How to extend (not mirror) a computer screen to a Chromecast?

Question: For about two weeks or more between May and June 2020, I was able to cast my entire desktop screen to my Chromecast device and get the ability to extend my desktop (not duplicate it) to the TV where I've got my Chromecast device installed. I thought, finally the Google developers have added this much requested feature to the Google Chrome browser's cast functionality! This was extremely useful to me, because I effectively had two separate screens to arrange my windows on--double the video real estate. Although the mouse moved somewhat slowly on the second extended Chromecast screen (the one on the right in the screenshot below), the audio and video on the TV were well synchronized. Somehow, the feature no longer appears for me this week when I go to cast my desktop from the Chrome browser. How can I get this functionality back?
Answer: Yes the procedure described in the article is not working on recent Windows 10 version. The trick is to make Windows believe that an external screen is connected.
I confirm that the BelegNeurion solution works nicely! This is my favorite solution (neat, no hardware required) but it requires device driver installation and the screen resolution is fixed to 1920x1080. It's a "geek" solution.
Another solution is to connect a dummy connector to the hdmi port of the PC (Amazon has plenty of them). Resolution can be set up to 4K.
An HDMI to VGA connector also works, in this case the resolution can be change up to 1920x1080.
And finally Dummy connector for VGA and USB C also exists.
Note: I also tried SpaceDesk: it works, but is much slower than chromecast (not suitable for video) and the sound is not transmitted to the TV.
Answer: than If you have an Android-based TV - try to use Spacedesk app:
https://spacedesk.net/
Server part app - on PC, client|viewer - on TV.
Technically, it works better than Chromecast and has more features.
Comment: I's not an Android-based TV. It's a regular TV with a Chromecast device.
Comment: Thanks I did not know about that!
Answer: I found the solution!

CAVEAT: If simply trying the steps below won't pull up the second
screen in step 6, then you must convince Google Chrome that there is a second
screen. Here's where a dummy HDMI plug, "headless ghost", or other
display emulator comes in handy if you do not have an actual second
screen.
NOTE: it seems a few PCs actually behave as though a monitor is connected, and won't need a dummy HDMI plug. I'm not quite sure if this
has to do with these ports behaving as analog ports. A machine that I've found does not need an HDMI dummy plug is the HP EliteBook 8470p.
Some examples of dummy HDMI plugs are:

Sample HDMI Dummy Plug from Amazon
Sample HDMI Monitor Emulator from Walmart

To convince Google Chrome that there is a second screen,
A.    Plug in either a real HDMI cable or a dummy plug into the
computer's HDMI port. If using a real HDMI cable, you must plug the
other end into an actual monitor or TV.
B.    Follow steps 1 – 2 below.
C.    Confirm that you have a visible extended desktop on your second
screen by switching its input source to that second HDMI port.
D.    Perform steps 3 - 5. Note that step 6 won't be possible at this
stage as your screen will appear on the Chromecast, just not extended
as a second separate display.
E.    Repeat steps 3 – 5 again, just to stop the Chromecast.
F.    Repeat steps 3 -5 a third time and voila! Step 6 is now possible.

Assuming your computer is convinced that it has a second screen attached, then to extend your desktop with Google Chromecast,

Make sure to hold down the Windows button on your keyboard and press P.
Then select Extend from the right flyout menu that should appear (this is what will open up the second monitor as an option for you to extend your desktop in the next few steps!)

After that, proceed to your Google Chrome browser on your PC and click the three-dot menu at top right and select Cast...
A popup menu will appear, but don't select your Chromecast device just yet! Instead, at this point you will choose what source content to cast. Hover over to the bottom of the popup menu and click the Sources button, and from its drop-down list select "Cast desktop" so it becomes checked.

Now you can click in the popup menu on the name of the Chromecast device to which you want to cast your screen. IMPORTANT: avoid switching steps 4 and 5 around because you can inadvertently cast the wrong source content to your device!
You will then be presented with a choice of Screen 1 or Screen 2 which you will cast to the Chromecast device, so choose Screen 2 here and click Share. That's it!

