Human race or human breed?

Question: What is the difference in between "race" and "breed"?
Could I say "human breed" instead of "human race"?
Comment: _Breed_ is restricted to domesticated animals (and plants, for those who understand plant breeding). _Race_ is a term for various perceived and/or asserted socioeconomic groups (which may or may not be social groups, and may sometimes include physical identifying characteristics). Like _caste, class, level,_ and many other terms, it's used to classify and sift human society. So the question is which metaphor you imply: humans are animals to be bred, or humans are all the same race (i.e, _race_ = _species_). I like the second one better, personally.
Comment: @JohnLawler The question of whether a *race* is applied to a group of humans e.g. *the Eskimo race*, or whether it is applied to humans in general is interesting. In the days of apartheid South Africa all visitors to the country were given a landing card which asked you to describe your 'race'. Some people used to write *human* in the space provided.
Comment: Depends. What do you want to say? _Humans breed other humans, all of whom, whatever their race, would be some breed of human, a strange breed of mammal._ We need more context to be able to help.
Comment: At least as I have seen it used in older works, breed implies a variety that has arisen through deliberate selection (hence the verb to breed), while a race is a variety that occured naturally.  However, contemporary sociopolitical constraints have seen the last usage replaced by 'variety'.
Answer: At the simplest level, it's race for people, and breed for animals.
From Oxford Dictionaries - race (emphasis mine): "Each of the major divisions of humankind".  [It then goes on to give lots of specific meanings, just in case anyone thinks that's too simplistic a definition of a highly-charged word.]
And from the same dictionary - breed (emphasis mine again): "A stock of animals or plants within a species having a distinctive appearance and typically having been developed by deliberate selection."
However, the same entry then gives a second definition for breed: "A sort or kind of person or thing".  So you could say, for example, that people who spend Saturday night posting to Internet forums are a special breed of people.  That's a metaphorical usage - it doesn't imply that the people share common DNA.
And no, you'd never say human breed; the stock expression is human race.

Human race or human breed?


PHP: svn_update and shell_exec("svn up ".DIR)

Question: Whats the difference between a svn_update and doing a echo shell_exec("svn up ".DIR);
Answer: My guess is the big block of text that says:
This function is EXPERIMENTAL. The behaviour of this function, its name, and surrounding documentation may change without notice in a future release of PHP. This function should be used at your own risk.
Answer: I would expect the 2 operations to be the same as the svn php module tries to be a "native" svn client.
There could be differences in the versions of the 2 clients (php client versus command line client).
You may want to check if the svn user used for both commands is the same. In the php client you must use svn_auth_set_parameter to setup the credentials of the svn user
I hope this will help you,
Jerome Wagner


Discount on shipping rate coming from ups

Question: I want to give 15% discount on shipping rate that I get from ups in magento.any idea ?
Answer: We use an extension provided by Amasty that allows this.  You can set it to work for all orders, or orders to certain destinations or orders totaling over a certain amount...
This is the link to the extension
http://amasty.com/shipping-rules.html
And on magento Connect:
http://www.magentocommerce.com/magento-connect/shipping-rules-by-amasty.html
I am not affiliated with the developer nor do I receive any benefits from recommending their extensions
Comment: Thank you.I have bought another extension by webshopapps but it is not working.I asked for support but didnt get any reply
Comment: Does it support for different store.for eg in one store we want to give 15% discount nd on other we want to give 20%.
Comment: Yes, you can specify per store and per customer group


Special relativity and quantum mechanics

Question: I know how the Dirac's equation about the relativistic quantum mechanics works. Can anyone tell me how can one combine special relativity and quantum mechanics as a whole - special relativity is valid only under constant speed while the speeds of electrons in quantum mechanics vary with positions.
Answer: You assert that

special relativity is valid only under constant speed while the speeds of electrons in quantum mechanics vary with positions

This is a very misleading statement.  What is true is that via Poincare transformations, special relativity describes precisely how the observations of different inertial observers are connected.  It is also the case that Poincare transformations do not connect the spacetime observations of accelerated observers to those of inertial observers.
However, this does not mean that special relativity has trouble describing the physics of accelerating particles.  Quite to the contrary, there is, for example, a relativistic version of Newton's Third Law for massive particles which says that if you choose some inertial frame from which to perform observations, then in such a frame one has
\begin{align}
  \mathbf F = \frac{d\mathbf p}{dt}
\end{align}
where $\mathbf p = \gamma m\mathbf v$ is the relativistic momentum of a particle of mass $m$ and $\mathbf{F}$ is the net external force experienced by that particle.
As for relativity and quantum mechanics, I think it would be overly-ambitious to give some sort of a self-contained account of how one combines the two; I'll defer to the wiki on relativistic quantum mechanics?
In my personal opinion, however, if you want to properly describe the interplay of relativity and quantum mechanics, you might as well make the conceptual leap to relativistic quantum field theory (QFT).  I'll again defer to wiki and the countless other posts on physics.SE detailing this vast subject.
Comment: Yes, this is one of the most annoying memes out there (for me anyway). Special relativity is perfectly fine with accelerating objects. [It is even perfectly fine with accelerating observers.](http://en.wikipedia.org/wiki/Rindler_coordinates) What it is *not* ok with is *curvature*. The question you need to ask is: "am I doing experiments over small enough a region for gravitational tidal forces not to matter?" If yes, then you have special relativity, no matter how you are moving. This is just the famous Einstein equivalence principle.
Comment: Thank you all for the replies.
Does that mean in Minkowski space - no significant gravitation, no matter there is acceleration or not, special relativity works?
Comment: @LisaLee Yes, it sure does.


I've had my home brew sitting at 19c for 4 days now but there are no bubbles

Question: I've had by home brew sitting at 19c for 4 days now but there are no bubbles showing in the air Lock I opened the lid slightly to look inside I can see the yeast floating on top and there's a fairly strong smell of beer. Do I need to do anything or just leave it.
Comment: It was Krause. I took the gravity reading and it came out at 1.012 so I've transferred it into the pressure barrel now. The seal around the rubber on the lid of my fermentation bucket had part of it missing so I think that's probably where the co2 was escaping through.
Comment: Describe "floating" you see krausen or dry yeast like you just pitched a dry pack?
Comment: Seems about right. i brewed a beer this week with an OG of 1.050. After two days at 68 was down to 1.01 and activity essentially done.
Answer: Take a gravity reading. Write down the gravity. Is it already in a reasonable range for a finished beer? Anything of 1.014 or lower should be fine.
Did you take a reading of the Original Gravity (OG)? Is the new gravity reading different? If yes, your yeast is working. If there is no change between the OG and the current gravity, your yeast may be dead. You should pitch another yeast.
The yeast may have died if you pitched it when the wort was too hot or if the yeast was uite too old or improperly stored.
Comment: This is my first time making a home brew so I just followed the instructions on the beer kit and there was no mention of taking an OG reading. I'll take one no tho and se hat comes up if it's high I'll look into getting new yeast. Thanks.
Answer: You don't need to do anything.
Airlock activity is also a measure of how well your bucket/carboy/barrel/etc is pressure-sealing.
If there's yeast activity (foam, bubbles, etc.) and a beery-aroma, it's going well.
I would wait 2 weeks before doing anything.  I expect you could then take a gravity reading and find the beer is finished.  If you really want to, take a reading at 1 week.  But be sure keep everything sanitary.
Also, if it's 4 days in and there was no yeast (not pitched, or no vitality), it would probably be already colonised by something nasty anyway.


Customize Salesforce1 List Views

Question: In default list views, the "link" to click on a record is the record's ID. This annoying limitation is avoided with a formula field which generates a hyperlink which has a more readable label. My issue is that these hyperlinks do not render correctly in Salesforce1 (for iOS). In the iOS app, instead of generating, for example, a tappable "John Doe" in the list view, we'll see (a href="/a00......" target="_self"....)  etc. This is perfect for the website, but the mobile app can't handle it. What is a way to hide this? Two options I can think of are 1. custom list views for users on mobile, or 2. formula fields that somehow check if the user is on mobile.
Is either of these possible?
Below is the offensive image.
Answer: This is a known issue and has been successfully reproduced by Salesforce support as confirmed here.
As for workarounds, I think "formula fields that somehow check if the user is on mobile" is going to be quiet difficult so I'd advise on creating a custom list view for mobile. Depending on which HTML5/ responsive CSS framework, there are components available for listview which you can parameterise as per your requirements.
Comment: Custom mobile list views sound great, thanks. Could you direct me to documentation that describes how to effect this? Certain Salesforce docs ask you to go to a section called "Mobile Configuration", but I don't see this in the most recent Salesforce implementation. By the way -- developing for Salesforce1, iOS.

Customize Salesforce1 List Views


Would it be possible to a world to have multiple ages across itself, but in the same time?

Question: I mean, would it be possible to a single world to have different historical time periods depending on the location? Like a village/kingdom that have a middle-age-like knowledge and technology, but a city far from there having a modern or futuristic technology, but both in the same time period? Or does if a world advances to another age every place throughout it also advances, with no exceptions?
Comment: I would say that we *are* in this kind of world. Just compare a random poor African country with western Europe or the United States. Do you think the whole world could access this webpage if they wanted to ? Some don't even know what a "webpage" is.
Comment: Further, some people cannot fathom a webpage at all. They've never even heard of  a computer.
Comment: It already happened on earth. Europeans had cannons while the inca had spears. They lived simultaniously. Ir a more modern approach: Think about North Korea. Sone estimate that North Korea is 40 years behind in technologucal development.
Answer: I see three types of situation in which you may have societies of very different levels of technological advancement surviving at the same time on one world:

Differential advancement: This is where geographic isolation and differences in the resources and opportunities available to societies mean they develop technological capabilities at different rates. Read Guns, Germs, and Steel by Jared Diamond for several interesting historical case studies of this phenomenon.
Cultural choice: This is where a society, or those in power in that society, deliberately isolates themselves from other societies and outlaws certain technological advancements. The Japanese period of isolation from the 1630s until 1853 is one example, or the Amish, living 19th century lives in the present-day US, are another.
Cultural theme park: This is a world in which someone has contrived to create a situation where different cultures at different levels of advancement live side by side. You see this in the story Cathouse in Man-Kzin Wars I or in the Riverworld Series by Philip Jose Farmer.

