup card, ideally with another bank, travelling abroad and relying on contactless does put you at the mercy of tripping an automated block on your card - or even cards if they're from the same bank, including joint cards with a partner.
Comment: As a UK resident I take cash with me, but very, very rarely need it. Not long ago merchants would refuse small transactions by card (because they had a poor contract that charged them a fixed fee per transaction). Now it's swung the other way, and many mobile and street traders *won't* take cash.
Comment: **Comments have been [moved to chat](https://chat.stackexchange.com/rooms/147573/discussion-on-answer-by-chris-h-uk-does-one-need-cash-in-the-uk); please do not continue the discussion here.** Before posting a comment below this one, please review the [purposes of comments](/help/privileges/comment). Comments that do not request clarification or suggest improvements usually belong as an [answer](/help/how-to-answer), on [meta], or in [chat]. Comments continuing discussion may be removed.
Answer: I visited Orkney a while ago, admittedtly a very rural part of the UK, and was in a few situations where I had to pay cash. I don't remember exactly where, but I believe the Italian Chapel was one place, where the entrance fee could only be paid with cash and I had at least one situation in Kirkwall, where a shop usually was accepting cards, but the card service was currently out of order.
Since you especially mention camp sites, that would be a kind of location where I would not 100% rely on card payments.
Comment: If "a while ago" was pre-Covid, things have changed a lot in the UK.
Comment: @JackAidley No, a while ago was this January.
Comment: The 3 campsites I'm most familiar with all take cards, with one really preferring them - but I know some of the most basic backpacking sites don't.  Also if your campsite sells ice-creams in reception, and has a minimum card spend, be prepared to be disappointed if you don't have cash
Answer: In my recent travels to England, (I have not been in the other parts of the UK in those trips) I could pay with cards or contactless at all places but one hotel. An other hotel had connection problems on check-in but preferred bank payment later over cash at arrival.
I would still not travel without a few pounds in cash, as connection problems do occur at random times, also when you can not wait till they are sorted.
I have, also before covid, had travel to England in which I did not use cash at all. But I have been in too many situations where cash made life easier.
Just like at home, I can go months without getting cash but then I have a few things to buy where only coins will work like a fending machine which is a bit older, a roadside fruit selling spot, or where all cash is accepted, like the odd market trader who does not do cards or even a butchers or a bike shop only accepting cash.
Comment: Vending machines are a good point too; I have coins with me when I'm most likely to need one. UK snack vending machines are (IME) quite likely to be coin-only, cards are sometimes accepted, notes very rarely indeed
Comment: Cards at vending machines become more common, with acceptance of bank notes going down. Card readers are cheaper to build, and you don't need to return coins. Some machines refuse to give change for coins.
Comment: We are now in a situation where you can expect either cash only, coins only, cards only or even contactless only, or machines that combine two or more. As you can not predict what you are going to find, best have some coins and a card that also allows contactless.
Answer: When I was in London (might not be representative of the whole of UK) a couple of weeks ago, I didn't use any of the cash I had brought (as @Willeke says in her answer, it's good to have), and I even visited one restaurant where the staff told me they only accepted cards (I saw some shops had signs to the same effect).
Answer: Now that my UK trip is over, I can add my own anecdotal story. TL;DR: I didn't have any GBP cash on me at any point in time, except for some small coins that my children found and which we took home as souvenirs.
Longer version: I didn't really plan to go cashless, having read all the advice from more experienced travellers and UK residents. However, I kept asking myself every day "Do I need cash for today and tomorrow?" and each time the answer was negative.
We stayed several nights at a campsite near Dover that accepts online card payments. Then we stayed with friends which was of course for free. I researched all car parks in advance and they were all either cashless or free of charge. That is, except for one in Conwy, Wales, where I messed it up and entered a car park in a marina, which cost GBP 3, cash only. Since I had none, I drove away in search of another car park and found one in front of a school, free of carge, of course empty because it was August, and even closer to the town centre and to the Conwy Castle.
Another cash-only situation was a woman selling balloons on a London street. However, the fact that I had no GBP cash was a great excuse that even my children could understand.
Restaurants, supermarkets and petrol stations of course all accepted cards. The only minor inconvenience I encountered was that contactless card payments didn't work above certain threshold. In my home country, if the amount is €50 or less, a PIN is not needed for a contactless payment; if the amount is higher, one needs to enter PIN as well. In the UK, my contactless transactions above a certain amount were simply rejected and I had to stick the card into the reader and enter the PIN, something I haven't done for ages. Not sure how that would work with an NFC payment by smartphone, since I don't use that.
Again, I'd like to stress out that this is just anecdotal and I don't advise anyone to travel around the UK with no GBP cash. It's just something I did, without actually intending it. Maybe if we visited some really remote places like the Scottish Highlands or even some outlying islands, having no cash might have been an issue. The remotest place we visited was the Snowdonia National Park and there we encountered no issues.
On the way back home when we stopped at a local bakery in rural Germany that only accepted cash, it reminded me that the UK is really much more cashless than most other European countries.
Comment: Re phone payments - Google Pay and Apple Pay both have no limit, unless one is set by the card issuer.
Answer: I agree that the use of cash has declined dramatically in the UK in the past few years but I find that there are still occasions when you need it.
Donations in churches or museums spring to mind. Many just have a donations tin by the door.  Paying for car parking often requires coins in some locations. And tipping. In some restaurants if you pay the tip by credit card (as part of the whole bill) the staff don't always get it, or at least they don't get as much. Whereas if you tip them in cash. It's more likely to go where you want it to go. (So some waiters have told me).
Comment: Even churches now typically have a machine by the door that accepts donations by card. And the tradition of tipping is dying out, both for restaurants and taxis. These days I very often go out without any cash, and £100 from an ATM lasts me for months. In fact, ATMs are becoming rather scarce as a result. Nearly all car parks allow you to pay by phone, though if you're using them a lot you'll need 3 or 4 different apps.