NOTE - You can also make the Cast button remain on the Chrome browser toolbar always for easier access (otherwise it will disappear later and you'll have to find the feature again hidden under the three-dot menu.) To keep the Cast button always on the toolbar, right-click the Cast button on your toolbar after you've already begun casting per the above steps. The choose, "Always show icon".
Some caveats about extending your desktop via Chromecast, from my experience:

You may experience a slight video lag on the extended monitor; I say it is negligible considering the advantages you gain by this feature (you just shouldn't expect high performance screen refresh rates on it!)
At times, when you are not using (casting to) the second monitor, certain popup windows may try to appear on that side and leave you clueless unless you either re-display the extended desktop or use the Windows + P keyboard combination and switch to "PC screen only".
Comment: I found a solution for Windows 10. The trick is to add a virtual display and then use Chromecast to stream to that display. To create the display, install this driver: https://www.amyuni.com/forum/viewtopic.php?t=3030. You will then see another display in Display settings. Set that to `extend`. Now you can stream to that display via Chrome.
Comment: This can only ever work if your PC is, for whatever reason, detecting a second display, I think?
Comment: This is no longer working on Windows 10 2004
Comment: My answer above is indeed still working for me on Windows 10 Version 10.0.19041 Build 19041 with plain old Google Chrome Version 87.0.4280.88 (Official Build, 64-bit). I would stay away from downloading third-party software drivers to make your Chromecast work--especially from unknown sources like the proposed amyuni.com.
Comment: This doesnt seem to be working for me. No matter how many times I do windows + p and then extend, when I click on my tv it automatically casts without asking me Screen 1 or Screen 2.
Comment: It is March 2, 2021, and this is still working for me. I am now on Google Chrome Version 88.0.4324.190 (Official Build) (64-bit), Windows 10 Version 10.0.19041 Build 19041. My laptop is an HP EliteBook 8470p. Frankly, I can attest that for some reason I can't get it to work on a Dell Latitude E7440 laptop I also have. It is running the exact same version of Chrome, but an older Windows 10 Version 10.0.18363 Build 18363. I'm not quite sure yet what the matter is. Another HP EliteBook 8440p laptop with that same older Windows build version also fails. Is it my newer Win 10 works better?
Comment: I'd like to follow up on my comment above (from March 2, 2001) about the Dell Latitude E7440 not Chromecasting an extended desktop: I finally got it to work using a dummy HDMI plug I bought from Amazon (https://www.amazon.com/gp/product/B07C4TWZRM/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1) and following my steps in the post above!
Answer: Windows 10 20H2 Build 19042.870
First off, using Chrome to Cast to your 2nd monitor/display via the extend is working fine! Disclaimer here is: Windows NEED to detect your 2nd monitor/display. If Windows do not detect yr 2nd monitor/display, Chrome Cast will not be able to cast from 2nd monitor/display.
What you can do is to purchase a DUMMY VGA/HDMI/USB C plug to fool Windows into thinking you hv a 2nd monitor/display connected. Then in Windows, EXTEND your display.
Start Chrome, click Cast, Cast Desktop, Click Chromecast Device to Cast, Select Which Screen to Share (remember to enable share audio)
That's all. This is as per @ShieldOfSalvation posting!
Comment: The "Share audio" checkbox is something new I just began to notice today! I don't believe I had seen that before. In fact, it's not in the screenshot that I posted earlier, titled "Share your entire screen". It's a new checkbox that appears at the bottom left of that dialog box.
Answer: Apologies for not finding this post in time, here is my video from 2016 https://youtu.be/rMPAlWMvfsY
Still works, just need a dongle.
In short - You can fool you need a Dongle from HDMI/DVI/DP/USBC to VGA, VGA is the key as it is an analog signal that the computer cant check for, so when you have a VGA dongle and hit detect displays, it will add a new display, without it being there, from there you can use that extra screen to be set to be cast to Chromecast, Teamviewer or any other share medium.
Comment: While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference.  Link-only answers can become invalid if the linked page changes. - [From Review](/review/late-answers/1154245)
Answer: I ran into the same scenario with my laptop...  I have seen some answers regarding the VGA monitor port, and forcing your OS to think that you have a second display, but I do not have a VGA monitor port (only an HDMI), so this was not an option.  If you are wanting to use your extended display just to monitor something in the background, but you are working on your main screen, I may have a good answer for you, without having to buy any additional equipment (dummy adaptors, etc...).  I didn't like the idea of having a dongle plugged into my laptop every time I wanted to extend the screen.
If you hold down your Windows key + Tab, you can create another desktop.  After you create a new desktop, open Chrome on the new desktop to whatever page you are wanting and cast it through Chrome.  Then, use your Windows + Tab function again and go back to your original desktop you were working on.  Now, you are casting the content from your other desktop, while independently working on your original desktop.  Though you are not truly "extending" your display, you can monitor content in the background on your 2nd display.
Hope this helps!!
Comment: I hadn't tried your Windows key + Tab idea before, but it does create a separate virtual desktop for me which is useful in its own right. As you said, however, this virtual desktop just doesn't extend my display onto a second monitor using the Chromecast. Bummer! Great idea, though, for someone that just needs the extra real estate but doesn't need to see both screens simultaneously.


