Detection of Nitrogen and Sulfur in Lassaigne's Test

Question: In Lassaigne's test for the detection of nitrogen and/or sulfur in organic compounds, we prepare Lassaigne's extract. In this extract, NaCN is formed if nitrogen is present, and $\ce{Na2S}$ is formed when sulfur is present. NaCN on treatment with $\ce{FeSO4}$ and $\ce{FeCl3}$ gives a Prussian blue precipitate of $\ce{Fe4[Fe(CN)6]3}$, and $\ce{Na2S}$ on treatment with $\ce{Na2[Fe(CN)5NO]}$ (sodium nitroprusside) will give a violet color due to the formation of $\ce{Na4[Fe(CN)5NOS]}$. However, when both are present, NaCNS is formed. NaCNS can be detected by an $\ce{FeCl3}$ test, which gives a blood red color of  $\ce{Fe(CNS)3}$ (ferric sulfocyanide). My question is, when both nitrogen AND sulfur are present, will the individual tests for nitrogen and sulfur (Prussian blue and sodium nitroprusside test) be shown? Doesn't this depend on whether the sodium extract contains NaCN, NaCNS, or $\ce{Na2S}$? Is it possible to have a mixture of all 3 (NaCN, NaCNS, and $\ce{Na2S}$), in which case all the above mentioned tests will be answered?
Comment: Why do you want individual tests? It will give either NaCN or Na2S if either is present and NaSCN if both are present and you can distinguish NaCN, Na2S and NaSCN. Na2S is yellow solid and while NaCN and NaSCN are white, they can be distinguished by their solubility (NaCN is 63.7 g/100ml and NaSCN is 139 g/100ml at rt) and once you identified the sodium salt, you proceed with corresponding reaction. Tbh, Lassaigne test has become obsolete and nobody uses obviously due to its hazard factors.
Answer: Please note that nobody uses Lassaigne's test of the 18th century to test $\ce{S}$, $\ce{N}$ and halogens in organic compounds. It is unsafe and really obsolete method of qualitative analysis. I know that it is still taught in high schools in certain parts of the world. Just looking at the chemistry's point of view, your hypothesis is partially correct. One may see the formation of thiocyanate. However, all those products which you mentioned are dependent on the relative ratio of sodium metal and the organic compounds. If an excess of sodium metal is used, thiocyanate ion is destroyed during fusion and you get the regular products. In short it is possible to simultaneously detect $\ce{N}$ and $\ce{S}$ with the proper amount of compound to sodium ratio during fusion.
Reference:
Tucker, S. Horwood. "A lost centenary: Lassaigne's test for nitrogen. The identification of nitrogen, sulfur, and halogens in organic compounds." Journal of Chemical Education 1945, 22(5), 212-215 (DOI: https://doi.org/10.1021/ed022p212).
Comment: I do not know the exact answer because a general answer cannot be stated, what if a molecule has two S for each N or any other ratio. The above reference suggests to vary the amount of sodium and see the effect for each compound.
Comment: So it will show a positive test for all 3 tests (ferric sulfocyanide, sodium nitroprusside, and ferric ferrocyanide) if the correct ratio of sodium metal to organic compound is used?


can you wipe over your feet once with cotton socks?

Question: When i make wudu do it have to be "leather" socks like the Hadiths or can i just wipe once over my cotton socks? Jazakallah khair for your help.
Answer: According to the Mazhab of Imam Abu Hanifa (A.R.), the following conditions must be met:

It stays up for itself on the feet.

A person can walk with these socks (alone) to a distance of three miles without the socks being torn.

It is so thick that water is not able to seep through the socks to enter the feet.

A person is not able to see his feet through the socks.

According to the Mazhab of Imam Malik (A.R.), it states that these socks, (cotton or cloth) must be made with leather inside and outside so that it resembles the leather socks (Khufain) and a person may be able to walk upon them. Only then, it would be permissible to make masah over them.
According to the Mazhab of Imam Shafi (A.R), it states that an allowance would be granted to perform masah over these socks, (cotton or cloth), when the following conditions are met:

They must be thick and heavy to the extent that it is not transparent and nothing can penetrate it (like water) and one can walk continuously upon them.

The sole of these socks must be made with leather.

According to Imam Ahmad (A.R), he says that the following conditions must be met in order that a person performs masah on these socks:

It must be very thick, to the extent that no part of one's foot becomes visible.

It remains firmly gripped onto the feet and a person is able to walk continuously with these socks.

It is the unanimous agreement (Ijmaa of all the Imams of Fiqh) that if the socks are thin and water can seep through and enter into them, masah would not be permissible upon them. (Bada'ius Sana'i)


how to derive the ac emitter resistance of transistor amplifier biasing?

Question: I went through many textbooks and websites and I found out the derivation for AC resistance for diode which is \$r_d=26mV/I_{F}\$ where as for ac emitter resistance it is \$r_e=25mV/I_E\$ for which I didn't get any derivation or explanation. So, I wanted to request the forum for mathematical derivation or are they same with a valid reasons.
Answer: It's just a linearization of the Shockley equation. The large signal model for a diode is:
$$I_F=I_S\cdot\left(e^{\cfrac{V_F}{n k T/q}}-1\right)~~~~~~~~~~~~~(1)$$
\$n\$ is the emission coefficient and is by default set to 1. In many small signal BJTs, the default is fairly accurate. With many diodes, it's not, and is usually larger -- not unlikely from 1.6 to 3 or more. But in the two cases you mentioned, I'm pretty sure it was taken as the default value.
The value of \$\frac{k T}{q}\$ is a matter of basic physics and ambient temperature. It's called the thermal voltage and is, at room temperatures, somewhere from \$25-26\:\textrm{mV}\$.
\$I_S\$ is known as the saturation current and for BJTs is typically in the area of \$2\times 10^{-14}\:\textrm{A}\$ for discrete, small signal BJTs. For discrete diodes, it's usually more, often by a factor of 1000 or more.
Linearization of the above equation is pretty easy:
$$\begin{align*}
D\left(I_F\right)&=D\left(I_S\cdot\left(e^{\cfrac{V_F}{n k T/q}}-1\right)\right) \\
 \\
\textrm{d} I_F&=I_s\cdot D\left(e^{\cfrac{V_F}{n k T/q}}-1\right) \\
 \\
\textrm{d} I_F&=I_s\cdot e^{\cfrac{V_F}{n k T/q}} \cdot D\left(\cfrac{V_F}{n k T/q}\right) \\
 \\
\textrm{d} I_F&=I_s\cdot e^{\cfrac{V_F}{n k T/q}} \cdot \cfrac{\textrm{d} V_F}{n k T/q}~~~~~~~~(2)
\end{align*}$$
At this point, it is helpful to recall equation (1) above and note that the "-1" term is negligible in almost all cases. So we can substitute \$I_F\$ into equation (2) above, giving:
$$\begin{align*}
\textrm{d} I_F&=I_F \cdot \cfrac{\textrm{d} V_F}{n k T/q}~~~~~~~~~~~~~~~~~~~~~~~~~~(3)
\end{align*}$$
Trivial algebra now yields:
$$\begin{align*}
\frac{\textrm{d} V_F}{\textrm{d}I_F} &= \frac{n k T}{q I_F} ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~(4)
\end{align*}$$
Since \$n=1\$ and thus \$\tfrac{n k T}{q}\approx 26\:\textrm{mV}\$ at room temperature, equation (4) reduces to the equation you've been seeing. Whether it is 25 or 26, is rarely important. (It might be if you are using this equation to measure the ambient temperature or if you are struggling with circuits that otherwise depend upon this parameter's exact value in exact circumstances.)
In the case of the BJT, the usual much simplified active region equation is:
$$I_C=I_S\cdot\left(e^{\cfrac{V_{BE}}{n k T/q}}-1\right)~~~~~~~~~~~~~(5)$$
But a similar derivation occurs. And since the emitter collects both the base and collector currents, which travel across that PN junction, the use of \$I_E\$ rather than \$I_C\$ is appropriate. But those two values are usually so similar, you may find either form used, in practical applications.
In the case of a small signal BJT, I'd be more likely to use the equation since it more usually applies (ignorant of better specifications.) In the case of diodes, excepting diode connected BJTs, I'd usually be suspicious of it as the emission coefficients are usually not 1, but larger.