Differences that result from Differential advancement or Cultural choice always get overcome eventually. There are still uncontacted tribes in the Amazon living essentially stone-age lives, but there is little of the planet that is so remote these days that technological society does not reach it. Europe eventually found the cultures of north and south America with devastating consequences for those cultures. Japan's isolation was ended when the Americans under Admiral Perry sailed modern war ships into Edo in 1853 and forced Japan to open to outside trade.
If you opt for a natural explanation for why different societies co-exist with radically different levels of technology, you must have a satisfactory reason why the more advanced societies do not enslave or dominate the less advanced societies. Technological advancement reaches a point called the Line of Empire (see Bruce Sterling, Shaping Things), where it's possible for a more advanced society to dominate less advanced societies militarily, culturally and in terms of trade. The pull of resources and increased living space  will mean that a society who can walk over a neighbour is very likely to do so.
This is why authors resort to the Cultural theme park – they must posit an outside power with inscrutable motives that imposes the situation on the world. In the Man-Kzin Wars Cathouse story, there are walls between hexagonal compounds that tile the surface of the planet, each compound contains an environment and a culture from various historical periods and alien races in Larry Niven's Known Space universe. The mystery of who set up the zoo is central to stories like this.
The worst case of the cultural theme park is the average 1990s Dungeons and Dragons setting, where a Dark Ages Viking society lives next to a high-medieval society or a gun powder using European Renaissance society. No attempt is made to explain why the more-advanced society simply doesn't stomp their less advanced neighbours and take their stuff.
So the choices seem to be opt for deliberate cultural or geographic isolation, and understand that the differences in development are temporary and some societies will come to dominate others in time, or build yourself a theme park with more or less rationale.
Answer: Easily, though out history there are many examples of one civilization been decades and even centuries more technological development then an other.  Good examples would be Europe and the American Indians, Pre-industrial Japan and the US, the Mughal and Europe.
Here are some possible explanations for why this might happen.
First post-apocalypse: end of the world happens Society collapses, eventually though people come together and try to rebuild. You could have a scenario one group of survivors is made up of Engineers, scientists, doctors, and other intellectuals. These people would have the knowledge to speed up human technological development as they rebuild Society.
In contrast if their neighboring group of survivors was primarily composed of regular non-intellectual people, then you would have a case where one group was able to rebuild much of the technology we have today in this short amount of time, well the other regressed to a more primitive level of Technology and was unable to reclaim anything close to our current level of Technology for centuries.
Second closed borders: in the early stages of colonization Japan shut its borders forbidding any foreigners save the Dutch from entering Japan on pain of death. This protected Japan from foreign influences but it also closed it off to foreign trade. Because of this Japan missed out on the early stages of  the Industrial Revolution, when do United States eventually showed up with its modernize Fleet and demanded that Japan open its borders to trade, it was little the Japanese could do but yield. They knew well enough that swords and arrows cannot stand against modern machine guns. In your world a similar situation might happen with one country closing off its borders to trade and missing out on several technological inventions that are spreading throughout the rest of the world.
Third lack of contact: the American Indians had little contact with Europe or Asia because of the gigantic ocean that separated the old world from the new. Because of this similar to the way Japan missed out on the early stages of industrial Revolution, the American Indians missed out on several inventions that came from Asia to Europe specifically gunpowder. Something similar could happen with your two countries an ocean or a non passable mountain range could prevent contact with one another allowing 1 to develop technologically very quickly and the other to develop much slower.
Comment: Also of note, Mughal != Mongol
Comment: Did you mean to compare the Mongols and China? Just wondering, becuase you seemed to be comparing nations that historically had interactions.
Comment: @XandafTheZenon No but I did compare post China Mongols with Europeans during the Mongol conquest of Russia. But the Mongols and China is another good one.
Answer: (Please excuse me if this answer is not what you're looking for; it's my first one.)
An example that might help us understand the situation you are proposing is that of the Inca civilization and the Spanish conquistadors. The Inca were not quite living as one would in your stereotypical (European) middle-aged town, but for our purposes, we can dismiss that in order to draw lines parallel to your scenario. In contrast, the Spanish had developed muskets and war tactics that would send the Inca into disarray during battle. During The Age of Discovery these two contrasting worlds collided, and while the conquistadors were not exactly flying around in F-18s, they were certainly frighteningly much more developed than the indigenous Inca. A display of these technological differences was Battle of Cajamarca, in which the Spanish Conquistadors won by a ridiculous margin.
The biggest (and pretty much only) factor in the separate evolution of technology in these two societies is the large distance in between them, not to mention the (enormous) Atlantic Ocean. So, I would recommend something similar to an ocean in between your two societies.
To tie my point together- yes, it's completely feasible for societies to develop on drastically different technological tracks, but it has to be for a reason (whether that is an unsurpassable mountain range, the stormy terminator of a tidally locked planet or a large body of liquid). In most cases, when there are two towns right next to each other, one of which is chock-full of rockets and robots, and the other brimming with spears and serfs- the people in the latter town are all going to adopt the former town's technology (with the exception of super-strict luddite religions or the like).
Answer: I would say there are a few ways that different technological ages would exist, and in fact to some extent they already exist that way.
Geography. There might be no communication between peoples. The higher-technology people can't get to the poorer tech people's place or don't know about them. This is the case before Columbus brought the full invasive force of Europe to them. Perhaps the high-tech people are concerned with space travel, and just have forgotten that any people outside their city even exist. This sort of thing might explain Atlantis.
Economic and cultural reasons. There are places on earth where a few, isolated tribes have stone age technology even now. For some reason we respect their culture and don't try to bring them into the 21st century. Certain places in Mexico are suffering with diseases of malnutrition that haven't been seen elsewhere since Medieval times (Arizona Republic, 2016). For some reason our vaunted generosity has failed to bring succor to them. Maybe the technologically rich take care of their lesser brethren, with deliveries of food and basic medicine but refuse to let the Sleepers learn anything about it or possess any technology.
Superstition, prejudice, war, subjugation. There might be a fear of technology or a refusal to study to learn its secrets on the part of the poor technology people. They might do this for religious reasons, or they might just find schooling somehow degrading. The abundant technologists might feel that their technology is somehow sacred and the others not suited to use it. Perhaps one nation abused the technology, and are forbidden to have anything above medieval level technology or they will be bombed to the stone age. Perhaps the people in one area are 'most dangerous game' and the high-tech people are cannibals or just hunt them for fun. Defensive technology would be forbidden, and any technology is hard to develop while on the run.


Is it possible to optimally transmit solar radiation by a system of multiple optical fibers?

Question: As we all know, if an optical fiber is designed with the right material and properly bent, light can be transmitted along the fiber with very little attenuation.
Figure 1 shows the interaction of radiation F with a conical structure. The cone is a hollow mass with an internal reflector layer or a homogeneous mass of transparent material. Due to the non-ideal incident angle of radiation F (as shown in figure 1), radiation F will be easily reflected back or transferred out of the cone.
In the case of the above cone is a system of multiple optical fibers (figure 2). Optical fibers are arranged as a multi-branch tree: Start by an optical fiber at the bottom, then it is gradually branched at the upper rows and the top row will have the most fibers. Optical fibers have one narrow end below and the other larger end above. The optical fibers are separated by a vacuum layer so that total internal reflection can occur optimally. It can be noticed that radiation F will be easily transmitted along the fiber inside the cone.
We find that with the same incident angle and interaction position, radiation F in the two figures will have two different directions. It can be seen that a system of multiple optical fibers will have a much larger acceptance angle.
The key is to find the right material for the fiber and bend the fibers properly (without bending too strongly) so that radiation F is not refracted out of the fiber during transmission.
Extending from radiation F, can we use the above multiple optical fibers system to optimize solar energy harvesting (similar to Winston cones or Fresnel lenses but with higher efficiency)?
Comment: What do you mean optimally transmit? Interesting idea. I think you could imagine replacing the multiplexing layers with a single optical element like a lens and have the same fundamental concentration by Étendue conservation.
Answer: It should be possible, but with mirror or lenses you also can concentrate sunlight, so for what will you use id?


Naval warfare in medieval fantasy

Question: So before cannons how would naval warfare work in a medieval fantasy setting? I only know of Greek fire and flaming arrows. Would soldiers still wear armour or use light clothes in case they fell overboard or the ship sank? In my story there is a high presence of magic so maybe that could compensate for a lack of gunpowder. Mainly I need to know what types of weapons would be used ship vs ship. Thank you to all who help!
Comment: This isn't about worldbuilding. This would be a better fit for [history.se].
Comment: Misclicked on review. This is certainly for [History.SE] and is not about worldbuilding.
Answer: Well, you might consider the shape and tactics of the Venetian navy. They did very well with archers and infantry and ramming techniques. I don't know if they had greek fire or not, but they certainly rivalled the Byzantine navy and eventually acted quite independently of that Empire. They held sway over the Adriatic and eastern Mediterranean for several hundred years and later kept the Ottoman navy at bay as well. Many of their ship designs seem to evolve from earlier Roman designs. And the Roman navy is another place you can look for inspiration.
Answer: Ships wood fire catapults and ballistas at each other for far range.
Up close ships would try ramming each other once one ship rammed another if the other ship didn't sink they would send over borders. Which would take over the ship.
No  it's unlikely these guys would wear heavy armor. If your ship was doomed you you could have abandon it set it on fire and then set it on course straight for the enemy ship suicide bomber Style.

Naval warfare in medieval fantasy


Will formatting a USB drive remove the Ubuntu Bootloader installed in it?