Stability of Bohr Orbits

Question: In class we had been taught that Rutherford's model was unsuccessful because it failed to show that the orbits are stable because the electrons would lose energy because of electromagnetic radiation.

Bohr had suggested that electrons would move in orbits having well defined energy levels, but my question is that what would prevent these electrons from emitting radiation and collapsing into the nucleus.
Comment: I'm not sure I would say that Rutherford's model was unsuccessful at all. It addressed the question of where the protons were quite well - the demonstration that there is a nucleus is pretty important, and still quite useful. That did leave the question of the electrons. Given that we still do Rutherford scattering, I would say it was pretty successful.
Comment: I think you should do some reading first. I'm sure you'll be able to answer your own question. But to be honest, as far as I know, Bohr just said those orbits are stationary states and are stable. Full blown quantum theory addressed this question with complete rigour. Imo
Comment: Well, there's something "special" about those orbits. Quantisation is key. https://en.m.wikipedia.org/wiki/Bohr_model
Comment: ok, but what stops them from emitting radiation?
Comment: But, quantum theory came after Bohr's theory, why did the scientists accept Bohr's theory at that time?
Comment: Because they had nothing better. Why do we accept general relativity and quantum mechanics even though we know they are incompatible? They are still both useful theories.
Comment: Yeah exactly. For lack of a better option, and even though it was replaced by quantum mechanics, the Bohr model worked well for explaining the spectra of hydrogen. It's not a bad model..but it was replaced by something better later. That's how science works.
Comment: Could any of you guys also add the quantum mechanics explanantion..
Comment: @TanmayKulkarni That is like "please tell me all quantum mechanics and stuff" - a super unspecific and broad question.
Answer: Not every model is perfect. Rutherford's model suffered from the problem of electromagnetic radiation but answered important questions about the structure of atoms by showing the existence of a positively charged nucleus.
Bohr improved on this by realising that if electrons could only have certain energies then he could explain other things such as the spectra of hydrogen. As far as I am aware he did not attempt to explain why electrons are fixed in these orbits - this was only explained later by the advent of quantum mechanics. However, this does not mean that Bohr's model was useless since it accurately predicts useful things.
We do not have to know everything about why a model works in order for it to be useful. Einstein did not know why the speed of light was constant but he assumed that it was (based on all the available experimental evidence) and derived special relativity, which is an extremely useful theory. Nowadays we have quantum mechanics and general relativity which we know are incompatible but they are both exceptionally useful theories so we do not ignore them.
Answer: Few people continue to investigate this issue, but it was an active field of research in Bohr's days. Bohr himself basically sidelined the question with a postulate of stability, and quantum mechanics avoided it altogether by postulating that the classical laws of physics do not hold on the atomic scale.
In the course the 20th century, successive generations of scientists did manage to formulate a so-called nonradiation condition based purely on the mathematical application of Maxwell's laws. They found that, although an orbiting  point-charge must radiate, this is not necessarily the case for distributions of charge. Goedecke published on this subject in 1964 and ended with the suggestion that there might be "a theory of nature in which all stable particles (or aggregates) are merely nonradiating charge-current distributions whose mechanical properties are electromagnetic in origin."
Around the turn of the millennium, Mills brought this work to its full conclusion : if a point-charge in orbit around a nucleus must radiate, then the electron is not a point-charge. Instead, it is a sphere of distributed spinning charge and mass centered on the nucleus — a physical electron is like a soap bubble that encompasses the atom's core, in the world according to Mills.
Whilst this idea may seem outlandish to the current generation, it seems to be a useful extension of the Bohr model as it allows accurate calculation of a wide range of atomic and molecular phenomena, including such things as the ionization energy of $\ce{He}$ and other atoms.