Oh. Keep in mind that the thermal voltage is in millivolts. An emitter current of \$1\:\textrm{mA}\$ suggests \$re \approx 26\:\Omega\$. You need to keep your multipliers straight. (When you see \$\tfrac{26}{I_E}\$, then you must either read the 26 as being millivolts with \$I_E\$ being in amps, or else you must read the \$I_E\$ as being specified in milliamps.)
Comment: so why is  bulk size of a diode not in the equation which obvious affects \$V_{ce}/I_e\$ at the same current when saturated or \$\Delta V_e/\Delta I_e\$ when tested for this impedance and does not include hFE when know that is factor as well as base impedance.
Comment: Bulk size of the diode will affect capacitance, but isn't in the Ebers-Moll model.   The base spreading resistance and beta are also not part of the model (and usually very minor in effect).
Answer: "*....where as for ac emitter resistance it is \$r'e=25/I_E\$ "
From jonk`s excellent explanation - and in conjunction with the above quoted sentence - we can derive a very important property of the quantity r'e. The TRANSCONDUCTANCE of a BJT is (with sufficient accuracy)
$$\frac{dV_{be}}{dI_c} = g_m = \frac{I_c}{V_T} (\text{with } V_T \sim 26 mV)$$
Note that we have \$g_m = 1/r_e'\$.
The transconductance \$g_m\$ plays the major role for finding the voltage gain of a transistor stage; \$g_m\$ can be calculated also using the small-signal parameters \$\beta = \frac{dI_c}{dI_b}\$ and \$r_{\pi} = \frac{dV_{be}}{dI_b}\$ as \$ g_m = \frac{\beta}{r_{\pi}} \$.
However, for practical design purposes, who knows the values for \$\beta\$ and \$r_{\pi}\$? Therefore, it is common practice to use \$g_m = \frac{I_c}{V_T}\$ because our design starts always selecting a proper \$I_c\$ value.


Book series where the main characters have powers associated with smells

Question: There is a book series where the main characters (one boy and one girl) have powers. If I remember correctly, the boys powers are associated with the smell of oranges, and the girls powers are associated with the smell of vanilla. What book is this?
Comment: When did you read the series? In what language? How many books do you remember there being in the series? What sort of powers did they have?
Comment: I ready this series, but I only remember there being 4 or 5 books in the series. I also don't remember the specific powers.
Answer: Probably the "The Secrets of the Immortal Nicholas Flamel" series by Michael Scott.
The Warlock, 5th book of the series has some textual evidence, but I am assured that it exists throughout the books.

From this site's summary (emphasis mine):

Josh is one of the main characters in book. He is Sophie's twin brother and one of the twins of legend. He has a pure gold aura, and when he uses it, the smell of Oranges fill the air. Josh and his sister split during the story, each taking one side. To Josh, he picked the correct side, but to Sophie he didn't. They become reunited at the end of the book. Sophie Sophie is also a main character in the book. She is Josh's twin sister and is the other twin of legend. She, unlike her brother, has a pure silver aura, and when she uses it, the air fills with the smell of vanilla.

A summary from Goodreads of the book (bolding theirs):

The twins of prophecy have been divided—the end has begun.
Alcatraz
  Although their ally Dr. John Dee has been declared utlaga, Machiavelli and Billy the Kid will follow the plans the Elders have laid before them: they will loose the monsters of Alcatraz on the city of San Francisco, thereby triggering the end of the human race.
Danu Talis:
  The Shadowrealm that Scatty and Joan of Arc have entered is far more dangerous than they could ever have imagined. And they haven't landed here by chance—the warriors were called for a reason. So were Saint-Germain, Palamedes, and Shakespeare. The group was summoned because they must travel back int time to Danu Talis to destroy it. For the island of Danu Talis, known in humani myth as the lost city of Atlantis, must fall if the modern world is to exist.
San Francisco
  The end is finally near. Josh Newman has chosen a side, and he will not stand with his sister, Sophie, or with the Alchemyst, Nicholas Flamel. He will fight alongside Dee and the mysterious Virginia Dare.
Unless Sophie can find her twin before the battle begins, all is lost—forever.

And, from Google Books:

Her face moved left and right as she sniffed the air. "You can still smell the magic.""Vanilla," Isis said."Orange," Osiris added.
Comment: Nicholas Flammel? Is this a Harry Potter book?
Comment: @user13267: [Nicolas Flamel](https://en.wikipedia.org/wiki/Nicolas_Flamel) is a historical figure who was also an alchemist. He first showed up in fiction in *Indiana Jones and the Philosopher's Stone* which was released two years before the first Harry Potter book.


UIWebView localication

Question: I'm developing multi languages application and intent to have DONE button localized. This button is appear in UIWebView when user clicked on dropdown ("select" tag) with multiselect:

By default, this is always in English. However, if you set CFBundleAllowMixedLocalizations to YES in .plist file, you will be able to get it in current SYSTEM language:

Here, DONE is written in Chinese, when text in select in Arabic. It is because current system languages (i.e. set in Settings) is Chinese, but inside app I selected Arabic so app is using Arabic bundle with Arabic strings file.
I wonder, is any possibility to localize "DONE" button to current Application's language, but not system language?
Answer: Don't use "Application's language". Simply use the system language, like all other applications do.
I'm sure it's possible to somehow achieve it, but you're simply taking the wrong approach to localization.

UIWebView localication


Professional investment planning for small net-worth individual in bearish market

Question: I would like to find an investment planner that can help me achieve moderately aggressive growth when the stock market is expected to go down. Perhaps he would use shorts and other more short-term techniques. I would only be looking to give the person control of between $50K to $100K. My guess is that I can find local professionals that would be worth the cost. Am I right? How would I find a good planner?
Comment: For what it's worth. The entire market isn't bearish just because the aggregate may be. There are always bullish investments in a bear market to be found.
Answer: You're going to have a hard time finding a legit investment planner that is willing to do things like take short-term positions in shorts, etc for a small investor. Doing so would put them at risk of getting sued by you for mismanagement and losing their license or affiliation with industry associations.
Answer: There is no magic bullet here.
If you want professional management, because you think they know more about entry and exit points for short positions, have more time to monitor a position, etc... (but they might not) try a mutual fund or exchange traded fund that specializes in shorts.
Note:  a lot of these may not have done so well, your mileage may vary


Trying to make WYSIWYG on my page

Question: Being unlucky with this:
A WYSIWYG Where it's possible to limit the text/height
What should I do, i would like to have WYSIWYG, but the WYG part, (what you get) is inside a fixed sized box.
And the editors out there makes scrollers/expands in height when you write more than the height you set the editor to.
So what can i do?
Answer: overflow: hidden in the right place should do the job.
For CKEDitor, see  CKEditor: Class or ID for editor body

Trying to make WYSIWYG on my page


How the bubble size in a champagne bottle evolve?