Question: I have a PC, where I installed Ubuntu on the hard disk, and the bootloader on the USB drive. Please don't ask me why :P  . I Want to know the answer if the formatting is done normally, using the right click popup menu and clicking on the format button, either on Windows or Ubuntu
Comment: Do you want to remove the Ubuntu bootloader, or do you want to keep it in the USB drive? - I would suggest that you install the bootloader into the internal drive. See this link, https://help.ubuntu.com/community/Grub2/Installing
Answer: No. Formatting will not typically remove the MBR. The MBR and the partition table both live in the first 512 bytes of the drive. Wiping the first 512 bytes of the drive will eliminate both which is not what you have in mind. To eliminate only the content of the MBR (Master Boot Record) issue the command `sudo dd if=/dev/zero of=/dev/sdX bs=446 count=1 where /dev/sdX is the drive you wish to wipe the boot record from.
Be careful to confirm that you have the right device beforehand by checking the output of sudo fdisk -l or lsblk or lsscsi
Ideally, in your situation I think what you wish to do is to install grub to the MBR of the Hard Disk
Source: https://www.cyberciti.biz/faq/howto-copy-mbr/
Comment: So, right clicking on the drive and formatting won't remove the bootloader in any OS, right?
Comment: Not to my knowledge. Formatting is a high level process typically done on a partition and not on an entire device.  I try to never say never because all things are possible given the right (or wrong) conditions.
Answer: mkusb provides a surefire method to restore a working USB drive to it's original condition.
The Wipe menu gives options to zero out the first megabyte of the drive, (wiping out the MBR), restore the MSDOS partition table and then partition the drive with a FAT32 file system.
The script uses dd to do this, but it wraps a layer of safety around the process using a simple GUI with checks and double checks.
https://help.ubuntu.com/community/mkusb/wipe


How to make only my first letter of my first paragraph Drop Cap without changing all next paragraphs with drop caps?

Question: I am struggling with Drop Caps and character styles. When I try to make my 1st letter of my 1st paragraph in my body text style Drop Cap, all other paragraphs a beginning with Drop Cap.
How can I do it only for the 1st letter, maybe with character and nested styles?
Answer: You need two paragraph styles: one with Drop Cap, one without(let's call it "Body text").
I recommend setting the one with Drop Caps with Next Style: "Body text".


Determining the Thevenin Equivalent

Question:
How could I proceed with this circuit to determine the Thevenin Equivalent? I have the values of \$R_1, R_2, R_3, I_2\$ and the time value for g. I first tried to figure out the total voltage for the equivalent by ignoring \$R_2\$ and \$R_3\$, but that did not seem reasonable to me in the end. I tried to solve other examples of (somewhat) similar circuits, which had multiple resistors in series and/or parallel, but I'm still not sure how the ones in this one should be viewed. Perhaps \$R_3\$ and \$R_2\$ should be added as parallel, and then added parallel to \$R_1\$? What comes to the total voltage, the time value throws me off since I do not have much clue on what to do with it, so I thought I could just figure it out via total resistance.
Comment: Is there anything that you haven't understand?
Comment: Read [here](https://electronics.stackexchange.com/questions/86260/thevenins-theorem-for-the-circuit-with-controlled-voltage-source) and [here](https://www.allaboutcircuits.com/technical-articles/thevenin-theorem-dependent-source-circuits/). Report back if you still don't understand.
Comment: @Unknown123 Read through them, but I simply do not understand this concept. Ill try to look for more material from the web, but do you have anything to add to my problem?
Comment: I got the voltage now, working on the resistance.
Comment: What concept did you don't understand? If you don't understand how to find the Thevenin Voltage then I guess there's something wrong with your understanding on circuit analysis, nodal analysis in my case works easily.
Answer: First, I'll redraw the circuit.

simulate this circuit – Schematic created using CircuitLab

1. Calculate the Thevenin Voltage \$\mathrm{V_{th}}\$
Do circuit analysis to calculate \$\mathrm{V_{th}}\$. Here, I will do Nodal Analysis by choosing \$\mathrm{V_b}\$ as ground.

simulate this circuit
Known
\$
\begin{align}
\mathrm{V_{th}} &= \mathrm{V_a}\\
\mathrm{V_{R2}} &= \mathrm{V_x-V_a}
\end{align}
\$

Inspect Node \$\mathrm{V_x}\$
\$
\begin{aligned}
\mathrm{I_a} &{}+{}& \mathrm{I_b} &{}+{}& \mathrm{I_c} &{}+{}& \mathrm{I_d} &{}+{}& \mathrm{I_e} &= 0 \\
\mathrm{-(g\!\cdot\!V_{R2})} &{}+{}& \mathrm{I_2} &{}+{}& \mathrm{\frac{V_x}{R_1}} &{}+{}& \mathrm{\frac{V_x-V_a}{R_2}} &{}+{}& \mathrm{\frac{V_x-V_a}{R_3}} &= 0
\end{aligned}
\$

Inspect Node \$\mathrm{V_a}\$
\$
\begin{aligned}
\mathrm{I_f} &{}+{}& \mathrm{I_g} &= 0 \\
\mathrm{\frac{V_a-V_x}{R_2}} &{}+{}& \mathrm{\frac{V_a-V_x}{R_3}} &= 0
\end{aligned}
\$

Derive it by yourself

2. Calculate the Thevenin Resistance \$\mathrm{R_{th}}\$
Calculate the Short Circuit current between \$V_{ab}\$, then do circuit analysis to calculate \$\mathrm{R_{th}=\frac{V_{th}}{I_{SC}}}\$.
The \$\mathrm{V_{th}}\$ value will be the open circuit we calculated before. Also, I'll do nodal analysis again the same as before, but this time instead of \$\mathrm{V_x}\$ I'll rename it as \$\mathrm{V_z}\$, because I would like to note that it is different in this situation.

simulate this circuit
Known
\$
\begin{align}
\mathrm{V_{th}} &= \mathrm{-I_2R_1}\\
\mathrm{V_{R2}} &= \mathrm{V_z}
\end{align}
\$

Inspect Node \$\mathrm{V_z}\$
\$
\begin{aligned}
\mathrm{I_a} &{}+{}& \mathrm{I_b} &{}+{}& \mathrm{I_c} &{}+{}& \mathrm{I_d} &{}+{}& \mathrm{I_e} &= 0 \\
\mathrm{-(g\!\cdot\!V_{z})} &{}+{}& \mathrm{I_2} &{}+{}& \mathrm{\frac{V_z}{R_1}} &{}+{}& \mathrm{\frac{V_z}{R_2}} &{}+{}& \mathrm{\frac{V_z}{R_3}} &= 0
\end{aligned}
\$

Calculate \$\mathrm{I_{SC}}\$
There could be two choices from \$\mathrm{V_z}\$ calculated above.
\$\displaystyle\mathrm{ I_{SC} = -\left( -(g\!\cdot\!V_{z}) + I_2 + \frac{V_z}{R_1} \right) } \qquad\lor\qquad \displaystyle\mathrm{ I_{SC} = \frac{V_z}{R_2} + \frac{V_z}{R_3} }\$

Calculate \$\mathrm{R_{th}}\$
\$\mathrm{R_{th}=\frac{V_{th}}{I_{SC}}}\$

Derive it by yourself
I'll choose the second one. And I got the expanded form here. It still can be simplified.

Still not convinced?
For example, lets assume the value of each resistor and current source.
\$
\begin{align}
\mathrm{R_1} &= \mathrm{R_2} = \mathrm{R_3} = 1 \,\mathrm{k} \\
\mathrm{g} &= 1 \,\mho \\
\mathrm{I_2} &= 1 \,\mathrm{A}
\end{align}
\$
The Thevenin values would be

Read explanation about negative resistance if you didn't know about it.
Here on this SE site: What is negative Resistance or from Wikipedia.
Lets simulate it using time-domain or dc-solver analysis in Circutlab below. Tutorials.

simulate this circuit
As you increase  \$\mathrm{R_{load}}\$ from \$0\$ towards \$\mathrm{498.5\,k}\$ the current and voltage are increasing towards positive infinity. At \$\mathrm{R_{load} = 498.5k}\$ those are undefined. CircuitLab won't let you to simulate it, the undefined point will be shifted because of the voltmeter internal resistance, remove it if you want to see the effect and very accurate model. And then, as you increase it again towards \$\infty\$ the current will be increasing from negative infinity towards zero and the voltage will be increasing from negative infinity towards \$\mathrm{-1000\,V}\$. It's similar to the graph of \$\mathrm{tan(x)}\$ with its domain from \$0\$ to \$\pi\,\mathrm{rad}\$.
P.S. I use Microsoft Mathematics to solve the equations above. Although its development has been stopped and has limited functionality. It's free and can be useful as a light offline substitute for Wolfram Alpha or Symbolab.

Determining the Thevenin Equivalent


Hello, i have a porcelain with these symbols on it. can someone help me decifer them ? Huge thanks

Question: the titles say's it, i have some old porcelain form china at home, and i would really like to know the meaning of the symbols in the photo attached. thank you!
Comment: Image grabbed from https://www.gotheborg.com/marks/20thcenturyjapan.shtml
Answer: Your porcelain is from Japan as it says Great Japan (大日本) .... This mark was intended to be read from the right then up to the top & left.
[item/photo#]: "1090. Mark: "Dai Nichi Hon" (Great Japan) over a sign meaning "roof/house" and the number "three", early 20th century. It is generally accepted that marks that includes "Dai Nippon" in Japanese characters on the whole date to the Meiji (1868-1912) period, reflecting the greatly increased nationalism of that period.
I found that pottery info at the webpage below & you can send a photo that pottery expert for more info.... https://www.gotheborg.com/marks/20thcenturyjapan.shtml
Answer: 日: (Sun, Pinyin:rì)
本: (Origin, Pinyin:běn)
大: (Big, Pinyin : dà)
三: (Three, pinyin :sān)
日本 :(Japan, Pinyin : rì běn)
Comment: Thanks, any idea if it's a concept of some sort, how it is red ?