G.H. Goedecke "Classically Radiationless Motions and Possible Implications for Quantum Theory" (Physical Review, Volume 135 number 1B, July 1964)
Randell L. Mills, "Classical Quantum Mechanics" Physics Essays, Volume 16: Pages 433-498, 2003
Answer: As you might have studied Bohr's model includes various orbits. An orbit is a Defined as a particular region in space where the electron moves and has constant energy.
A particle that has constant energy will not collapse in the nucleus. He even went forward to calculate these specific distances from the nucleus he came to the conclusion equation mvr=nh/2pi where the terms stand for their general meaning. By this equation we get to know about specific distances from the nucleus where the electron can be present without losing any energy.
See the thing is Bohr's model was only created for hydrogen, what you are actually seeing is the adaptations made to the Bohr's model that help us calculate values like velocities, energies, radii, etc.
The Logic behind the discarding of the Rutherford model was that the fact that classical electromagnetism states that ACCELERATING  charges product EM waves and hence an electron would lose energy.(Circular Motion is a motion where constant centripetal acceleration is needed hence the electron actually experiences the constant acceleration). This constant energy loss would collapse the structure proposed in almost 10^(-8) Seconds as you might have studied. But this does not happen.
Comment: The same flaw applies to the Bohr model as does the Rutherford model. Bohr just ignored it because he realised the model was still a useful one.

Stability of Bohr Orbits


Is it possible to clear history in windows command prompt?

Question: When using CMD, one can press upper arrow key to retrieve old commands.
Is it possible to erase the history of these commands?
If so, how could one do so?
Comment: `Alt-F7` - see [this reference site](https://ss64.com/nt/syntax-keyboard.html). Alternatively, you can exit and start `cmd` again.
Comment: @AFH You should make an answer from that comment.
Answer: There is a simple keyboard short-cut, Alt+F7.
There are very few questions on cmd that are not answered on this reference site.
Alternatively, the history is cleared every time you exit and restart cmd.
Answer: The command doskey /listsize=0 should do the trick.
Comment: Indeed it does. Interestingly, it has the additional effect of disabling history for the rest of the session (or until the list size is changed again).
Answer: Unlike  Linux-based CLIs, In Windows all histories are cleared when you close CMD.
So, don't worry about that.
Comment: First of all, welcome to Super User!  We always appreciate the contributions from our community members, but can you please do us a favor?  You may want to edit your answer so that it includes the suggestion related to Clink.  At the moment, your initial input simply described what had already been said within the officially accepted answer.  Thanks for your help.
Comment: Yes I was mostly concerned about that because I remember in Linux I was able to retrieve the commands even after reboot.
Comment: Yeah, However or In addition, you may (again) find useful to reuse commands you have typed for next session. To be able to use them you have to install third party program called [Clink](http://mridgers.github.io/clink/). To use it just click Batch file **clink.bat**  in program folder and to delete histories, delete file **.history** in folder *clink_0.4.8/profile/*  It is smaller program and you can just download it in ZIP because it is portable. _This will be helpful also for other who find it :-))_


Internet Recovery failed with -2002U code

Question: I have a MacBook Pro 2012 that when I try to recover from the internet using WiFi, I always get the error to call Apple Support with the code -2002U. I tried several times to solve this problem without any luck. Does anyone know how to fix this?
Answer: After several attempts using WiFi, I tried to hook up the MacBook Pro 2012 using an Ethernet cable directly to the router. This time it was able to connect to the Apple servers and the Internet Recovery worked.


Silently pass parameter through Rails form

Question: I need to pass a parameter through a Rails form and access it in the controller. I would use a hidden field tag, but I don't want the user to be able to edit this data in the HTML and submit it. How can I secretly pass this parameter and access it on submit with parameter[:my_parameter]?
Answer: While a user can view hidden field data in almost any modern browser, they can't directly modify it in the browser itself.  They can, however, build up form-data and pass it using some other client of their choice (curl, fiddler, etc) - there isn't really anything you can do about this.  I suggest not relying on such a method for sensitive data.
A possible workaround would be to encrypt it in some form in advance, which can allow you to determine if it has been tampered with when posted.  It can be tricky to do this right, and it's not a common practice for passing parameters.
Perhaps if you can clarify what exactly you are trying to accomplish, someone may be able to suggest a suitable alternative.
Answer: The best practice for something like this is to not pass the parameter through the HTML, but instead to tie it to the user's session, and store the actual sensitive data in a database or cache. You could try to encrypt it, but, if the user has access to the data, then they have the ability to try to decrypt it.