Question: How the bubble size in a champagne bottle evolve ? Is it fixed or is evolving with time ?
Comment: this is not interesting comment
Answer: When the bottle is closed, part of the gas is dissolved in the liquid. The portion that is not dissolved is responsible for the pressure. Any additional gas leaving the liquid increases the pressure, what prevents the continuation of the process.
When it is opened, the pressure is atmospheric, and the fraction of stable dissolved gas is certainly much smaller that the actual content.
But for the molecules of gas form a gas phase (a bubble), it is necessary also to form an interface liquid-gas (the bubble spherical surface).
Where only very few molecules gather, the free energy to escape to form a gas is not enough to form the surface (that requires some energy), and they return to the dissolved status.
Where their number is big enough, the critical radius is reached, and a stable bubble is formed. Once that happens, the dissolved gas in the neighborhood can migrate to the existing bubble, that grows as a consequence.
It is a process of nucleation and growth, common to many changes of phases.
Comment: The time of travel is short. But the tendency is that existing bubbles grow.
Comment: thanks. But then, when does the bubble growth stop ? (why does it growth for ever ?)
Comment: They grow and go up because are lighter than the liquid. Once at the surface, the gas goes to the atmosphere.
Comment: thank you. So, are you meaning that during the travel to the surface, *their volume increases* ?


Engine Vibration 2007 Honda Accord

Question: 2007 Honda Accord vibrates really bad when put in drive or reverse, but when in park it doesn't.  Even turning the AC on makes the vibration/noise worse. Any advice?
Comment: Welcome to Motor Vehicle Maintenance & Repair! Is the check engine light on? If so, have you read the codes? If you haven't, get them read and please let us know what they are. At a guess, I'd suggest you might have a bad coil/plug/injector on one cylinder, but that's a guess ... you really don't give us much to go on here.
Answer: This is a hard one with what little information there is to go on. Sounds like a misfire but usually (especially with a 4 cylinder engine) you would feel it at idle in park if it was a totally dead cylinder. First thing I would do is unplug the coils, crank the engine over for a few seconds and listen to it, I've seen plenty of these Hondas partially loose compression on a cylinder due to timing issues and with a good ear you'll hear it clear as day when cranking it, it'll sound consistent for the most part but you'll get a random jump is crank speed for every revolution, it's hard to explain over text but I hope this helps..
Answer: I've fixed this problem on many cars.  Clean the throttle body shutter with throttle body cleaner and a toothbrush.  With the engine running and the a/c on, spray throttle body cleaner through the port just in front of the throttle body shutter, in short spurts, for five minutes.  Replace the air filter.
Change the crankcase oil, substituting one quart with Marvel Mystery Oil.  (@Paulster2 - there is nothing in the Terms and Conditions of this forum that disallow product names, so stop reprimanding me.)
Put cleaners in the gas tank.  On 7/8 of a tank, add 32 oz. Marvel Mystery Oil, 32 oz. Liqui-Moly Fuel Injector cleaner, and two gallons of lacquer thinner.  Rev the engine in neutral at stop lights with quick pumps to the floor.  On an open highway, floor the pedal on a hot day, 50 - 80 MPH, at least five times.  Do not refill the tank until very near empty.
Change the oil again after 300 miles.  You may have to continue adding a few ounces of MMO and change the oil every 1000 miles if your crankcase is particularly dirty.
If the problem persists, change the oil and add Restore to the crankcase.  This step will correct the loss of compression discussed by DieselMech.

Engine Vibration 2007 Honda Accord


Schema Definition For Audit-like Asset Tracking (Entity Framework)

Question: I'm having trouble designing the relationships between entities in my schema. The issue is that each Asset (projector,laptop,etc...) can have a change in purpose/location (among other things), and if I wanted to get the most recent of these values which it would currently involve copious joining etc.... To keep track of these changes I have defined a schema like so:

Meaning that an AssetInstance represents a use of the Asset, i.e being loaned out, sent to repair. Each AssetInstance is responsible for recording the location,shelf (if applicable) and purpose of the Asset. However if I use this approach and I want to get the current user of an Asset with AssetID = 1, I would need to join Asset,AssetInstance then sort by Date and select the top one, then join that with User to get their name.
Ideally I would like Asset to keep retain the most recent Purpose,Location,Shelf and User it had associated with it. I realize this could be accomplished by adding these relationships to Asset and then in my code after a new AssetInstance is created for an Asset update the Assets columns appropriately.
To me this seems far from ideal and I'm wondering if there is a better way to do it.
Thanks :)
Answer: I think you could see the design from another viewpoint
Lets say AssetInstance is a reallocation of shelf,location and purpose.So I prefer calling the table Transaction or Relocation-Transaction.
As shelf,location and purpose are naturally properties of Asset we will have relation of them in asset Table. Last location, shelf and purpose are saved in these foreign keys in asset table.
relocation-transaction table contains the history of relocation, so we will insert a row in that when the asset is re-locating(perhaps even in first place of asset creation).
I think it would be a good idea to have a base table for transactions, may further actions on the asset will be needed to have traced and reported, like asset out of ordering, selling ...
A draft model would be something like this:

Hope this helps.
Comment: +1 This is a way of doing it I hadn't thought of though I'd rather have the purpose,shelf and location be stored in Asset (exactly like you've done) and then just keep all transactions in 'AssetInstance'. Simply because people are likely to be searching the database and having one AssetInstance table simplifies this.


Can I make use of my wife's unused personal tax free allowance to reduce our income tax?

Question: I'm working full time (UK) and my wife is unemployed. Is there a way (legal of course!) for us to take advantage of her personal tax free allowance? If we were each earning only half my salary, we would be more than £2000 better off each year after tax.
Comment: You can't do anything about you salary, but you can make sure any income generating assets are held in her name. For example you can use bare trusts to hold dividends paying shares etc
Answer: I wasn't going to answer this as I have only superficial knowledge. But since nobody else has, here is what I believe to be the case.
No, there is no simple tax accounting way of doing this. Once upon a time there was a 'married person's allowance' which essentially allowed you to claim more if you had a non-taxpaying spouse, but I believe that has been ended for people born after 1933.
The way people often try to make this happen is to pay their spouse for some kind of work, and then claim the payment as an expense. However the Inland Revenue is of course on to this. You will get away with it only if it is work that is real and necessary to your business; so you might be able to make it work for bookkeeping, but not for cleaning. And she would really have to do the work.  And expect to be audited heavily if you try it.
TL;DR No
Comment: It would have to be work directly associated with your own employment; paying her for housework wouldn't be a legitimate employment expense so would come out of your taxed income, defeating the purpose.

Unless you're self-employed, you will find it very hard to do this.
Comment: Thanks for your answer. Would it be "legal" to pay my wife for housekeeping? If so, what are the costs/steps involved in doing so?
Comment: Pretty sure No. But you can check with the IR.
Answer: As DJClayworth's answer explains, normally you can't transfer personal allowances between people, whether married or not.
However, as of April 2015, it's now possible to transfer a small part of your allowance to your husband or wife under certain circumstances. The main criteria are that

One spouse needs to be earning less than the personal allowance
The other spouse needs to be paying basic rate tax only

The amount of allowance that you can transfer for the 2015-16 tax year is up to £1,060, so your spouse would end up saving £212 in tax at the 20% rate.
The full conditions are here:

You'll be able to claim Marriage Allowance if all the following apply:

you're married or in a civil partnership

your annual income is £10,600 or less, plus up to £5,000 of tax-free savings interest

your partner's annual income is between £10,601 and £42,385

you were born on or after 6 April 1935

(for people born before 6 April 1935, there's a separate but confusingly similarly named "Married Couple's allowance")


Open and Short Circuit questions

Question: I am confused on the terms Open, Short, and Closed when talking about circuits. As far as I know:

a) Open circuit means the wires are cut off so there will be no current flow, but there is voltage.
b) Closed circuit means the wires are connected so there will be flow of current, but there is no voltage
c) Short circuit also refers closed circuit.