Answer: One thing that may be worth not overlooking... public loos (bathrooms).
It did seem between different trips in 2015, 2019, and 2022 that there has been a marked decline in the number of places that charge (especially after covid??).  But there still were some, even at one or two medium sized transport locations, parks, and in big cities (TFL for instance has a neat link list of borough by borough sublistings, and some still show as pay).
More definitely take cards or services like Google Pay than in the past, but there are still some that don't.
So a small bit of money can help add a little flexibility, especially when traveling a lot in thrifty ways.
Comment: Some railway stations allow contactless for loo but some may only be cash from memory (2023)
Comment: As someone who refuses to pay for toilets on principle, if you are in a pinch then most pub toilets are quite easy to access, especially the larger chain pubs like Wetherspoons. They are also likely to take cards if you want to buy a drink or some food to be polite. But they rarely have access codes like cafes and other places do. Outside of London, larger supermarkets also tend to have toilets which are essentially public.
Answer: I should put this in as an actual answer, rather than just in comments…
To be safe, you need two physical cards, duplicated to your phone if you can, on two payment methods [VISA/Mastercard etc. Amex is not popular in the UK & you might struggle to use it at all] and £20 - £50 in notes, plus a fiver in coins, for just in case.
The recommendation for duplicating onto your phone, ApplePay etc, is because of the high 'zero check' limit on NFC card payments. Some places it's only £40 but others it's £100 on just a tap, no PIN or any ID check required. That means your card is worth a lot if stolen, whereas your phone is no good without your PIN/fingerprint/face.
Your cards/phone must be capable of NFC, as well as chip & PIN - ATMs, of course, are all chip & PIN. Signature went out 20 years ago & no-one will be able to handle that any more.
London buses, for instance, have no way to pay by cash or regular card on board. It must be NFC. You can buy a pre-paid, auto top-up tourist Oyster card online or from train/tube stations to get round this requirement.
In bigger towns/cities you will find many places, pubs, bars, cafes etc have gone card only since covid. Even the homeless guys on London streets selling the Big Issue magazine have NFC readers these days.
In London I only take my phone with me, but I keep a spare 20 rolled up in a tiny metal case on my keyring, for just in case. [I also have a pound-sized bit of metal for supermarket trolleys on my keyring.] Out in the sticks, I'd have physical cards & more cash as well, possibly up to £100 if I was going to less densely-populated areas. In a large city, you're rarely more than 400 yards from an ATM, but out in the wilds there may not even be one in each village.
As already noted elsewhere, right out in the Lake District or the Scottish Highlands, you may be outside phone reception, & relying on finding some wifi to get your ApplePay working is risky.
[Comments note this may not be such a difficulty. ApplePay apparently works without a phone/net connection.]
Car parks have been covered elsewhere, but just to note, the entire car park system in the UK is a nightmare. Some only take cash; some only NFC, some you need an app or make a phone call to an automated system & give the non-human clerk your card details. Many times you will only find out which when you get there. Systems are being updated constantly & one that was cash last year could be pay-by-app this year. Google Maps isn't keeping up.
Comment: Apple pay does not need internet to work, altough you wont get the notification until you get under coverage again, but the payment gets processed also in absence of internet service
Comment: Ah, OK. Didn't know that. Never been anywhere yet it became apparent.
Comment: I also add that you can choose one card to be used in "express-cards" when phone battery is depleted but iphone still allow for transactions on the card set as `express`.
https://support.apple.com/en-us/HT212171
Comment: That's probably more detail than we need. OP hasn't said they even have a compatible phone.
Comment: It depends how prepared/paranoid you want to be. You'll probably be fine with a single Visa/Mastercard, and some cash for emergencies. Even if you can't pay by contactless, you can normally put the card in a slot. The only exception is public transport in some places, as you say, but most cities outside London still take cash (as well as other weird app-based or smartcard-based systems).
Comment: @StuartF - Covered in more depth in other answers, but 'probably' is fine if you're half an hour  from home, not if you're in another country & your card gets stopped.
Comment: Out of interest, how long has that £20 been there ? Because I live in London and I don't use cash at all. The only businesses who insist on cash in my experience can be described as "dodgy geezer coming round on a Sunday to fix the fence". Which I am trying to avoid.
Comment: Local car wash, one Chinese who want cash for delivery unless you want to pay 30% more for JustEat. For either of those we just go to the nearest ATM, 50 yds.  Otherwise it's ApplePay & has been since covid persuaded everybody to get NFC. Before AP I just used to keep £100 in my wallet along with my cards, every time it ran out I got another 100, as & when. I've needed the 20 once, before Apple Pay, forgot my wallet, needed fuel. Last time it saw the light of day was when I realised it was still a paper note, so swapped it for a plastic one. The only cash I carry now is that emergency 20.
Answer: I have travelled extensively, including in the UK, and my personal experience is that you should always keep two small cash reserves separate from your credit and debit cards.
One for discretionary spending, such as with street vendors, and for use in situations when you might not want to be seen taking your cards out (A couple of notes in a back pocket, for example), and one in case your cards are stolen.
My experience of the UK is that almost everywhere prefers cards. Visa is the favorite. American Express and equivalents are widely accepted, but not always. This holds true just about everywhere from Taxi and market stands to big tourist destinations.
Some places accept Applepay, some accept Paypal, few accept crypto.
Almost everywhere accepts cash still.
Comment: I agree with everything except 'some places take ApplePay'.  These days if they have an NFC reader, they take Apple & Google just as easily as a card. In London that's all I carry with me, my phone [& an emergency 20 on my keyring, which I've only used once in 20 years].
Comment: Very occasionally I've come across businesses with cheap card readers that will take bank cards but not Google/Apple, but I agree it's fine on most card readers.
Comment: London is London, outside of it things can be very diffetent
Answer: Yes
Regardless of how many shops do or do not have a credit card reader, the one thing that you do NOT want to happen in general, and abroad especially, is to find yourself without any mean to pay.
This means that you should be looking for redundancies to cover for extraordinary events:

Multiple cards, from multiple providers:

A card can get blocked, or you can trigger the spending threshold.
Not all providers are accepted everywhere.

There may no available card reader:

There's always a few folks who resist credit card; I got to jump the taxi queue once because nobody before me had cash on hand and the driver didn't accept cards.
A shop near me underwent some renovations for a week, a few times in the morning as they were moving tills they could not accept cards as they were not connected.
Card readers fail.

For all those reasons, I recommend traveling with at least two cards (Visa & Mastercard are great in Europe) and a few hundred dollars/euros in cash, though perhaps not all in one place, and not all in your person.
And do remember to check what your spending limit/cash withdrawal limit abroad is with your bank, and possibly give them a heads up about your travel plans so they pre-clear your cards and/or raise the limits.
Comment: While nicely written I do not feel this answer adds anything to the existing answers, but maybe for the amount of money, (which seems much for me, a few nights in a hotel not just a small bit of spending money.)
Comment: It depends where you are but in a city, you'll easily find a place that takes a card, even if the first one doesn't. There may be somewhere which has a broken machine but unless you're in the middle of nowhere you can go somewhere else nearby. Going to the trouble and expense of acquiring an additional credit card or large amounts of foreign currency probably isn't worth your while. But it depends how much of a worrier you are and if they thought that you might be stuck will impede your enjoyment of your trip.
Comment: @StuartF: It depends where you are, and where you incur expenses. An example of issues I've gotten into is having a hotel paid, but the hotel wanting an authorization on a credit card for in-room snacks/mini-bar. It was a pain, exhausted after a day of travel, to argue/negotiate with the manager when my credit card didn't work because I had exhausted the spending limits... and I dearly wished I had another then. I swore to myself: never again :)
Answer: The pier at Clevedon only accepts cash, no idea why.
The cynic in me thinks that any outfit who only wishes to deal in untraceable cash is doing so for nefarious reasons.
Comment: Merchants always have to pay a small commission from card payments, so on every transaction they lose a few % to banks. That's why some opt to not pay that commission and only take cash. (but nefarious reasons are a legitimate reason too, for example to avoid paying taxes in revenue)
Comment: @wanaryytel Business bank accounts charge for depositing cash anyway, so together with handling costs, this isn't really true.
Comment: It does depend on how much the trader gets in in cash, how they pay their suppliers and pay for their needs, but the butcher and the bikeshop owner told me they rarely bring cash to the bank.
Comment: In the Lake District there are a lot of cash-only places due to a lack of broadband/mobile connectivity (they don't allow mobile masts just anywhere in a National Park), I've had the same issue in the Highlands and Islands in Scotland.
Comment: (@JackAidley) banks also charge for getting change  out. The amusement arcade at a peier will take a lot of coins, give out rather less (and most of those through change machines) so by taking payments in cash (mainly notes probably) they're turning coins into cash. Then pay a few suppliers in notes and they avoid banking fees in both directions. Plus they'll want to take cash anyway to separate kids from their pocket money - child accounts with cards aren't all that easy.
Answer: Tour Guides or Tipping