How to trigger screenshot on every validator/Assert pass and fail using testng listener

Question: I want to take screenshot for every validator/assert pass and fail steps. Let me know if we have testng listeners can help me to implement this scenario as I am new to selenium webdriver and testng automation testing
Answer: You can take screen-shot by following :
<code>        File screen = ((TakesScreenshot) driver).getScreenshotAs(OutputType.FILE);
        BufferedImage img = ImageIO.read(screen);
        File filetest = Paths.get(".").toAbsolutePath().normalize().toFile();
        ImageIO.write(img, "png", new File(filetest + "\\Screenshots\\"+ "ScreenshotName" + ".png"));
</code>
Here, Screenshot will save in "Screenshots" folder(Which you need to create) under project workspace by using Absolute path. If you want to save on other location, You can locate path for same.
If you want TestNG validator on every @Test whether its pass or fail, you can implement it in AfterMethod.
<code>@AfterMethod
public void testStatus(ITestResult result) throws IOException
{
    if (result.getStatus() == ITestResult.FAILURE) {
        testResult = "Test Fail :" + result.getName();
        testResult = "Details of Fail Testcase:" + result.getThrowable();
        }
}
</code>
You can define above Screenshot utility in testStatus method, which will take screen shot if @Test got fail.
Comment: how to get screenshot at every step or for every method, problem is  many test cases which has 300+ methods I don't want to call takeScreenshot() method in every method that would be headache, any better way to do it like using `@aspect` annotation or something like that .. any pointers on this would be really appreciated
Comment: Yóu can create singe function, and after you just have to call ít by single liné.


Keyboard shortcut for "Invert selection"

Question: Is there a dedicated windows keyboard shortcut for the "Invert Selection" command?
Comment: Not a keyboard shortcut, but you can hold Ctrl and select all files with your mouse.
Answer: Windows 8 + 10:
Alt+H for Home tab in the ribbon, then S and I for Invert Selection.

Windows 7:
Alt+E to open up the Edit menu, then I for Invert Selection.
Comment: Seven years later, this is still inacceptable. Windows 10 gave us 1-click invert Selection, but Windows 11 took it away, then Windows 11 22H2 took it away *again*, and they *still* can't give us a keyboard shortcut for it!
Comment: I want to know why MS had to make it a 2 character keyboard shortcut? There aren't THAT many options on the home ribbon tab that we can't have "I" or something. Alt-H-I ???
Answer: An alternative method, which is what I use, is to add a registry entry for your shell right click menu. Therefore, when you select a file or folder(s), all you need to do is right click them and go to select > invert selection.
Add the following to a txt file, and then rename it to .reg
<code>Windows Registry Editor Version 5.00

[HKEY_CLASSES_ROOT\*\shell\Select]
"SubCommands"="Windows.invertselection"
"Position"="middle"

[HKEY_CLASSES_ROOT\LibraryFolder\Background\shell\Select]
"SubCommands"="Windows.invertselection"
"Position"="middle"

[HKEY_CLASSES_ROOT\Directory\shell\Select]
"SubCommands"="Windows.invertselection"
"Position"="middle"
</code>
This can expanded upon if you want, by adding more options to the subcommands, but that is entirely up to you.
Answer: Using AutoHotKey (AHK v1) -

For the Windows 11 23H2 Windows File Explorer -

<code>#If WinActive("ahk_class CabinetWClass")

^i::                      ; Ctrl + I
MouseMove 750, 150        ; see Note 1
Send {Click}              ; click on "see more" option
WinWait, PopupHost ahk_class Microsoft.UI.Content.PopupWindowSiteBridge ahk_exe explorer.exe,, 1 ; 1s
If ErrorLevel             ; if "see more" fails to open in 1 second
    Return                ; do nothing further
; Sleep 500                 ; alternative to WinWait ; see Note 2
Send {Up 3}               ; select "Invert selection" option
; Send {Enter}            ; see Note 3
Return                    ; The end

; Note 1: location of "see more" option on screen
; MouseMove - move the mouse cursor to x,y coordinates on 'Screen'
; use Window Spy to determine coordinates for your own screen(s)
; might want to change "CoordMode" if you have problems, visit
; help page: https://www.autohotkey.com/docs/v1/lib/CoordMode.htm

; Note 2: wait 500ms for window to respond to keys
; wait times (in milliseconds) don't work sometimes,
; try changing them to see what is sufficient for your PC spec/performance.
; Sometimes it doesn't work at all on the 1st try
; (ex: when a new explorer instance is first opened after login/restart)
; but works on subsequent attempts. *shrug* no idea why.

; Note 3: uncomment this line after test of previous steps
; or leave it as is if you prefer to do this step manually,
; especially if you notice inconsistent selection of options.