Is my knowledge (a,b,c) about the question correct?
There is also another thing which confuses me:

d) Voltage is the force that makes the current flow. How can there be current but no voltage or voltage but no current? (from formula: \$V = I \cdot R\$)

Please explain a, b, c and d so it won't bother me anymore if I am going to solve circuits..
Answer: For (a,b,c) that's more or less correct. In general, there doesn't have to be a voltage/current just because there is a short/open, there just can't be any voltage in a perfect short and there can't be any current in a perfect open.
Another way to re-word these two terms is that a short circuit has 0 resistance (R=0), and an open circuit has infinite resistance (R=infinity).
So in Ohm's law, \$V = IR\$.
If \$R = 0\$, then \$V = 0\$.
If \$R = \infty\$, then using some mathematical trickery:
$$
I = \lim_{R\rightarrow \infty} \frac{V}{R} = 0
$$
As far as the force analogy goes, if it's useful think about you pushing on a building. Just because you are applying a force doesn't mean the building is going anywhere. These type of analogies tend to break down when dealing with theoretical 0's and infinities, so I wouldn't rely too heavily on them but rather look at the mathematics.
Comment: +1 for answering the later part of the question, and showing that Ohms law still applies. Questioner do the math. Draw a circuit `V10 — r=1 — R=1 — and back to start` then: repeat at least 6 times: Multiply the value of R by 10 and recalculate. Observe what happens as R gets big. Now do the same, but this time make R become smaller.
Answer:

simulate this circuit – Schematic created using CircuitLab
Answer: I may be wrong since I did not study this in English, but I see a major difference between b) and c).
From a very practical point of view, a closed circuit is good, a short circuit is very bad !
A short circuit is, for instance, connecting a wire directly between the poles of a battery or power supply. Whereas a closed circuit is just a "normal" load between the poles.
Oops (let me be careful here, I don't want to get sued or anything), don't actually, physically, do it at home (or anywhere else), the wire might melt, you might burn yourself, start a fire, cause the world to stop revolving, etc...
From \$V = RI\$ : in theory, for short circuit R=0 (this is never actually the case unless you got superconductors) so that I becomes "infinite". Actually again, your battery will deliver its max current, heat up and die quickly, your PSU will either do that also or shut its output down if sanely designed.
Regarding d), once again, this is the theory: as mentioned above, a wire does have a resistance so there is some voltage (potential) difference when current runs through it. Similarly, there can be leak currents that contradict the "voltage but no current flow".
I learned with hydraulics analogies, it was rather evocative, but that's a little too long to elaborate about here.
Answer: You are right about open circuit, the wires are disconnected.
In case of short circuit as well as closed circuit, the wires ARE connected but the difference is that in case of short circuit, the resistance between the connection is extremely low so very high current flows as per ohm's law, whereas in case of close circuit, the connection offers considerable resistance, hence no high current issue.
Answer: There are a few misconceptions in your question, and it might help you understand things if we clear those up. For a) and b), replace "there is" with "there may be". The way you have defined an "open circuit" means that there is effectively an open circuit between any two points in a circuit that are not connected together by an ideal wire. When there is not an ideal wire connecting two points in a circuit then it's possible that those two points will have a different voltage. So, removing and ideal wire from a circuit means that it becomes possible for the voltages at those two points to differ.
Your definition of a "closed circuit" is any two points connected by an ideal wire. By the definition of an ideal wire the voltage at those two points must be the same (the voltage difference must be zero). Current might flow through the wire, depending on where you connected it in the circuit.
For circuit analysis, it can be helpful to use a zero-ampere current source to represent an open circuit and a zero-volt voltage source to replace a short circuit.
The reason that Ohm's law doesn't apply here is that the closed circuit is created using an ideal wire, and Ohm's law only applies to resistors. The ideal wire, by definition, has no resistance.
When considering these circuit analysis concepts it's important to remember that we're talking about ideal circuit elements rather than real circuit elements.
Answer: A short is a current path with no resistance. All that is determined by what you are measuring across. The reason why you can read current and voltage across a battery is because batteries have internal resistance.
Battery by itself is open.
When you put your leads on battery, you complete or close the circuit. Shorts bypass the load straight to ground but is not a complete circuit, like circuit was shorted, you owe him some load. You give him the load and make it even, so the deals is now Closed.
Closed circuit is when everything is connected in the path to ground or reference.
An open is when there is NO current flow.  You can have a voltage reading across an open but there is no current because there is no physical path.
Best advice is to take a PSPiCE class. There you learn to plot a circuit using coordinates.

Open and Short Circuit questions


Difference between "site:current-user:field-foo" and "account:field-foo" in setting a Rules condition?

Question: What is the difference between "site:current-user:field-foo" and "account:field-foo" in the Rules "Add a new condition"?
Comment: It depends on the context of the rule...what event are you responding to?
Comment: that's "After saving a new user account"
Answer: As I mentioned in the comments the variables you have available to you in a rule depend on the context it's in.
[site:current-user] is a global token available to any rule and will always return an object relating to the currently logged in user.
The [account] token is there because you're responding to the "After saving a new user account" event; that event passes an object relating to the user object that has just been saved (for obvious reasons).
I guess the main distinction is that the former will always give the currently logged in user, while the latter could be a different user object altogether (if, for example, an administrator had created the account).
Answer: In the "After saving a new user account" event, [account:field-foo] is referring to the account being saved, while [site:current-user:field-foo] is referring to the currently logged-in user, which possibly is the user who created the new account, or caused the account to be automatically created.
In the case of an administrator creating an account for another user, [site:current-user:field-foo] would be for the administrator account.


How to enable Chatter Communities?

Question: I want to use chatter with customer portal. I searched on google and found that this can be done using Chatter Communities. Any idea how to enable chatter communities?
Answer: Since Chatter communities are outdated, and maybe someone might stumble upon this post for how to enable chatter in communities (since this is one of the top google results for this question). In lightnign communities, chatter is integrated out of the box, However, for guest users, there is a setting that enables them to use chatter,  under community administration in the preferences tab:
Answer: I think its in Pilot. You need to submit a request
You can find the link for the request in the FAQ.
Third Question from the bottom
Answer: I asked this question to salesforce this week.  Communities is coming out after ALL the instances are done upgrading to summer13, sometimes around the middle of June.
Release notes make it sound like it's available when the release happens, but it's not.

How to enable Chatter Communities?