Quantile-Quantile Plot for Negative Binomial Distribution

Question: I am performing regression analysis in R on count data which are negative binomial distributed. I would like to use a quantile-quantile plot as a tool to diagnose the fit of my models, but I am struggling to find an appropriate method for my data. A Normal Q-Q plot would not be appropriate since the residuals are not expected to be Normally distributed, so I think I need to compare my residuals against theoretical quantiles from the negative binomial distribution. I have tried using the 'qqPlot' function from the 'car' package, but I can't seem to get it to work properly.
Comment: What are your residuals?  Usually such a model concerns the *conditional expectation* of the response distribution.  If you are, say, subtracting that from the observations, then you won't have anything remotely like a negative binomial distribution--you will have plenty of negative values.  If you are using deviance residuals, they won't even be integral and again cannot possibly have a negative binomial distribution.
Answer: Normally distributed residuals can be generated using the randomized quantile residual method. For example
library(MASS)
fit <- glm(Days ~ .^4, family = negative.binomial(2), data = quine)
library(statmod)
res <- qresid(fit)
qqnorm(res)
qqline(res)

Here the MASS package provides the negative.binomial GLM family and the quine data example, while the qresid function is part of the statmod package.
Answer: The package performance has this and other features for negative binomial regressions.
https://easystats.github.io/performance/
By using check_model() with the model, you will have several outputs that may help you understand the performance of your model.
Also, recently discovered DHARMa package, which uses a simulation-based approach to produce residuals plots. It works with complexed models (hierarchichal or mixed models) but, can also lead with negative-binomial ones. Take a look: https://cran.r-project.org/web/packages/DHARMa/vignettes/DHARMa.html


What is the difference between maximal flow and maximum flow?

Question: I have tried a lot on internet, but I am unable to get a good answer on the difference between maximal and maximum flow in case of network flow. Anybody has an idea? with example would be really helpful.
Answer:
A maximum flow is a flow that attains the highest flow value possible for the given network.
A maximal flow is a flow whose value cannot be increased without decreasing the flow along some arc.

All maximum flows are maximal flows. Not all maximal flows are maximum flows.
Figure 3.9 in the Bang-Jensen and Gutin textbook, first edition illustrates an example of a flow that is maximal but not maximum.
The difference is very much analogous to the difference between a global optimum and a local optimum.
Answer: Both terms refer to the same thing. In the Wikipedia article on the maximum flow problem both terms are used equivalently. There you can also find some references to scientific publications that use both terms.


Can I enter Czech Republic with Schengen visa issued by Greece?

Question: I am an Indian passport holder. Can I enter Czech republic with Schengen visa issued by Greece? Are there any additional formalities/documentation required for the same?
Answer: With a visa valid for the "Schengen states" (in the national language of the issuing country) you can in principle enter any Schengen country.
You may not misrepresent your itinerary and intentions to get a Schengen visa, and if there is suspicion that you did so your visa may be annulled (cancelled). It is up to you to avoid this suspicion. With a multiple-entry visa that has already been used once for the reason it was granted, the problem does not come up. With a single-entry visa that has not yet been used, it is difficult.
For instance, if you have a 10-day visa from Germany and you arrive in France with a ticket to Germany on the next day, then you can easily show that you will travel on to your main destination.
If you have a 10-day visa from Greece and you arrive in Finland, making this plausible will be much harder.
Comment: Great explanation, short and sweet! (+1) Note that the technical terminology for the cancellation of a visa obtained by fraud is "annulment" (and it's not the same as a revocation). The stamp (in the local language) would reflect that.
Comment: @Relaxed, edited.
Answer: Yes, but you must prove that you will spend the same or bigger number of days in the country that granted you visa, Greece. Otherwise you can be declined to enter. The proof can be prepayed room at the hotel, reservation of tickets from Czech Republic to Greece & finally from Greece to your homeland.
Comment: The "main destination" rule is more complex than that, and it comes into play when it comes to applications. One might plan on 8 days in the Czech Republic, 9 days in Greece, and then postpone the travel from the Czech Republic to Greece by a day or two, as long as the basic premise of the trip remains the same.


My cheese is too mushy

Question: This past weekend I wanted to try to make my own cheese for the first time. I gathered the typical ingredients: milk, rennet, and citric acid and went to town. Making Riccotta and Mozzarella seemed to work out alright, so I decided to try a harder cheese; however, in order to do that I had to press the cheese so that it compressed into the proper form. It was difficult to determine how much weight to put on the curds to get them to form. Too little force and it gets mushy, too much and I felt like I would compress it into a singularity! Has anyone done this before that could offer some weighty advice?
Comment: It's important to mention that you actually need to worry about the pressure, not specifically the weight being applied -- so various recommendations about using 10kg for 8 hrs aren't useful without knowing what the surface area is of the container they were using.
Answer: How are you pressing the cheese?  Are you using a cheese press?  I have done some traditional cheese making and it does take a lot of force.  Not quite to the point of creating an atomic reaction though.   For something simple, look for a Dutch press.  It is a lever based press that helps.  A screw press can also work.
Comment: I was using exercise weights, but I wasn't really pressing down on them for too long - maybe my hyperbole was a bit much :) - I will keep an eye out for a cheese press next time I'm at the department store.
Answer: I think if it is too mushy, you can safely assume you need more weight. If it it compresses to nearly a singularity :), you just didn't make very much cheese.
Answer: If you slice the cheese into 1 inch slices, sprinkle it with salt and let it weep for 3 days at room temperature, the whey will be drawn out. Then cover it with brine and refrigerate. A press is not necessary.

My cheese is too mushy


Why doesn't gnosis safe bubble error messages instead of generic gs013 error

Question: One annoying thing about working with gnosis safes is the generic "GS013" error code when the transaction from the gnosis safe fails; it makes it difficult to debug why the underlying transaction failed. Why doesn't the contract bubble up the underlying error in the contract call to make issues easier to debug? The call and delegateCall in execute should also return a return value that includes the error message with why the underlying transaction failed.
Answer: It was done primarily for gas optimization. It's not worth putting the logic into the contract to be executed on-chain when you can run it off-chain where it's cheaper.
Debugging the transaction is relatively easy with Tenderly, you just need to input the transaction hash or simulate the transaction.
You can also use SimulateTxAcessor to simulate the transaction, this method always reverts and returns data in success:bool || response.length:uint256 || response:bytes format and you can decode the error from the response.
Comment: I was wondering this same thing a while ago. While this makes sense, it would _also_ make sense to include a message to devs in the GS013 error that this could be (/likely is?) masking other errors that can be discovered by simulating on tenderly. Even better: generate a tenderly link that will run that simulation for the user to click on.


Is a sinusoidal electromagnetic wave just a special case of electromagnetic radiation?

Question: The only solution of the wave equation that we are taught in elementary level Optics describes only sinusoidal oscillations.
Is this a mathematical tool for us to describe electromagnetic radiation or is there something fundamental that dictates an electromagnetic wave of sinusoidal form to be produced?
If the first is the case, what other types of electromagnetic waves are there?
Comment: The basic reason that sinusoidal waves are concentrated at the early level because any periodic motion can be decomposed to an infinite series of sinusoidal waves related to the principle time period and properly chosen amplitudes - this is the Fourier's Theorem; so indeed there are periodic waves which are not sinusoidal.
Comment: Thank you for your comment. So is any periodic wave equally valid and much more complex functions are equally probable; or is the majority of electromagnetic waves we observe sinusoidal?
Answer: The term wave equation normally denotes a very specific object, which is the partial differential equation
$$
\frac{\partial^2}{\partial t^2}f = c^2\nabla^2 f,
\tag 1
$$
where $\nabla^2$ is the Laplacian operator, usually on 3D, and $f$ can be e.g. the scalar potential, or some component of a vector field. In many situations one can simplify this to waves that propagate along a single dimension, in which case it reads
$$
\frac{\partial^2}{\partial t^2}f = c^2\frac{\partial^2}{\partial x^2} f,
\tag 2
$$
and it is still called the wave equation in that form. In its essence, the wave equation is an equation of motion: you supply the configuration of the wave $f(x,0)$ at some initial time $t=0$, and the wave equation will tell you how that initial configuration will evolve.
It is hard to know from your question, but you've probably only been exposed to some particular solutions of $(2)$, likely of the form
$$
f(x,t) = A\cos(kx-\omega t),
\tag 3
$$
or related constructions. This is not a 'wave equation', it is only one particular solution of $(2)$. We often concentrate on this specific solutions in introductory courses, but this is only to help build intuition, because they are easy to analyse.
In fact, there is a huge infinity of different solutions of the wave equation. In one dimension, for example, for any two arbitrary functions $F$ and $G$ of a single variable you can build a solution to the wave equation, of the form
$$
f(x,t) = F(x-c t)+ G(x+ct).
\tag 4
$$
In two and three dimensions the solution space is even bigger.
On the other hand, part of the reason that introductory courses focus so much on sinusoidal solutions is that they are indeed very useful solutions for describing physical phenomena, both by themselves or by building other solutions as superpositions of sinusoidal oscillations. Some phenomena (like AC power propagating down a high-voltage line, or a highly coherent laser beam) are very well described by a sinusoidal oscillations; other phenomena (like, say, sunlight) a bit less so. Whether the 'majority' of solutions are sinusoidal or not, however, depends on whether you spend your time looking at lasers or AC power line, or whether you look at other phenomena with different behaviour.
Comment: I have replaced the part 'the wave equation' with ' the only solution of the wave equation' to be more precise.
Comment: The sinusoidal solutions are the eigenvectors of the energy operator, so are representative of individual photons.
Comment: @Whit3rd No, that is incorrect. You can have a single-photon state that is not an energy eigenstate (and therefore not stationary), simply by superposing single-photon states at different frequencies. The sinusoidal solutions are also not eigenvectors of the energy operator, they're the translationally symmetric eigenfunctions of the Helmholtz operator on coordinate space (not on Hilbert space) and they're therefore completely decoupled from the hamiltonian - you can have e.g. a single photon with well-defined energy in a cylindrical Bessel resonance.
Comment: (And, in any case, discussion of QM is out of the scope of the question as posed.)
Comment: Monochromatic radiation IS an interesting special case, regardless of the reason.


Do Latin loanwords conserve their gender?