#If
</code>

For ribbon UI enabled File Explorer (in older Win 11 and 7/8/8.1/10 systems) -

<code>; Source: https://www.autohotkey.com/boards/viewtopic.php?f=76&t=27564
#If WinActive("ahk_class CabinetWClass")

^i::                      ; Ctrl + I
ControlGet, hCtl, Hwnd,, SHELLDLL_DefView1, A
PostMessage, 0x111, 28706, 0,, % "ahk_id " hCtl ; edit, invert selection
return

#If
</code>


Looking for a specific presentation template

Question: Can somebody help me to find this template:
Answer: You are looking at the default style for the prosper class, one of the older classes for presentations. The documentation, in …texmf-dist/doc/latex/prosper, includes example .tex files for the default and for eleven other styles, some of them quite attractive. It's also possible, and not too difficult, to make your own styles, and there are additional styles, without examples, in …texmf-dist/tex/latex/prosper.
The examples should be compiled with latex (not pdflatex or lualatex or xelatex), and the resulting .dvi file converted to .ps and then possibly to .pdf.
If you like prosper, you should also look at HA-prosper, which added some fixes, and at powerdot, the successor to HA-prosper.


Why is the community moving CV questions to the homepage today?

Question: Officially the Community is saying that this is being done to reevaluate for questions they recently activated (reactivated) the answers as to which are good or bad.  But how are new comments, upvotes and downvotes going to be used to decide good from bad.  If answers get classified as good, neutral or bad how is that to be used?  I have noticed that I have been a beneficiary of this because some of my answers from 2012 are being reviewed that otherwise would not be and I have been getting additional upvotes.  Is this something that has been done periodically to try to clean-up answers?  I am trying to decide whether I think this is good or bad for the users.
Comment: You mean like [this](https://i.stack.imgur.com/k3mUm.png)? That's been happening at least a few times every day that I've been on the site. A bot bumps posts that haven't got an upvoted or accepted answer to bring them to the front page. It has been happening for at least 5 years, and probably a good deal longer. When traffic is very slow it bumps more, when traffic is heavy it sometimes won't bump a post. However, what shows up on the post when it gets bumped has recently changed to make it more explicit.
Comment: Also, take a look at: http://meta.stackexchange.com/questions/19738/who-is-the-community-user
Comment: You might have a misunderstanding of what the "Community" user is: it's not any of us, either individually or collectively.  It's a "bot"--software that acts somewhat as if it were a very privileged user, in order to automate some useful moderation tasks.
Comment: Posting volume is definitely down from what it was say a month ago (which is why we see more bumps from the Community bot at this time of year). I think that typically it drops by about a third from end of November to end of December but I could probably find a SEDE query to get exact figures (though I think you have enough privilege to see the [site analytics page](http://stats.stackexchange.com/site-analytics) -- and then click the "weekly" checkbox). The script for this particular Community task would not need to be especially complex - it just needs a few conditions on when it acts.
Comment: ctd... answers already. There's certainly *a* script that puts them there but I don't think that's regarded as associated with the Community bot.
Comment: I can't tell what you're referring to from your description. Who are "they"?
Comment: I am referring to the community which you can see has moved several old posts to the CV home page.  This has been going on today and all day yesterday.  Scan the home page and you will see.
Comment: Thanks Bill Huber.  I am not very clear on some SE terminology. So does that mean I should add "bot" after Community in my question. I have already gotten the answer i was looking for.
Comment: @Glen_b I have not seen this before.  But it has happened a lot this week.  It doesn't seem to be a particularly slow week but maybe it is a little slower because of the holidays. I think zaq has given me the answer I was looking for.
Comment: If this is an automated process it must be a complicated algorithm. Doesn't a real person need to activate it.
Comment: Today is New Years Day (January 1, 2017).  I guess a slow day by some standard.  I am seeing suggestions for things to do. This may be common but I have not noticed it before.  For example a list of hot questions is given for Network questions, Meta posts and even old questions from sites that include sites other than CV (e.g. Mathematics).  Is this Community bot at work and if so is it a regular thing that is done especially on slow days?
Comment: @Glen_b I went to the analytics page changed it from daily to weekly and changed the range to Jan 01 2016 - Jan 01 2017. All posts have a definite drop in December that that seems to carry through January.  All questions are not surprising lower but correlated over time with all posts. Last of all, all answers is fairly flat throughout the year.
Comment: @Michael (in reply to the comment posted 2017-01-01 19L57:43Z) If you're referring to the "hot network questions" list in the right-hand sidebar -- that sidebar has been there continually for ages (three years, at least), and the HNQ list itself has been around for several years (at least) before it appeared in the sidebar. That element of the sidebar has been there every day you've been on site in the last couple of months. It's not actually something for you to do (other than being a list of potentially interesting questions to look at), since hot questions usually have several good ...ctd
Comment: Note that if you set the list to go from Jan 1 2016 to Jan 1 2017 you will have the last week having only 2 days in it (2016 having 366 days = 52 weeks and 2 days), so it will naturally be low. It would be better to do either 364 days (52 weeks exactly) or 371 days (53 weeks exactly) or to ignore the final one. That said, the low part at the end of December normally does continue into the first couple of days of January, with Dec31/Jan1 being a particularly low dip
Comment: @Glen_b All I did was to try what you suggested and I reported what I saw. It was the first time I tried it. I don't understand why you get all up in arms with your comments.  You sound defensive for no reason.
Answer: Many find it discouraging to post answers and get absolutely no response, positive or negative. The campaign of Community-bot is to remedy it; the notice it places under a post is to invite readers to evaluate the answers that have been neglected so far.