OpenStreetMap Neighborhood Boundaries

Question: Is it possible to download neighborhood boundaries from OpenStreetMap? I see on maps that they often indicate neighborhood names; is there a way to retrieve their coordinates and boundary polygon coordinates, much like the Zillow Neighborhood Boundaries data set?
Comment: What countries and regions are you looking for ?
Comment: Starting with major cities in UK, France, Germany. Chiefly London, Paris, Berlin, but ultimately looking for all major cities in Europe. Israel too.
Comment: What are you trying to do with the data? It is definitely possible that a company like Maponics might have what you're looking for. Can you provide any more details?
Answer: New service from MySociety last week will get Administrative Boundaries for your chosen city.
Example Paris

http://global.mapit.mysociety.org/area/29746.html
UK has better data source as using the OpenData from Ordnance Survey
Example:
Birmingham, UK
note different url request
http://mapit.mysociety.org/area/2514.html
Comment: But it looks to me like what you refer to is only displaying the boundaries of the city (Paris), not of the neighbourhoods within the city. Or am I missing something? Sorry, I never worked with MySociety before
Comment: @NeStack different countries have different hierarchy for administrative boundaries. https://wiki.openstreetmap.org/wiki/Tag:boundary%3Dadministrative#10_admin_level_values_for_specific_countries
Answer: Not from OpenStreetMap, but you can download Administrative Boundaries in many formats from the Global Administrative Areas database. They are heirarchial in nature (country>province> town>village, etc) and relatively acurate for popular places. http://www.gadm.org/
@Mapperz has identified the best answer to the question...However, I don't feel that OSM is a the best resource available for this type of task.
Comment: FYI -- their footer states: `These data are freely available for academic and other non-commercial use. Redistribution, or commercial use, is not allowed without prior permission.`
Comment: For the US they only provide the country outline, states, and counties, which is available on census.gov - https://www.census.gov/geo/maps-data/data/tiger-line.html
Answer: FYI, in my experience, OSM rarely has boundaries for neighborhoods, it usually just has a centroid point label.  Other sources would probably be better for boundaries. If you want to check, though, you can look up areas with Nominatim, and see what is in your areas of interest.  How Nominatim works and how to query is more fully described on its OSM wiki page
Answer: For the US and Canada you can download Neighborhood data (shapefile format) here:

US: http://dinehere.us/neighborhoods.html
CA: http://foodpages.ca/neighborhoods.html
Comment: FWIW the DineHere.us neighborhoods shapefile has no identifying information for the geographies, and the shapes have obviously been put through a very reductive simplification process. I'm not sure how anyone would effectively put them to use.
Comment: That is great, but OP is looking for European cities according to comments.
Comment: Also, this is a HTML sitemap, not shapefiles that you can download and use
Comment: There are shapefiles on the top right corner: Click on " Download as shape file
" http://foodpages.ca/neighborhoods.tar.gz

OpenStreetMap Neighborhood Boundaries


Declaring element with ANY name in DTD

Question: Is it possible to declare an element in my DTD which can have ANY name? So far I have learnt that ANY can be used only for the data type as in:
<!ELEMENT element-name ANY>
Any help will be most appreciated.
--
Ali
Answer: No, the ANY keyword only applies to the element's category.  What would the semantics of ANY as a name be, anyway?  Whatever it is that you're hoping for, DTD doesn't have it.
Answer: Yes, but not for the reason that I think you want to.
See 3.2 Element Type Declarations of the XML 1.0 specification, productions 45 and 46,  at
http://www.xml.com/axml/target.html#elemdecls
ANY is an XML keyword, and part of the contentspec, and means that the content of the element can be anything that doesn't break the rules of XML.
You could use the name 'ANY' to name an element type, but it wouldn't have the semantics of the ANY keyword, when used in a content model.

Declaring element with ANY name in DTD


What first aid options do astronauts have during an EVA?

Question: If an astronaut gets unconscious or ill I suppose the colleague will help her to the airlock. But are there any other measures that could be taken already outdoors? Do they have duct tape to seal a leak in the suit? Can any kind of first aid be provided?
Comment: A suit for an EVA is made of many layers for thermal isolation and protection against micro meteroites. Thus the air tight layer holding the pressure can't be the top layer. How would you place duct tape to seal a leak in that special layer? Removing all upper layers with your swiss army knive?
Special emergency gloves to be worn over a leaky glove may be possible. But there should be an additional flange at the suit to mount this glove and seal it.
Comment: A prototype of a [rescue ball](https://en.wikipedia.org/wiki/Personal_Rescue_Enclosure) was build but never flown. A much larger version of such an enclosure to fit a suited astronaut may be possible but heavy and bulky.
Answer: None, really.  If you examine the EVA Cuff Checklist (the one that is actually taken outside, shown here on Suni Williams' suit - note customized cover)

you will see that all emergencies end in either TERMINATE EVA or ABORT EVA (ABORT EVA being faster) if working the procedure doesn't fix the problem. In other words, get back inside where you can actually do something about the problem.
Here's the Cuff Checklist table of contents.

Here's a sample of one of the procedures.

If you get the Suit Pressure Emergency alarm:
Check the hardware gauge to confirm the reading (a redundant sensor to the one that sets off the alarm). If it shows low, check that your valves are closed and the O2 Actuator is in the proper position.  Then abort the EVA. (The check mark means check something and fix it if it's wrong. If you find a valve open and closing it works, you're done.)  If the hardware gauge shows the pressure is OK, ignore the alarm. But keep checking that gauge!
You can read the whole thing here.
Comment: I imagine it could be made possible in some future program.  But in today's ISS program, adrenaline is not taken outside, and I am not sure the suit is self-sealing if a needle is jabbed through it.  Your question asks what options do they *have* which to me means, today. A lot could be done, hypothetically, but it's not in effect today.
Comment: Would it be possible for a fellow EVA-astronaut to administer an injection of adrenaline and other stuff that might get a knocked out astronaut conscious for a while again by biologically provoked super stress? Not anything at all but to check the pressure and bring the guy home into the airlock? Imagine that a kilometer from the habitat on Mars.
Answer: To answer the medical aspects of this question, all Apollo spacesuits had a hypodermic injection port in the right thigh.  It was protected by a flap on the outer ITMG layer (p. 14):

Access flaps constructed of a thermal-resistant cross section of materials are held closed by a system of snap fasteners and fire-resistant Velcro tape.  These access flaps cover the entrance closure and the UCTA connector and biomedical injection area.

and had a self-sealing patch through the pressure garment assembly (p. 8):

A biomedical injection patch is built into the right thigh portion of the TLSA to permit a crewman to self-administer a hypodermic injection without jeopardizing the gas retention quality of the PGA.

According to Apollo by the Numbers, lidocaine, atropine, demerol, and marezine were available as injectables.  Fortunately, they were never used.


how to disable the default toolbar of pdf page

Question: I need to provide some security of pdf page by disable the toolbar, no right click, no text selection, no cope-paste. Can anyone please tell me how to achieve this ?
Answer: I don't think you can do it. To my knowledge there is now way to programatically disable the toolbars or any of the features you are describing. And it is certainly the wrong way of providing security to a document. Maybe you should look at alternative formats.
As a general rule of thumb, if someone can see it on their computer, they can duplicate it.
Answer: It is not possible to disable the toolbar or right-click functionality in Adobe Reader (or any other reader) -- this is not a feature of the PDF specification . However, it is possible to disable content (text, images, etc) copying using a number of different PDF libraries. You can also prevent printing, page extraction, document modifications, etc.
Two PDF developer libraries that would offer you this functionality are:

Quick PDF Library
Solid Framework


What is the source of my father's Hebrew name? It is כלאוונא

Question: I am going getting a GET from my wife and the Rabbi asked for my father's Hebrew name.  I told him it was כלאוונא, the same as my great-grandfather. The rabbi said he had never heard of the name.  After some research, we found it mentioned in the Talmud without any explanation we could understand.  Can you shed any light on this?  Thanks.
Comment: This question has two close votes for being "off-topic, Hebrew." I don't agree -- it asks for the meaning of a specific Jewish name in the context of Judaism.
Comment: Explanatory note to supplement @IsaacMoses' -- it's a good idea to [edit] the gemaras where the name appears as well as its pronunciation into the question, because [1] those are important pieces of information [2] comments are less visible (and they don't last forever), questions do.
Comment: Mark Cantor, welcome to Mi Yodeya! Where did you see this name in the Talmud? Please [edit] the source into your question; that would make it a great deal easier for people to investigate it.
Comment: Please [edit] your sources, including as much bibliographical information as you can, into your question.
Comment: How do you pronounce that name? Shouldn't you give the GET to your wife, not you getting a GET from her?
Comment: Here is a link to his headstone in Hebrew. http://s15.postimg.org/tg31l1ctn/marker.jpg
Comment: It appears in the talmud with a space between the first 2 letters and the last 5 letters. Here are the links where it appears in 2 documents below from halakhah origin of names:http://www.hebrewbooks.org/pagefeed/hebrewbooks_org_8990_163.pdf
http://www.hebrewbooks.org/pagefeed/hebrewbooks_org_8440_182.pdf
Comment: here is another page, but it doesn't give the talmudic citation http://www.hebrewbooks.org/pagefeed/hebrewbooks_org_8990_121.pdf
Comment: The family has always pronounced it Cluvnah
Comment: @RebChaimHaQoton - you are correct
Comment: My grandfather came from Gruzdziai, now in Lithuania.  He was a "Cantor" and a Shochet.
Comment: @MarkCantor it looks like the spelling on the headstone is with a 'ח', not a 'כ'.
Answer: This appears in the Talmud in Nazir 7a. It is a combination of two word, כל meaning all or each and אוונא which means a region or section of land. It appears in a very technical discussion about taking a Nazarite vow which was given a term equivalent to a distance to another location. The term אוונא refers to a standard for measuring land.
From the discussions in the sefarim you link to (which are about names for a Get) it is not related to that discussion, rather it is a name associated with the Hebrew names of Lapidus or Saadia.
Comment: I am not sure this explains the meaning of the name though.
Comment: It's not about the GET, we have wanted to know more about this Jewish name for some time.  He died before his 9 children had children, so they named the first male of each of his children's children with the name  כלאוונא
Comment: @MarkCantor, My only point was that it is in the context of a Get that such names get attention in Jewish books.
Comment: @Sarge, I couldn't really find much about that, but that part of the question really goes off-topic, IMO. Jewishly, it is the foreign language name that substitutes for one of those two Hebrew names. Why people connected to either of those names, I don't know.
Comment: You may want to remove this answer, as it is based on a typo.
Answer: Although it is not a common name, even in the Talmud, it is mentioned in Oholei Shem under כלאוונא where he says look at חלוונא. Looking further in Oholei Shem I can not find חלוונא. However in Kuntras Hasheimos Hachadash - note 13 & 14 he mentions a connection to Lapidus and Saadya.
The name does exist as you can see from this Lzecher Nishmas.
Answer: The Aruch Hashulchan In Even Haezer 129 which discusses the names and spellings has this name by the letter ח and he goes through the spelling,and the name itself.
Comment: Im not sure what "*he goes through the spelling,and the name itself*" means.
Comment: The Aruch Hashulchan does that for all names in the section of names regarding gitten,he goes through the correct spelling.


When a block that has a transaction with change in it is successfully mined, is the change transaction included in the newly mined block?

Question: If not, how is the change transaction processed?
Answer: There is no change transaction type.
Transactions are defined as a list of inputs consumed, and outputs created, and the values assigned to those new outputs. The creator typically makes an output for a entity they wish to pay, and another output which pays excess change back to them.
Any amount of Bitcoin left un-used by the outputs is interpreted as the fee for the transaction.
Answer: Transactions have to spend all their inputs immediately. Essentially, any transaction splits the inputs to (at least) two destinations: the transaction's intended recipient and yourself (change).
Say you have an input with 1BTC and want to spend 0.5BTC:
<code>                         ------- 0.5BTC (recipient)
1BTC  ---transaction----|
                         ------- 0.5BTC (change to yourself)
</code>
Any portions of the input that you don't designate to a destination become part of the transaction fee which will be claimed by the miner.


Recalling memories by electrical impulses?

Question: I've read that in an experiment, a pair of stimulating electrodes were inserted into a blind man's visual cortex, and upon passing electricity, the phosphene phenomenon was produced. Is it the same with memory? Can each and every memory, of an individual be recalled by a similar method? And if so, in which region of the brain will the stimulating electrodes be inserted?
Comment: As far as I know, we do not know very much about the whole mechanism of memory. My guess is: no. You would be better off trying hypnosis.
Comment: You might be interested in reading the June 2013 issue of Scientific American which contained an interesting article on forming of memories..
Comment: it is important to first map the memories. Usually a memory involves a lot of different perceptions. So i guess it is not that easy. visual cortex is a specific anatomical location that is involved in a very specific task.
Answer: There have been quite a few studies on stimulating the brain with electrical pulses and it has been found to be successful too. Three papers of particular interest I found were:
1) In a study titled "Explaining How Brain Stimulation Can Evoke Memories", it was found that electrical stimulation in the temporal neocortex can cause neurosurgical patients to spontaneously experience memory retrieval. The subject's brain in the study when stimulated enabled him to recollect memories from the time that he was in high school. They concluded that there are patches of neurons encoding memories for differnt periods (in this case the high school). (reference)
2) In the second study titled "Memory Enhancement and Deep-Brain Stimulation of the Entorhinal Area", it was observed that

entorhinal stimulation applied while subjects learned the locations of
  landmarks enhanced their subsequent memory of these locations: and the
  subjects reached these landmarks more quickly and by shorter routes,
  as compared with locations learned without stimulation. (reference)

3) In a third study titled "Stimulation of Entorhinal Cortex Promotes Adult Neurogenesis and Facilitates Spatial Memory", it was found that stimulating a specific region of the brain leads to the production of new brain cells that enhance memory. (reference)
In two on these studies, memory is enhanced by stimulation to the entorhinal cortex and in the first study the neocortex was stimulated. So, in a broad sense the  cerebral cortex is generally used for these studies.
This is a continuous field of research as a treatment to diseases like Alzheimers (reference).

Gray-Brodman-Entorhinal Cortex (Wikipedia)

Neocortex location (reference)


Login screen display not working

Question: I'm running a Kali Linux 2 system with Intel HD Graphics 5500. It originally came bundled with GNOME 3 however I quickly changed to XFCE due to preferences. However in about 2-3 weeks of usage I noticed that whenever I locked the computer and more specifically whenever the display turned itself off, when I went to turn the display back on the screen was completely red, then if I press any key it instantly turns blue and I can blindly log back into the computer. I have no idea what is going on here and any help would be much appreciated.
Answer: That is the default behaviour of slock, one of the screen-lockers used by default by xflock4 which in turn is the screen-locker script used by Xfce power manager, if I'm not mistaken, and other apps/widgets part of Xfce.
When the computer is locked, it turns off the display and when you wake it, you need to type in your user password to unlock it. If you hit enter when waking or if you enter the wrong password, the screen turns red and when you start entering your password, it turns blueish.
xflock4 tries to use xscreensaver first and gnome-screensaver second as locker commands so if any of those suits you better, make sure you have them installed.