Question: When I asked my teacher for the gender of Mensa, she replied that it is feminine, because the Latin word mensa is feminine. When it comes to words that share the same spelling in both German and Latin, is this generally true?
Comment: I see a strong pattern of gender preservation, even regarding exceptions to word-ending gender rules. For example, the general rule regarding words ending in -ik is that they are feminine (e.g., _Musik_, _Statistik_). Compare with the exception _Atlantik_. In Spanish, for exmple, they correspond to Music**a**, Estadístic**a** (f), but Atlántic**o** (m). When you see the Spanish counterparts, the exception makes sense.
Comment: There is also a rule for words ending in _-ei_ being female (e.g. [de] _Polizei_, [es] _Policí**a**_). The exception, _Papagei_, (parrot) is cognate with the spanish word Papagall**o**.
Comment: I can only speak authoritatively for Spanish, which is my native tonge, but I'm sure it also applies (largely) to the other romance languages.
Answer: It is largely true, but there are exceptions with false etymologies or old words that have been modified a lot or words that look like the opposite Latin gender to a non-expert.
For example:
fenestra (Latin, female) -> Fenster (German, neutral), both mean window.
arcubalista (Latin=archer, male) -> Armbrust (German=crossbow, female, false etymology arm+breast).
Comment: Note: Those words do not have the same spelling in German and Latin.
Comment: These are not loan words from Latin to New High German, but descendants of ancient Germanic borrowings from Latin.
Comment: Also, in English the genders are called "masculine" (not "male"), "feminine" (not "female") and "neuter" (not "neutral").
Answer: Latin and Greek were both very alive in Germany in the 19th century. This might be one reason why this kind of argumentation and also the knowledge of which latin ending belongs to which gender is still very common. (And a word like "Mensa" might even be introduced during just that very time, but that's just a guess.)
A nice example of how this works in German is the plural of the word "Status". "status" derives from Latin and belongs to the rather uncommon u-declination, so its plural is "status" [ˈstaːtuːs]. This is still the "official" plural in German, too. (according to Duden, for example).
However, you'll hardly ever hear [ˈstaːtuːs]. Instead, people will use "Statusse", or - and now comes my point - if they try to be clever "Stati", assuming this would be a Latin o-declination.
So, the connection between -a -> female and -us -> male, plural -i is obviously very well known. Probably because its one of the first things you'll learn in Latin lessons, so it stuck with people. :) Another one is -or, which is always anticipated to be male.
Word like "Fenster" and "Mauer" came to German ages ago, so the connection got lost completely. Also, there's no -a or -us in there so you can't really see where it is coming from.
Comment: Thanks for pointing out the status one. One of my pet peeves. In addition the neutrum nominative plural ending on "a" is often confused with the feminine nominative singular.
Comment: Another example is _virus_, which is neutral in Latin, but looks like a masculine noun. Consequently, both _das Virus_ and _der Virus_ are in use.
Comment: As long as you don't use viri or, worse, virii in the plural ...
Comment: In Classical Latin, "status" actually has a short a sound, so the Classical Latin plural is not [ˈstaːtuːs] but [ˈstatuːs].
Answer: I think that it's important to distinguish between two types:

Words that were adopted into the german language in old times, maybe even during the roman empire. As the other contributors have pointed out, these words have changed and degenerated over the centuries and now have often different articles than their origins.
Words that found their way into German through Latin as the language of science in Germany until the early twentieth century. (I still have an old Greek dictionary from school with a preface in Latin. And I'm only 23.) Those words were - and still are - majorly used by the educated classes and have preserved their gender and orthography.

So - to answer your actual question - if you find a word that has exactly the same spelling in German and Latin, you can be pretty sure that it belongs to that second type and has preserved its gender.
As always: Be aware of traps!
Answer: I think it's true for words with the exact same spelling, although I think the overwhelming number of loanwords have changed spelling and pronunciation in their German version. For these cases there are exceptions, for example:

das Fenster - fenestra, -ae
die Mauer - murus, -i
Comment: @ladybug: "Except sanitation, medicine, education, wine, public order, irrigation, roads, a fresh water system, public health and peace, what have the Romans ever done for us?!" - But that wasn't a history lesson, IIRC.
Comment: Mauer und murus seems to be cognates but I am not sure you could invoke direct import.
Comment: @ogerard: I remember having learned that in a history lesson of "what have the Romans done for us". ^^


Need for Buffering Signals ?

Question: What is the need for Buffering signals ? Is it to provide the ability for the source or circuit to be able to provide high current amount ?
The buffer I was reading was the Op Amp unity gain amplifier.
Answer: When op-amps are used as unit gain amplifiers, they are often used to convert a high-impedance input (which doesn't have much drive) into a low impedance output which has more driven to interface with the next stage of the circuit.
The op-amp has such a high-impedance that it essentially draws no current from the input stage.
Comment: Thanks, to make sure I am understanding this correctly, by combining all the answers , it seems that due to the High Input impedance(of Opamp), R2 in the voltage divider becomes very large and hence the source impedance doesn't impact the output readings . And combining your answer , it is used so as to eliminate the source impedance and provide a zero(almost) impedance signal(at the output of the op-amp). Correct ?
Comment: @DallasCarter Correct about the voltage divider, that's a very common use for it.  The output is not going be zero, but very close to it; see [this answer](http://electronics.stackexchange.com/a/73695/1322).
Answer: Consider the following

simulate this circuit – Schematic created using CircuitLab
You have some signal (V2) and you have some load (RL). V2 has a high output impedance, its 1Kohm. What would the voltage be on RL ? It would be 0.5Vin.
If you are measuring some sensor, say a temperature sensor that is part of a system that is meant to shut down when the temperature reaches 100C, and 1V repreesents 100C, you would NEVER shut down in time. Because the system would think that the temperature is only 50C (if in this scenario its a linear relationship).
As Rs decreases more the value of RL, then most of the voltage would be seen by the load.
Rs << RL. A voltage divider formula can prove this.
Obviously you can't just physically a sensors output impedance, so instead, you buffer it or convert its high impedance to a low one.
And that's what a unity gain op amp does.
Comment: So basically it would always be advised to buffer the output from the sensor ? Also , if you do put the buffer, which would be after the sensor, wouldn't the source impedance(sensor) still exist ? as 1V would still be given at the buffer I/P through the voltage divider ?
Comment: @DallasCarter The source impedance would still exist. But with an opamp used as a buffer, the input impedance is incredibly high. So regardless, RL (the input impedance of the opamp) will be much larger than the output impedance of a sensor.
Answer: By definition, a (voltage) buffer provides high input impedance and low output impedance. That means that if you place a buffer in line with a signal, it essentially isolates that signal from whatever is on the other side. You're also right in that it can provide a higher current to a load, power amplifiers do precisely that.
Here's an example of when I used a buffer in a circuit once. I was building a VU meter and used the MSGEQ7 graphic equalizer IC (datasheet: https://www.sparkfun.com/datasheets/Components/General/MSGEQ7.pdf). If you look at the schematic, the L and R channels get mixed after having a 22k resistor placed in series with them, but I designed my board to play music at the same time the chip was receiving these inputs. In order to isolate the channels from each other, I placed a unity-gain amplifier (buffer) between the audio and the inputs to each of the IC pins, which effectively isolated them from each other.

Need for Buffering Signals ?


How to clear part loaded operating system

Question: Using a Noobs SD card I started to load my Raspberry Pi B+ for the fist time with Raspbian OS. Unfortunately I pulled the power cable mid operation as it was not the OS I wanted.
How do I start again and wipe what is there as a result of my stupidity?
Comment: assuming that pulling the power didn't corrupt the SD card, you can probably just restart.
Answer: Not to worry, it's easily done and the Pi is a pretty flexible tool for unplugging (unlike most PCs).
Sorting this is as easy as wiping the SD card and restoring NOOBS onto the SD card, then try installing it again.
On a seperate Computer:

Stick the SD card into your computer of choice.
On Windows: Double click on Computer
Select the SD card.
On Mac OSX: Double click on the SD card that appears on your desktop
On Linux: (May vary depending on distribution and whether you are a text or graphical user). Navigate to your SD card using your File manager or the terminal. It should be under /home/[your name]/[Your SD Card].
All Platforms: Select everything on the SD card and do Shift+Delete.
Go back to https://www.raspberrypi.org/downloads/noobs/ to download a new copy of NOOBS (if you haven't already got it). Either version will work, but you'll need to keep your Pi connected to the internet for the whole network install as it won't tell you when it will/won't need it.
Extract the contents of the file it gives you onto the card.

Back on your Pi

Stick the SD card back into the slot and stick the power back in.
Follow the instructions to install your OS of choice.
Be happy :)

If you want a bit of help with NOOBs, don't forget the guide on the Raspberry Pi website is always a great tool.
Good luck with your endevours!
Comment: Thank you Mike you make it sound easy , then I have a chance of getting it right.R
Comment: You're most welcome :). Good luck and let us know how you get on.


A magnets north pole and south pole

Question: Why does the N pole of a magnet do not attract with the same pole but the N pole of a magnet attracts with S pole?
Comment: Because that's the way it is. Also read http://physics.stackexchange.com/questions/90164/physics-the-why-vs-how-question
Comment: Also see what Richard Feynman had to say about a very similar question: https://www.youtube.com/watch?v=KLoNA9lMb6A
Answer: Suppose we start by considering electric charges. We all know that like electrical charges repel and unlike charges attract. The situation with magnets is more complicated because there are no such things as magnetic charges. The bar magnets that we are all familiar with are magnetic dipoles not magnetic charges.
If we switch back to electricity for a moment, electrical dipoles also exist and they behave like a pair of charges very close together:

So the electric dipole has a positive end and a negative end. These are analogous to the north pole and south pole of a magnetic dipole. With this in mind it shoukld be obvious that two electric dipoles will attract if their unlike ends are pushed together and repel if their unlike ends are pushed together:

And the same argument applies to magnetic dipoles. Although magnetic charges don't exist a magnetic dipole behaves as if it has a north magnetic charge at one end and a south magnetic charge at the other. So they attract and repel in the same way that electric dipoles do.
Answer: Various ways to think about it. Here's one;
The energy of the overall system is lower if the poles are closer.
This creates a potential gradient (a hill), in which

external work must done, energy put in to separate the poles, but
energy is released if the poles are brought closer. Release energy is kinetic

Imagining an infinitesimal variation in relative position, any released kinetic energy in the direction converging the poles produces a positive feedback, i.e. more energy is released. Attraction is consequently observed.
Comment: Of course one question leads to another; in this example it's why is the potential energy sloped that way -
ultimately there must be some underlying assumptions for any model, but these should be simplified and minimised. That's hopefully a bit more helpful than just saying "because it is".