But how are new comments, upvotes and downvotes going to be used to decide good from bad. If answers get classified as good, neutral or bad how is that to be used?

There is no classification other than normal votes, up or down. The hope is that useful answers will have a positive score while useless or wrong answers will have negative score. We know it doesn't always work that way, but that's the ideal, and the bot is trying to get the site closer to it.
Comment: Thank you for your answer.  It is very helpful.  I definitely like the intent.  I found that several of these post were several years old and some including my answers had lots of upvotes.  So it is not just bringing up post that have not been given a fair shake.  Also old questions could have answer that were excellent at the time but as statistics evolves they may seem to be obsolete. I don't think that they should get a downvote in that case. Is this the first use of this?  Is it continuing or has it just been given a short trial period?
Comment: This idea could also be used to bring back questions that received no answers and didn't get bounties or the bounties were never awarded.  Some of these questions might deserve another look. Others may not and so the readers get another crack at evaluating the question.


The style of whiskies produced in England?

Question: An English relative sent me a map of distilleries based in England today, which I'd never thought of before. Of course Scotch can only be made in Scotland, which begs the question of what these English distilleries are doing. Do they try to mimic the style of Scotch, generally, or something else?
Comment: They produce the same drink, whisky and the process is very similar in any distillery around the world. The intricacies in taste and effort to create come mostly from the ingredients used and slight differences in equipment (stills). Scotch obviously has a very high cultural effect and the regional behaviours play a part tops.  The style again varies basically distillery to distillery so I would say they are quite similar and I haven't noticed consistencies in Scotland that aren't mimicked in other distilleries around the world (japan/Sweden or England). I haven't been to them all though
Answer: Generally, they make something that doesn't resemble Scotch Whisky. There are many styles and types of whisky, or whiskey with an 'e', and it's made all around the world. There are distinct and famous styles from Ireland, Japan, and the United States. Sullivan's Cove from Australia, Mackmyra from Sweden, and Kavalan from Taiwan have all made award winning whiskys. There are at least 4 whisky distilleries in Toronto, Canada alone.
Not every whisky made in Scotland can even technically be called a Scotch, there are regulations that Scotch follows and some distillers will step outside of those bounds for creative reasons. Scotch regions are also individually recognized for their unique styles.
Often in areas where there is no strong history of whisky, each distillery will kind of do their own thing because the drinkers of the region don't have strong expectations of what the whiskey should be. This is especially true in today's global economy, consumers have tasted bottles from all over the world so there is no accepted standard in new markets. The distillers will borrow ideas from regions that do have a strong tradition, mostly they just try to make something that tastes good. So even though Japanese whisky, for example, was borne essentially directly from Scotch in the 60's, there are strong differences between them. Some of this is because whisky highballs are super popular in Japan, and whiskys will be blended with this in mind. Some of this is for technical reasons, different altitudes make for different distillation temperatures.
In conclusion, they different... probably.


How can a computer run an infinite loop?