Login screen display not working


Are business cycle fluctuations usually studied in a (New Keynesian) DSGE model, or can they also be studied in a growth model?

Question: I wonder if business cycle fluctuations are usually studied in a (New Keynesian) DSGE model, or can they also be studied in a growth model?
Answer: Business cycle fluctuations can be studied in many ways including also growth models. An example of using growth model to study business cycle would be this 1995 paper by Cho & Cooley. This being said growth models are not as common for examining business cycles as for example DSGE models so saying they are usually used for that purpose is correct. To my best knowledge no statistics on the relative use of this or that model for explaining business cycles exist but anecdotally, speaking from attending macro conferences, DSGE or various non-stochastic (when we talk about pure theory) New Keynesian or to lesser extent real business cycle models remain popular.
Comment: @BeckBatucada I don't think there are some simple clear cut rules where to use which model. This requires some soft skills as you can often modify/mold a model in various ways. I agree that OLG is probably suited more for some examination of business cycles from long term perspective but you can make a multiperiod versions of New Keynesian models as well.
Comment: Thanks for you answer. Yeah I am a bit confused, because indeed in papers I read, they talk about BC (business cycles) both in DSGE as in OLG frameworks. However I wonder if the definition of a cycle is the same in both approaches, as OLG typically looks at long term changes, rather than variations along those long term developments. E.g. a Boone and Quaghebeur 2018 paper, versus e.g. their Quaghebeur 2019 paper.

2018 Beyond rational expectations: The effects of heuristic switching in an Overlapping Generations model.

2019. Expectations and the Macroeconomic Dynamics of Fiscal Policy.
Comment: Gasteiger and Zhang (2014) is another example of a ramsey growth model approach
Comment: I just wonder why in some cases they choose a growth model, and in other cases a DSGE NK model


How to see previews of old quicktime movies (old codec)

Question: Since I updated to Yosemite, the .mov previews in Finder no longer show up for videos with an older codec. (When I double click the video, it starts a conversion process.) Is there any way I can see previews of my videos without needing to convert them all? Even if there's a software I can download to see the previews, that is fine. It's too much work to convert all of the videos to see what they are.
Answer: Not sure about the Finder preview function. You would have to find a preview handler written for that specific codec (whatever it might be) and compatible with Yosemite. A bit of searching might find that, if it even exists.
If all you want to do is play the video try VLC at videolan.org it's free and well supported and will not try and covert it, just play it as is.


'Add a Page' Missing

Question: In a Sharepoint site, the menu that drops down from the gear is supposed to have an option, "Add a Page". I don't have this button. When I go to the wiki page where there should be a button "+ Add a New Page", there is nothing. In Sharepoint Designer, I can't edit the Home Page to add links to pages I create in SPD. I have every level of permission as well. This seems to be a common enough problem and there are plenty of answers around but I have gone through every one I can find and none of them have helped.
Answer: I found a work around which just makes me more confused about the original problem. I can write in a link on the Home page to a page that doesn't exist and then Sharepoint gives me the option to create it because it can't be found (e.g. [[Page]] would prompt a "'Page' doesn't exist, would you like to create it?" dialogue box). If there is no problem with creating pages though, why can I not find this button in the first place?

'Add a Page' Missing


Can I use the Sites folder for whatever I want?

Question: Not realizing that OS X creates a ~/Sites folder when you enable web sharing, I started putting my sites for MAMP within a ~/Sites directory, which did not exist on my new laptop. Is this ok, as long as I don't enable web sharing? Even if I did enable it, would something happen to the other files I put in there?
Answer: It's fine, just realize the built in apache server will share the files you have placed there unless you change the apple supplied httpd.conf should you start up web sharing.
Comment: Thanks just wanted to make sure it wouldn't take over and delete any of my files.
Answer: Any files you put in the ~/Sites folder will be treated the same way most web hosts treat the files. HTML, images & scripts (scripts may need special changes to htaccess & httpd.conf) will be served normally from http://my-Mac-name-here.local/~jjeaton/ .
So, like a typical webserver, whatever files you put in there will not be changed unless you youself edit/move/delete them.
You should also double check the permissions so that only you have read/write/execute permission and group & others only have read/execute.


My middle knuckle of right ring finger pain while piano practise. What can I do?

Question: This is the only finger that pains. What should I do?
Comment: It may need rest, or it may need treatment. I would try rest first, because it is cheap and easy and won't make the problem worse. If it doesn't work you'll need medical advice.
Answer: Have someone such as a Piano teacher or advanced player check your technique. You may be playing in a way that is causing harm to your finger.  If you don't have a teacher or advanced player you can reach out to you might try posting a video of your playing to get advise on your technique.
If the pain continues or happens when you do other things, seek medical advise. I have Psoriatic arthritis that started as a pain in a single finger joint.  Finger and joint pain shouldn't be taken lightly, especially for a musician. It is important to get diagnosed and start treatment as soon as possible if it is a condition that will affect your finger mobility.
Answer: I had similar pain in my right finger and it turned out to be something called "Trigger finger" which as a guitar player, caused pain when playing.
The pain usually starts at the bottom of the finger, but can cause knuckle movement to cause pain elsewhere in the finger.
I eventually had to to get surgery called "Trigger finger release" which opens up the bottom sheath around the tendon. Left unchecked, it gets worse over time, but the surgery for me worked. Best thing to do is ask your Doc what they think. Bottom line is don't ignore it.


Condensing furnace in garage or attic safe for winter?

Question: Is it ok to have high efficiency condensing furnace in the garage or attic of a house in an area that experiences below-freezing temperatures? I've heard that having a furnace in a spot without "conditioned air" can lead to frozen components that somehow break the furnace.
Comment: What make/model of furnace are you looking at?
Comment: Only Carrier at this point.
Answer: A condensing gas furnace needs to discharge it's condensate "waste" water.  If the furnace is in an environment that experiences freezing temperatures, this can become difficult. Typical condensing gas furnaces don't have provisions to protect the condensate line against freezing temperatures, so the line can freeze up and break.
This is one of the reasons you won't typically find furnaces installed in attic spaces in cold climates.
Answer: My high efficency condensing furnace in the attic freezes when it gets to zero or lower.  I have been going into the attic on cold mornings and thawing the line and the tank in the unit with a heat gun.  This winter I have built a shroud out of a large aluminum roasting pan, attached it to the side of the furnace  Put a heat lamp under it and plugged into a thermostat contolled receptable.  Supposed to get to -5 this next week.  Hope it works.


C#, Is it possible to avoid defining private fields and still use set/get on public fields?

Question: Like this:
public String text {get; set{
    // here comes some setter checking / setter logic
    // finally assign the value to the var without using the setter
    <is_there_a_"without_setter"_keyword?> text = value;
  }
}

Answer: No there is not a way to do that
Answer: No. The property is the exposed interface (not a C# interface) for controlling access to the private backing field. The special syntax introduced in C# 3 is what's called "syntactic sugar" - the compiler actually turns this into a private field that you never see with a get/set property to access it.
Edit: Properties themselves don't store data. They simply control access to data stored elsewhere, where one common case is in a private field.
Another Edit: This is the sugar syntax I'm referring to:
// When this is compiled, it actually creates a private
// field for you, which the property accesses.
public int MyProperty
{
    get;
    set;
}

Answer: If you define custom code for your setter, however simple, then you must also define the get method if you want it
The farther you can go without writing this custom code is
public String text {get; protected set}

which allows public read access and modifiction only in your classes and its descendant
Answer: There is no way to do that when you have custom logic to apply. If you have a simple property, you can do:
public int Foo { get; set; }

You can also set the visibilities of each:
public int Foo { get; private set; }

Answer: You cannot do that for an property where you do not define the backing field. When you define such a property, the C# compiler generates a private field with the name <PropertyName>k_BackingField in the MSIL. As other posters have pointed out, at most you can tweak the access modifiers on the property.
There are some rules regarding that too, which are mostly common sense:

Access control on get and set can only be more restrictive than on the property. i.e. You cannot have a private PropertyName and give it a public get or set.
You cannot definite both get and set to be private when the property is public. At most one attribute can be private.