A magnets north pole and south pole


How do I tell vim that some file extensions are synonymous?

Question: I edit C++ code in .cpp files, while template code goes into .tcc files. When I open such a file, no syntax highlighting is available. I tried set syntax=cpp in a a tpp.vim file in ~/.vim/ftplugin/, but it does not do anything, while in the editor it works. Is there some way to have vim treat specific extensions as synonymous with others?
Comment: Very useful question! Thanks!!
Answer: The preferred approach is to create a ~/.vim/filetype.vim, as mentioned in :help 43.2 and explained in Vim FAQ 26.8:

A better alternative to the above approach is to create a filetype.vim
  file in the ~/.vim directory (or in one of the directories specified
  in the 'runtimepath' option) and add the following lines:
" my filetype file
if exists("did_load_filetypes")
    finish
endif
augroup filetypedetect
    au! BufRead,BufNewFile *.x       setfiletype c
augroup END

Edit:
The did_load_filetypes shouldn't cause you any problems; it would only require a Vim restart after it is changed.
It is necessary in order to avoid multiple and unnecessary loading of this file, and it is inherited from the base file ($VIMRUNTIME/filetype.vim):
" Vim support file to detect file types
"
" Maintainer:   Bram Moolenaar <email@example.com>
" Last Change:  2014 Jun 12

" Listen very carefully, I will say this only once
if exists("did_load_filetypes")
  finish
endif
let did_load_filetypes = 1

For more information check :help new-filetype.
Comment: Why the `if` statement? If I leave it in, the autocmds are not executed for me.
Comment: @oarfish even if you restart Vim?
Comment: Ok, this is weird. I have to `au!` statements in the group, namely `au! BufRead,BufNewFile *.py        setfiletype python` and in the line below
`au! BufRead,BufNewFile *.tpp       setfiletype cpp`. But only the second one actually works. Editing `.py` files does not set the file type to `python`. I'm not exactly a vim-guru so I'm probably missing something.
Comment: @oarfish if you check the file I mentioned on the edit you will find that it already contains a line with `au BufNewFile,BufRead *.py,*.pyw  setf python`, so your first line is irrelevant. Your problem is probably elsewhere.
Comment: `"Editing .py files does not set the file type to python"` - what is the output of `:set ft` on that case? If it is not python, try following the procedure in [Vim-FAQ 2.5 - I have a "xyz" (some) problem with Vim](http://vimhelp.appspot.com/vim_faq.txt.html#faq-2.5)
Comment: Found the error! From some ancient time of vim noobism I head a `~/.vim/ftdetect/py.vim` file which set the filetype incorrectly. Removed it and it works.
Comment: Great solution! Perfect!! Thanks a lot! +1
Answer: You could do (in your global .vimrc):
autocmd BufEnter *.tpp :setlocal filetype=cpp

Comment: If you have multiple files you want to associate with a language, delimit the patterns with commas and no spaces between the commas: `autocmd BufEnter *.tsx,*.jsx,*.ts :setlocal filetype=javascript`
Comment: `autocmd BufEnter *.cuf :setlocal filetype=fortran` to treat CUDA Fortran files as Fortran
Comment: Upvoted this answer because it works directly from the .vimrc file.
Answer: One could also have: au BufRead,BufNewFile *.tpp setlocal filetype=cpp in their .vimrc such that upon opening a file with .tpp extension, filetype is set for C++
Comment: This seems to be basically identical to @edi9999's answer. Can you elaborate on why to use `BufRead,BufNewFile` instead of `BufEnter`?
Comment: @oarfish It doesn't matter too much I think, but I prefer `BufRead,BufNewFile` ahead of `BufEnter` because in the docs (`:help BufEnter`) it states:
"BufEnter:  .... Also executed when starting to edit a buffer, **after** the BufReadPost autocommands." where _BufReadPost_ is synonymous with _BufRead_.
In addition, I do not believe the `:` is required before `setlocal` so I also omitted that in my answer. (I might have interpreted this completely wrong, happy to hear further discussion)


Is it safe to store (hashed) passwords in a cookie?

Question: I've read some articles and questions on SO (e.g. here) that say you shouldn't store a user's password in a cookie. If the password is salted and hashed, why is this insecure?
In particular, why is it less secure than using sessions, the alternative usually suggested? If the user wants to stay logged in then surely this new cookie (with a session ID/hash) is exactly as secure as the one with the user's password? If the cookie is "stolen" on some way the attacker can log in as the user in the same way.
EDIT: The main crux of the question is the part about the user staying logged in, i.e. via a "Remember Me?" checkbox. In this case, surely there is only ever one session?
Answer: By putting the hashed password + salt in the cookie, you:

Open up an unlimited bruteforce vector
Allow the cookie to be copied and used by anyone (it always lets access; whereas a session does so for a period of time).
Make it harder to change hashing schemes, if it becomes relevant

Further, you generally need to store something else, to identify the user (like their user id, so you can look up their password and match it). This may lead other obscure problems.
So you're best just going with the session id approach.
Comment: I see your points. However, isn't there a brute force problem with sessions? If you have millions of active users (e.g. Facebook) then you could try random hashes until you get one that works? I suppose they're tied to IP addresses?
Answer: Among other distinctions, if a session is stored, you own this one session. If a pwd is taken, you own every session of that user from now on.
Comment: But surely that's irrelevant if the user wants to stay logged in permanently? There is, and will only ever be, one session.
Comment: Not necessarily. That's certainly the easiest way to implement it, but not the only one. For instance, you could have a rotating session key. If someone stole one, they might get the next one too, but that'd kick the original user out who would then relog in and your attacker is again left in the cold.
Answer: Sessions are usually keyed to IP addresses at some level somewhat preventing session theft.
Beyond that, the session ID doesn't contain any personal information; your password, even salted and hashed does. Passwords, salted and hashed as they may be, can be reused; session ID's can't. Once the session is over, it's over, you need a new session ID to be able to impersonate the user again.
Answer: How about letting someone else deal with and think about all these issues for you?  That is, use OpenID, Windows Live ID, Facebook Connect, etc.


My Blender is running slow

Question: I have used blender for a while now and it worked perfectly, but when I opened it up today it's being very slow/laggy. When I look around in 3d view it takes forever to respond, and everything is either delayed or slow.
Answer: Mine does this sometimes when my graphics drivers are out of date. Unless you installed a new plugin or something new to it, I wouldn't blame Blender for this. It is probably most noticeable in Blender but I am willing to bet that your graphics card drivers need to be updated. If you have NVidia, they usually have an app installed on your machine that has an update driver button. The app is called "GeForce Experience". http://www.geforce.com/geforce-experience/download
Answer: As William suggests, updating the drivers for the video card could help, but there might be other causes, too. Running slow could also mean that you have other issues. For example, if you run an application which does not manage memory correctly, you might lose a small amount (say a couple of megabytes) of available memory at a time, and eventually, there is not enough memory to for Blender to work properly.
Another situation in which this might occur is if the Blender file you're trying to open is too large. One reason this might happen is if you have multiple images packed in the file.
Another possible cause is if you don't have enough memory because of other processes running in the background. I've experienced all of these situations, myself.
For a better answer, it would be helpful to have more details about your system. What version of Blender are you using? What are the specifications of your hardware—what kind of processor, what clock speed, how much RAM, what kind of video card, how much video card memory, what operating system?
Also, what other software might be running at the time? I sometimes have multiple instances of Blender running, when, for example, I am linking or appending contents from one file into another. And instead of a calculator widget, I often make use of a spreadsheet for calculations. If there are multiple programs running, closing some of them can improve performance. Sometimes, when starting a new project I'll have a couple of instances of Blender running, and also have a browser window open to the internet so I can search for reference material on line, and I'll have one or more images open in GIMP.
Finally, I sometimes leave my computer powered on for days or weeks at a time, especially if I'm working on a large project. The longer the computer has been continuously on, the slower Blender (and other programs) get, and I find the speed improves by orders of magnitude, not only in Blender, but in other applications as well, by the simple expedient of saving all my work in progress, shutting down the computer, and rebooting it.

My Blender is running slow


How can I close an Epic with open defects?

Question: I have an Epic. Some defects are closed, but one defect is still opened and deferred until the next release. How can I close the current Epic since one ticket is still not closed?
Comment: Are you asking about a specific tool?
Comment: You haven't defined a tool. Each tool handles this differently. Without naming the tool, you'll get general best-practices answers, which may or may not help if your question is tool-specific. That said, JIRA is a ticketing system that links user stories to epics, and whether or not you're using JIRA, index cards, or some other tool, the "Scrum way" to address the situation is largely the same.
Answer: TL;DR
If the Scrum Team's Definition of Done for the epic requires this defect to be fixed before the epic can be considered fully complete, then the epic isn't really done until the defect is fixed. In that case, it should remain open. However, if the defect isn't on the critical path for your epic, you can work with the Product Owner to reassign the defect to a different epic, issue, story, bug, or chore (or whatever your particular tool allows you to call this particular unit of work) so that it no longer blocks your epic's status change.
You should never do this simply to close out an incomplete work item. You should only do this if:

the defect is deemed non-essential to the successful delivery of a working increment within the current iteration; or
the defect isn't really part of the Definition of Done for the current iteration, and deferring it won't endanger the current Sprint Goal.