Cash best

Food

Splitting payment with 'new' or old friends (easier than card/bank transfer, we don't have Venmo equilivent)

Cash for Parking is helpful

There are ways to pay by phone but not sure how using your euro card and providing your address via a phone call (though I think EU to UK has free roaming but I'm not sure)

Check your mobile roaming provider/Cash useful if in rural area with no mobile signal

Rural areas don't have great mobile coverage, so a physical card may be best not just one on your phone.

Aka mobile apps won't work if no mobile signal

See: @Dave Gremlin comment above: "In the Lake District there are a lot of cash-only places due to a lack of broadband/mobile connectivity (they don't allow mobile masts just anywhere in a National Park), I've had the same issue in the Highlands and Islands in Scotland."

This is the mobile coverage checker, if you know which network your mobile provider will use.
https://checker.ofcom.org.uk/en-gb/mobile-coverage
Supermarket Trolleys

Need 1 pound coins or adaptor
(For 1 trip, having some change might be worth it.  This avoids buying an adaptor unless the UK and EU use the same one which is unlikely...)
As mentioned by @Tetsujin
Trolley Adaptor: @davidbak This is one example: https://www.tesco.com/groceries/en-GB/products/306350899
Keep It Handy Trolley/Locker Coin
Though 1 review states: "considering Tesco sell these trolly coins you'd think they'd fit in every single Tesco trolly...but they don't...they only fit in some."
Comment: @Johnnyjanko that's becoming more common for well-mapped car parks, with some even requiring payment via an app despite very poor signal meaning you can't download it. Privately-run seasonal car parks on the approach to beach places often don't appear on maps, and are only slowly adopting mobile payment terminals where there's a signal
Comment: @DDS The problem is often you need to make payments via an app such as RingGo, which you would need to download if you didn't have it, and which needs to make a connection every time you make a purchase. Most council areas now seem to have standardised on RingGo, which at least means not downloading lots of different apps, but it still needs a weak internet connection to exchange some data. Some have machines that accept contactless payments, which would be OK with Apple or Google Pay even with no internet connection, but there is a lot of variation.
Comment: Cash for parking?? I've already checked some car parks at the places I want to visit and all of them are "NO CASH". Including remote places such as the Snowdonia National Park.
Comment: Note: At least apple pay does not need mobile signal to complete a purchase (it works just like a contactless card)

Does one need cash in the UK?


When did iota devs adopt ternary over binary?

Question: I read the Nov 30 posting , which was great. Just not clear when project became so aligned to ternary.
Comment: When isn't the same as why
Comment: Indeed I retracted my duplicate vote.
Comment: Thanks. I was hoping this forum wud be the antithesis of Slack.  Good data point
Comment: Can you please add a link to that "Nov 30 posting" to your question via [edit]?
Comment: I'm voting to close this question because it is about the legacy-network and it is a good idea to keep only Shimmer/ShimmerEVM and latest IOTA Chrysalis questions
Answer: Ternary is in the scope from day one. In fact cfb is working on this since many years. Here is a quote from cfb on the nxt forum (from 2014) :

We can count from the 30th of April 2014 when the name "Jinn" came to my mind - https://nxtforum.org/pub-crawl/rfc-project-'jinn'-(processor)/msg14306/#msg14306...
Or from the summer of 2012 when I wrote the ancestor of Jinn emulator (a framework called "Black Heart")...
Or from the autumn of 2010 when I rewrote server code from C to Java (for MMORPG "Apocatastasis")...
Or from the autumn of 2004 when I wrote first lines for the server part of my MMORTS (forgot its name)...
There is no a moment when I started working on Jinn.

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