However, this is all syntatic sugar provided by the C# compiler. You could provide the backing field yourself, and provide your get and set implementations for the property. You could choose to make the backing field public.
There is only a minor difference in the MSIL code generated for both cases ( The generated backing field includes an System.Runtime.CompilerServices.CompilerGeneratedAttribute attribute. So too do the generated get and set methods )


re-direct /Documents to a folder on a ubuntu server

Question: I hope this is an easy task to implement basically I 'need' to point the currently logged in user's documents,etc to a folder on the server. I know this an easy task on a windows server and client through group policy.
Comment: I think you want LDAP - https://help.ubuntu.com/community/ActiveDirectoryHowto - but I am not sure as I find your question vague
Comment: thanks for the quick response, I will re phrase, on a "windows" network you can configure the client to re-direct the users documents folder to an actual folder that resides on the server, thus giving the impression that the users documents 'follow' them as they login to different machines.
Comment: On Linux it is called active directory, see the link I gave you. I am not aware of any graphical configuration tool and it takes some time to configure.
Comment: RHEL has a graphical tool https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Deployment_Guide/ch-Configuring_Authentication.html
Answer: Just mount the network share on the Documents folder
Move any existing docs out of the local user Documents folder first.
Then add this to fstab
server.local:/srv/user/Documents /home/user/Documents  nfs  _netdev,auto 0 0

Change server.local to a valid dns name of your server.
Tell it to do the mounts now
mount -a

Now your local Documents folder points to your server location.
You would need your nfs server in place first, and nfs-tools installed locally.
This is a manual way to do it for one user on one workstation.  For multiple users or a generic version you'll need something smarter using LDAP or NSS.


Error when trying to fix "unmet dependencies" on Ubuntu Server 14.04

Question: I am trying to install php5-dev:
sudo apt-get install php5-dev

But I got this error:
You might want to run 'apt-get -f install' to correct these:
The following packages have unmet dependencies:
 postgresql-contrib-9.3 : Depends: postgresql-9.3 (= 9.3.9-0ubuntu0.14.04) but 9.3.5-0ubuntu0.14.04.1 is to be installed
E: Unmet dependencies. Try 'apt-get -f install' with no packages (or specify a solution).

So I run the command:
sudo apt-get -f install

But then I get this error:
invoke-rc.d: initscript postgresql, action "stop" failed.
dpkg: warning: subprocess old pre-removal script returned error exit status 1
dpkg: trying script from the new package instead ...
 * Stopping PostgreSQL 9.3 database server
 * Insecure directory in $ENV{PATH} while running with -T switch at /usr/bin/pg_ctlcluster line 255.
Insecure directory in $ENV{PATH} while running with -T switch at /usr/bin/pg_ctlcluster line 264.
(does not shutdown gracefully, now stopping immediately)

[fail]
invoke-rc.d: initscript postgresql, action "stop" failed.
dpkg: error processing archive /var/cache/apt/archives/postgresql-9.3_9.3.9-0ubuntu0.14.04_amd64.deb (--unpack):
 subprocess new pre-removal script returned error exit status 1
Errors were encountered while processing:
 /var/cache/apt/archives/postgresql-9.3_9.3.9-0ubuntu0.14.04_amd64.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)

My config:
Distributor ID: Ubuntu
Description:    Ubuntu 14.04.1 LTS
Release:        14.04
Codename:       trusty

I have no idea how to fix this... Can anyone help me out?
Answer: It fails to stop, so try to stop it manually:
sudo service postgresql stop

Make sure that apt cache is up to date:
sudo apt-get update

Try again to fix:
sudo apt-get -f install

If it doesn't work, wipe postgresql completely. See, what postgresql packages are installed:
dpkg -l | grep postgresql

Then, remove with sudo apt-get remove <package name>. Pay attention to the messages like "THE FOLLOWING PACKAGES WILL BE REMOVED", don't accept if important stuff is listed there.
Try once more to fix:
sudo apt-get -f install

...
Answer: The current version of postgresql-9.3 in trusty main is 9.3.7-0ubuntu0.14.04. To install 9.3.9-0ubuntu0.14.04 you need trusty-updates main.
Open the file /etc/apt/sources.list
sudo nano /etc/apt/sources.list

and check if you have enabled the trusty-updates repository at least for main
eg:
deb http://archive.ubuntu.com/ubuntu trusty-updates main restricted universe

Results from the Ubuntu packages search
Paket postgresql-9.3

    trusty (14.04LTS) (database): object-relational SQL database, version 9.3 server
    9.3.7-0ubuntu0.14.04 [security]: amd64 i386
    trusty-updates (database): object-relational SQL database, version 9.3 server
    9.3.9-0ubuntu0.14.04: amd64 i386


should I use the mean or weighted mean in this example?

Question: An investor calculated the percentages of each of three stock investments with payoffs as shown. Find the average payoff for the three stock investments.
what should I use in this case to calculate the average?
Answer: If you know how much money is invested in each of the three investments, it makes sense to do a weighted average, with the weights coming from $\frac{A_i}{A}$, where $A_i$ is the amount of money invested in investment $i$, and $A$ is the total money invested, so that $A = A_1 + A_2 + A_3$
Otherwise you could do a simple equally weighted average (i.e. average payoff = $\frac{1}{3}(p_1 + p_2 + p_3)$, where $p_i$ is the payoff for investment $i$.


Why does the application portal ask 'To what other programs/universities are you applying?' while applying in US universities?

Question: I am applying for higher studies in US universities. Every time I fill up the online form, there is a section where they asked which other universities or programs I am applying too? Why do they ask these questions? Do they check where I am applying?
What if I wrote a university name but later drop the idea to apply?
Comment: Does this answer your question? [Why do graduate school application forms ask applicants to name the other schools to which he/she is applying?](https://academia.stackexchange.com/questions/15842/why-do-graduate-school-application-forms-ask-applicants-to-name-the-other-school)
Comment: Also related: [PhD application asking what other insitution I am applying to, how to answer?](https://academia.stackexchange.com/q/5446/68109)
Comment: I think they're much more likely to use the information for internal statistics and for grant applications rather than the reasons mentioned in the Q&A linked by GoodDeeds. There might be some consideration of whether applicants are likely to go someplace else, but I don't think programs are going to play those games much when they think a particular student is a good fit. Maybe a bit different if you spend your entire interview talking about how great Harvard is.
Comment: Do you mean the online application of an individual university or something more general? There are generalized portals for application, but I thought they were used primarily for undergraduate applications.
Answer: The questions referenced by GoodDeeds covers most of your question - the why. For the rest:
In the US it would probably be impossible (illegal) for one university to query another about a student's application, so, no, they don't check.
And there will be no consequences if you list a school and then change your mind.
Don't feel any special obligation to answer such a question. I actually doubt that they use such a question to determine whether to accept you or not. It could be counterproductive for everyone.