Outside of those two key constraints, work items can be freely moved in or out of scope with the collaboration and support of the Product Owner.
Epics Aren't Required by Scrum; When Used, They Can Span Multiple Sprints
Regardless of your tool, it's important to understand that epics are a concept from "user stories," which are a way of representing and communicating about work contextually rather than as a set of detailed, fixed specifications. While user stories and epics are commonly used in agile frameworks, they are not a formal requirement of Scrum. That said, in a Scrum sense epics are a related list of Product Backlog items that may take multiple Sprints or even releases to completely deliver the features or full functionality they are meant to represent. A user story (or more formally, a Product Backlog item tied to the Sprint Goal that has been accepted for Sprint Planning) must fit within a single Sprint, but a theme or epic don't usually have the same rigor (such as INVEST) applied to them.
Your team may be conflating epics with features. Sometimes this is driven by tools, as ticketing systems like JIRA are (by definition) ticketing systems and not inherently designed for Scrum Product or Sprint Backlogs. Whether or not this is the case for you, it's worth asking the following question:

If you ship the feature(s) defined as an "epic" without fixing the known defect, have you met the Definition of Done for the epic?

If the answer is "yes," then the defect can simply be divorced from the current epic and treated as future work tied to a different epic, feature, or Sprint Goal. If the answer is "no," then the epic, feature, or story is simply not yet done, and should not be closed out until it satisfies the Definition of Done. In Scrum, as well as in other agile frameworks, work is either done or not-done; you can't treat unfinished work as "done."
On the other hand, the Developers or the Product Owner are certainly free to de-scope something that is not material to the current Sprint Goal, and the Product Owner can de-scope work that isn't needed for the current Product Goal. Whether that means deferring the work item for a future Sprint or removing it from the Product Backlog altogether is a decision for the Scrum Team to discuss together, although only the Product Owner can actually remove items from the Product Backlog.
Answer: What is an Epic to you, your team, and your organization?
Since there is no universally (or even widely) accepted definition of what an Epic is, there's no universally or widely accepted way for what it means for an Epic to be open or closed.
To some organizations, an Epic is a transient artifact. It's large and nebulous. Eventually, it will be broken down into smaller Stories that are valuable and the Epic will go away.
To other organizations, an Epic is a container for work. It provides a higher-level view over smaller work that a team (or teams) take on. In this case, an Epic is opened when the first unit of work under it is started and is closed after the last unit of work is finished. In this model, you may freely move work into and out of the Epic as you determine what is truly necessary to satisfy the stakeholders. An Epic may even spawn additional Epics of work for later on.
In the model for containers for work, there are even variations within that model. In some models, everything in an Epic is expected to be completed within an iteration. In other models, the work is expected to be bundled in a single release. In others, it's just a visualization and the work may be released at one or multiple points in time.
You need to understand who uses Epics and what they mean to various stakeholders to best understand how to use them to track and visualize the related work.
Answer: I know its tempting to close the Epic for the sake of momentum but I would suggest your backlog should reflect reality and the Epic not be closed until it is actually done.
Having open Epics with defects is also a good reminder when you create your release notes so you can easily find which defects are outstanding.


What does this wavy line across a barline mean?

Question: My daughter is playing Ocarina of Time, in an easy piano format.
The following sign appears at the end of the song:

Usually the song tells her to repeat and fade out but I have never seen this symbol before.  What does it mean?
Answer: It's not a standard musical notation.   If this is a piece that DOES end with a 'repeat and fade', I can accept it as meaning that.


replace favicon in mvc4

Question: I have provided <link rel="shortcut icon" href="~/Images/logo.gif" /> in my layout but i wish to replace the favicon on all the pages, even where layout is not used.
I replaced the favicon file with my favicon (present in the root folder) still it loads the default favicon on pages without layout.
What more needs to be changed in order to get my favicon.
a similar but slightly different question has already been asked but isnt answered by anyone.link
Answer:
It's better to convert your .gif to .ico, you can do this online
It's a convention to rename the icon to 'favicon.ico'

You can find more information on Wikipedia - look at the section 'How to use'
Your html should look like:

<link rel="shortcut icon" type="image/ico" href="~/Images/favicon.ico">

Check Mark Gravell's solution here
Comment: <Link is added to the _Layout file, in the head section.
Comment: if you use razor syntax you should take care of defining the href part with href="@Url.Content("~/favicon.ico")"
Comment: u can use gif image in mvc. it works in the pages using layout . it is rendered as ico only. i had placed favicon.ico in the root folder. it got solved just happend to be an issue with the server caching
Comment: favion.ico was in the cache i think, i cleared it and it took the new icon.

replace favicon in mvc4


Built-in Sound Recorder app | Nokia G50

Question: I have Nokia G50, running Android 13
I can not find any built-in sound recorder app. I am not referring to call recording app but one that can record audio that doesn't originate from the device.
Does anyone have an idea of such app comes with this model?
Is it an app that is provided by the OS or the device manufacturer?
Answer: There is a related article on Nokia Support - How can I record a voice clip on my Nokia smartphone?

You can use the pre-installed Keep app to record a voice clip. Alternatively, you can search for third-party apps in the Play Store.

Nokia relies on Google Keep for voice recording, but it doesn't have an independent sound recorder app.
While the AOSP has SoundRecorder app, the manufacturer can and do customize their Android ROMs, including removing AOSP apps.

Built-in Sound Recorder app | Nokia G50


Why does accelerating electron emits photons?

Question: I have read that accelerating or oscillating electron emits photons. But why and how does it so? And why only photons? There are other bosons like gluons, W and Z bosons, so why does electron emit only photons? And what is the mechanism ?
Comment: Given enough energy electron collisions will "emit" all of these particles, that's why we built LEP, the predecessor machine of the LHC accelerator, which was an electron-positron accelerator. "Why?" is not a good science question, but "How?" is basically what the standard model of particle physics describes. The details, however, deserve the phrase "It's complicated...".
Comment: Not just electrons, any _electrically charged_ particle will do this under _acceleration_.
Comment: What is your level of physics education. We need to know so we go in-depth but are still understandable.
Comment: Well i am doing mechanical engineering. But i have studied classical mechanics,  special theory of relativity, and introduction to quantum mechanics and particle physics. I know some of the concepts. But u can give the best explanation. I always manage to get the answers.
Comment: @XerenNarcy Uncarged particles will radiate under any acceleration too
Comment: possible duplicate of [How EM waves are produced by accelerating charged particles?](http://physics.stackexchange.com/q/46946/)
Answer: This might not be quite the answer you are looking for, but one useful way to think about it is: the accelerating electron emits photons because nothing forbids it from doing so.
By definition, because an electron has electric charge it is coupled to the electromagnetic field, and is able to produce excitations in this field which we can call photons. This is literally what it means to have electric charge, so there is no need for a 'mechanism' beyond that. The electron also has weak force charge (isospin), but no strong force charge, so right away we can tell that it cannot directly emit gluons simply because it is not coupled to this field (By the way, don't ask me why an electron has some kinds of charge and not others).
From this, the possible emission channels for an electron are those of the electromagnetic and weak forces: photons, and W and Z bosons. However, W bosons can be ruled out immediately: they have electric charge, so if the electron gave them off it would violate conservation of electric charge. But photons and Z bosons are both neutral, so those are okay.
What else is forbidden? Well, an electron that is at rest cannot give anything off, because that would be creating energy. But the principle of Lorentz invariance tells us that if an electron at rest cannot emit anything, then it also cannot emit anything when it is moving at a constant velocity, because one can always choose a reference frame moving at a different (constant) velocity without changing an observable like whether something was emitted.
Nothing so far has forbidden an electron that is accelerating from emitting a photon or a Z boson, so we should expect that both of these can happen. However, because a Z boson is quite heavy, to emit these without violating energy conservation the electron must be accelerating to a degree that is only possible in a particle accelerator (or similar natural process). Photons can be as low energy as you like since they are massless, so at the energy scales we are used to only emission of photons is possible.
This process of deciding if something is possible by elimination might seem odd, but it's actually an extremely useful perspective in particle physics. Indeed, it has its own Wikipedia page, and an associated saying popularized by Nobel laureate Murray Gell-Mann: "Everything not forbidden is compulsory."
Comment: A lot of things in particle physics can be summed up as "If nothing forbids it from happening, it can happen"
Comment: I would welcome any elaboration from the downvoter...
Answer: From a classical point of view, if you look at the field lines created by charge at rest or at constant velocity you'll see straignt lines. Now if the charge changes its speed (i.e accelerating), a "ripple" will appear and propagate along those field lines. Check the animation on this page :http://www.tapir.caltech.edu/~teviet/Waves/empulse.html. You'll notice that the lines remains straight "before" and "after" the little acceleration that the charge made, but in-between there's a variation that is percieved as an electromagnetic wave, and so a photon in a quantum point of view
Comment: This is a nice complement to my "quantum-y" answer. One can gain a lot of intuition for quantum theories by being able to convert between these two perspectives.
Comment: That's a very beautiful explanation.
Answer: The why is due to the principle of conservation of energy. When the field that is responsible for the acceleration is electric--the gravitational force is several orders of magnitude weaker than the electric force, and electrons are not nucleons--work is done by an electric field, whose quanta is your favorite boson--you guessed it--the photon!
The mechanism is called Brehmsstrahlung, which loosely translates to "braking radiation" in German. It is defined as the process of an electron undergoing an acceleration--negative values corresponding to decceleration--which causes a change in energy. The emmited photon, due to conservation of energy, has the energy as calculated from work.
The Wikipedia page has a very detailed and simple explanation of the process: Bremsstrahlung.
Answer: To get accelerated, any body as well as any particle needs energy to be applied to this body or particle. This happens through photons or more common through electromagnetic waves. This is obvious in particle accelerators and it happens in every acceleration too.
Throwing a ball with your hand you do not touch the ball in the meaning, that the molecules of your hand touch the balls molecules. The outer electrons of both bodies prevent this and this electrons intermediate through photons. At the end some energy is over given to the accelerated body.
During the time, the moving body is not influenced by any force, it moves straight and does not loose more energy as it get from the  surrounding world. This it called thermal radiation and it happens anytime.
When one try the slow down or to move a particle in circle or when ever the particle loose energy, when happens what you asked, it radiates photons. This is the inverse process to acceleration.
Gluons and W and Z bosons are intermediate processes inside the nucleons and are not observable in our dayly live.


Look (at) you in the eye

Question: On several occasions I read or heard expressions like "look you in the eye" (without "at" between the verb and the pronoun). This struck me as incorrect. Yet the same people who use this construct always use "look at" in most other contexts. So perhaps is it just colloquial and not used in formal discourse by convention ? Can the experts clarify ?
Comment: It is correct.  It means to gaze directly and frankly into the other person's eyes.  It does not mean to observe (as *look at* does) but to engage in a form of visual communication. It is often used figuratively. "Can you look me in the eye and say that?" when no actual looking is involved. In other words, Can you say that without averting your eyes, you shifty  b_st_rd.  It follows the same structural pattern as "look a horse in the mouth."
Comment: As a non-native speaker, if a sentence is constructed within the range in which native speakers understand what it means, we could regard the sentence as a grammatically correct sentence, even though it does not follow the normal rules of grammar. And let's think in this way if the sentence were written just as 'Look you', then, we could not define 'Look you' and know what it would mean, because without adding 'in the eye' that can have a few of different meanings, but just by adding 'in the eye' to it, the ambiguousness can be resolved and the meaning can be fixed in one way.
Comment: So, the sentence is understandable and acceptable as having a distinct meaning.
Answer: I think it's basically an idiom that does not follow the normal rules of grammar.
When "look" -- and "stare" -- are used in a phrase with modifiers, we generally drop the "at". "Look him in the eye", "stare him down", "look it over", etc.
I suppose this might be considered somewhat informal, but I wouldn't be surprised to see it in academic writing, etc.
Comment: I agree that it's an idiom, but would you really consider it "non-standard"? IMHO, I think it's pretty standard (and if someone said "look at him/her/me in the eye", I'd feel that that someone's usage is non-standard): **[look someone in the eye/face](http://www.macmillandictionary.com/dictionary/american/look-someone-in-the-eye-face)**: to look at someone when you are talking to them, especially to show that what you are saying is true.
Comment: @EwanDelanoy Informal and non-standard are not the same, I believe.
Comment: @DamkerngT. the dictionary you quote classifies it as "informal" though
Comment: @DamkerngT. Yes, poor choice of words, I just edited. By "non-standard" I meant "doesn't follow the normal rules of grammar", not "is rarely used" or "is limited to speakers of a regional dialect".

Look (at) you in the eye


Do the three Gunas apply only in the materialistic plane?

Question: In other words - Does Brahman and humans who have achieved enlightenment have these Gunas?
Comment: Yes, the 3 gunas only apply in the realm of Prakriti. In Vaikunta, Vishnu and the liberated Jivas have a *Shuddha Sattva* body.
Comment: From advaita viewpoint, only nirguna brahman is free from gunas. Saguna brahman has rajas as guna when (s)he creates, sattva as guna when (s)he preserves and tamas as guna when (s)he destroys.
Answer: I am answering from the perspective of Advaita Vedanta.
In Advaita Vedanta, AgyAna or ignorance is considered as endowed with the 3 Gunas - TrigunAtmikA. Other terminologies used for AgyAna are AvidyA and MAyA.
See the following flow charts, which I took from the concluding section of the book VedAnta SArah - Essence of VedAnta (Sri Sadananda Yogindra Sarswati; with commentary by Swami Amritananda):

The first flow chart is useful for our purpose. The characters are in Bengali and hence I am translating into English.
Definition of AgyAna (AvidyA/MAyA):
Four facets of AgyAna:

anirvachaniya

That means one can not explain what it is. It is neither Sat (true) nor Asat (false) and it is not even a mixture of both.

TrigunAtmikA

It is the embodiment of the 3 Gunas viz: Sattva, Rajah and Tamah.

GyAna Virodhi

It opposes GyAna or knowledge and it is destroyed by knowledge too.

BhAvarupa

That means it can be felt as true or can be experienced and hence not unreal.
The 2nd chart gives the power  of AgyAna viz: Avarana and Vikshepa but discussing those are irrelevant here.
So, in short, the 3 Gunas are part of AvidyA, which when destroyed, by knowledge, liberation is achieved. Therefore the Gunas exist as long as ignorance exists but not after that.
So, neither Brahman nor realized persons are affected by the Gunas.


The effective power bug made my iMessage not work

Question: I gave a "friend" my number and he sent the effective power code and now my iMessage doesn't work anymore I click on the app for iMessage and the app crashes
Comment: It doesn't work I tryed still can't go on to iMessage
Answer: Here's a few things to do for a fix:
If you receive one of these messages, there are a few possible fixes that have worked for us and for other people who have encountered the bug. If the Messages app was opened to the conversation with the person who sent the offending message, the Messages app can be reopened to this conversation. Sending a reply message fixes the problem.
If Messages was opened to the conversation list view, the app will crash when you attempt to open it. You can fix this by having someone send you a message or by sending a message to yourself. There are several options for sending a message to yourself, including sending yourself a message via Siri or through the Share sheet in any app.
To send yourself a message in Siri, tell Siri to "Send a message to myself." Siri will open up a dialogue where you can give her a quick message like "Fix" that'll be sent to your iPhone to clear away the malicious message.
Alternatively, you can open an app like Notes, craft a quick note, and use the Share option (the little document with an arrow) to message it to yourself. Sending yourself something though the share sheet of an app opens a new messages window where you can enter your own contact information.
Source: Macrumors
Comment: Why was this downvoted? I'll remove if it's an incorrect answer, but AFAIK these are the three known methods to fix this issue.


Linux username@server disappears after login as root

Question: On my linux server I signed in as the root user to access some files: su root...  After checking the files I went to sign back in as my main user su username.  In my terminal window it now says server ~: instead of username@server ~: to the left of the command line.  Is there any way I can get this back to normal?
Answer: Do a few exit commands.  Your su root started a subshell where you're root, and your su user started another.  Typing exit will end those subshells and bring you back where you started -- one level deep.
Comment: With "echo $SHLVL" you can see the subshell level.
Comment: It just exited me out of my remote connection... but I do think that I'm in a subshell.  Is there any command for me to view all the subshells I have running?
Answer: You just need to change the prompt. You can use the following command:

export PS1="\u@\h \w: "

I personally prefer the following as my prompt, but that's just me:

export PS1="[\u@\h \w]\$ "

You can prevent your prompt from changing by adding the statement to the relevant bashrc files. See also Bash Shell PS1: 10 Examples to Make Your Linux Prompt like Angelina Jolie for more info about PS1.
Comment: While this answer is correct about how to change bash prompts, it misses the point that to "switch back" to your own user after a `su`, you typically simply want to `exit` the `su`-shell, not start another one.
Comment: Why would changing users change my prompt?  This seems like more of a workaround than a fix.
Comment: Because the prompt will indicate as WHAT user you're currently operating, e.g. the prompt would look like root@myserver: when you're operating as the root user or someuser@myserver: when you're operating under the credentials of someuser.
Comment: David: Prompts are not global; They are user-specific. Therefore, changing users can change your prompt. In particular, the root user oftentimes has a very bare-bones generic prompt. What Michael suggested here is exactly the correct way to address your problem; It's not at all a workaround.
Comment: Since they are user-specific, why would changing to the root user change the prompt for the user "david."  I understand that when I am in the sub-shell for the root user that I would have a different prompt, but as soon as I exit out of that subshell I don't understand why it would change the prompt for the user david.  Didn't you just say that they are independent of each other?  I'm definitely a noob at linux, and I would appreciate it if someone could explain the error in my logic here.
Comment: 1) Prompt is governed by ENV variables (as above). 2) `su` starts a subshell and erases its parent ENV variables. 3) the child subshell inherits ENV varibles from the initialisation script for the user the shell as running as ( david ).
Answer: su switches users by spawning a new (sub)shell. So when you logged in, you started a shell. With the first su to root, you started a subshell as root. The next su to your user started yet another subshell -- note that at this moment you have three shells running. So instead of su-ing to your user from the rootshell, exit-ing back to your login shell will solve your problems.
If you have pstree installed, pstree -h will show you a graphical representation of what's going on.
Comment: That reads: -sshd---sshd---sshd---bash---pstree.   When I enter the root sub-shell I get: -sshd---sshd---sshd---bash---su---bash---pstree.  Since there is only one bash in the first reading would I be correct in assuming that I'm not in a sub-shell?
Comment: Exactly. So to get out of any unnecessary subshell, just `exit` until there's only one `bash` left.
Answer: Your prompt appears to be normally set by one of your shell startup files that is not being run when you su from root to your user. If you're using bash, man bash in the INVOCATION section might be enlightening. There are several startup files that bash might read, including but not limited to:

/etc/profile
~/.bash_profile
~/.bash_login
~/.profile
~/.bashrc

The PS1 environment variable is just an environment variable, and may or may not get set depending on which combination of the above files is run.
The su command has a -l (or --login) switch which may be abbreviated to just - that makes the new shell a login shell. This is probably what you want:
su - david

I always use su with this option.
Comment: That's correct. A plain su would not invoke the startup files whereas su - would.
I didn't know su - is short for su -l ! Thanks Greg for the info.
Comment: also, for 'sudo' people, "sudo -i" is *immensely* handy
Answer: if this is a bash shell, try:
source ~/.bashrc

if that doesnt work run echo $PS1 and see what has changed.
Comment: . ~/.bashrc does the same thing
Answer: su by itself does not create a new session of the shell. that's why it doesn't set up the shell as it normally does. however,
su -

will start a new login session


Is it possible to exclude an app from the Screen time activity?

Question: My statistics are skewed because of navigation applications (Maps/Waze), and I wish they were more accurate. Is it possible to exclude an application from Screen Time?
I tried to add Maps in "Always Allowed" but it still counts when I'm on the app.
Answer: Always Allowed doesn't exclude an app from Screen Time, all it means is the app isn't subject to any restrictions.
At the moment there is no way to disable a specific app from Screen Time, you can turn it off if it really annoys you or when Screen Time is displayed deduct the time from the app from the total time if you want to exclude the app manually.
Comment: At this time there is a range of settings accessed by "Always allowed". Like if you've set limit such that apps cannot be used during night time, you could allow some apps (by the app, not by category) to always be allowed.