Question: I understand the concept behind for and do/while loops, but I am trying to understand what is happening at the hardware level that allows a loop to run infinitely. Technically wouldn't it have to stop at some point because there are only a couple billion transistors in a microprocessor? Maybe my logic is off.
Comment: A running track has a finite size, but you can run around it as many times as you like.
Comment: "only a couple billion transistors". The same transistors get used over and over while the processor is running.
Comment: A [mechanical counter](https://en.wikipedia.org/wiki/Mechanical_counter) can only memorize a few digits, but it can loop forever. After 9999, we restart from 0000. Your argument only proves that in an infinite computation, the processor will eventually visit the same state twice, entering a loop.
Comment: See [this comment](https://cs.stackexchange.com/questions/99806/control-flow-statements-explained#comment212656_99806).
Comment: A Cray can finish an infinite loop in 3 seconds.
Answer: In a contemporary processor there is, among many other things, a register (digital electronic component to hold some bits) called the Program Counter (PC). It holds the memory address to the current machine instruction.
During normal flow of the program, the PC will always be updated to address the next instruction. However, any loop will be implemented with so called branch or jump type instructions which will cause the PC to address some other instruction, rather than the next one.
One, if not the, simplest infinite loop, is one in which the PC is updated again to the same instruction.
An assembly (MIPS) code example (beq stands for Branch on Equal):
LABEL: beq $0, $0, LABEL # Comment: If 0 == 0,then goto LABEL.

Answer: step 1: take a calculator
step 2: input a number
step 3: add 1 to the number
step 4: subtract 1 from the number
step 5: goto step 3
If you didn't eventually get tired or bored you would be switching between the 2 results forever. Computers don't get tired or bored.
Comment: Good answer, but it leaves out an important word that appears in some of the others: That is, "state." Your answer illustrates the fact that even though a real computer can only have a finite number of possible states, nothing prevents it from cycling through some subset of those states, over and over again forever.
Comment: I'd propose to remove state altogether, as it's not mandatory.
Answer: At the most basic level, an "infinite loop" only requires two transistors — an astable multivibrator will alternate between two different digital states indefinitely as long as it has power. In some sense, every other loop is just a bigger version of this. You don't need more and more transistors to keep operating for a longer period of time.
But what you might notice is that the astable multivibrator only has two states. Q is on, then Q is off, then Q is on again. It can only do two things before it repeats. There isn't really anywhere else for it to go. This is a general limitation: a system with a finite state can only go through a finite number of states before it repeats. The more transistors you have, the more "interesting stuff" you can do in your infinite loop. But with "only a couple billion" transistors there are trillions of possible combinations, which is quite a lot. And that's not even counting the RAM, which adds more billions (or even a trillion) more bits of state, making the number of possible states that the system can be in truly massive. As such, it's possible (even fairly easy) to write a program that, e.g. generates prime numbers, and keeps running for longer than the probable lifetime of the hardware (or even the solar system), without running out of different possible combinations of on/off transistors.
Answer: You only need two transistors for that, as demonstrated by the old joke.
How do you keep a (insert ethnicity here) busy forever?
Write "Please turn over" on both sides of a sheet of paper.
It's as true for transistors as for humans with a sheet of paper. In this case, two transistors make a bistable oscillator, and they'll toggle between true and false for as long as there's power there.
Answer: One thing other answers have missed is that CPUs have a clock source, an external device (usually a quartz crystal oscillator) which switches from 0 to 1 and back a fixed number of times a second. This clock signal is what makes the CPU go from one instruction to the next. If the clock signal is too fast, the transistors that "perform" the instruction haven't finished charging up and so the results are garbled.
Modern CPUs do very complicated things for performance reasons but this is the basic idea.
Maybe that is enough to understand how the CPU moves from one thing to another thing and then moves back to the beginning. But I will go into a bit more detail.
Inside the CPU are memory cells using a fixed number of transistors which can be overwritten with new values. There is also a fixed number of transistors that perform various computations, like addition, comparison, and so on. The clock opens the gate to allow data to "flow" from the memory cells, through the "compute" portion, and then the result is allowed back into the memory cells.
So if your instructions are as follows:
1. x = 5
2. y = x + 1
3. Go to step 1

the processor will end up in exactly the same state when it starts each loop. (Well, in theory.)


How is the destination MAC address filled during MPLS label switching?

Question: During IP packet forwarding, after a LPM(ip lookup) done on the FIB, it will fetch the Nexthop ip address and the egress interface to send out the packet. Now, ARP table lookup is done for this nexthop IP. If the entry is not found, ARP request is broadcasted on the outgoing interface, right. My question is, during a MPLS label switching how is the destination MAC address found ? Because we don't do the IP lookup at all at the intermediate LSRs.
Comment: MPLS or not, if ARP is used for IP-to-MAC resolution the ARP broadcast needs to reach the next-hop router interface.
Comment: @Zac67 To be more specific, my question is, during a MPLS label switching, we don't do a IP lookup, we just swap the labels and send it out. So, in this MPLS switching operation how it will get the information of next-hop IP addresses on which a ARP lookup has to be made ?
Comment: The only way I could think of is that ,when a LSR receives a Label from downstream router during label distribution, it notes down the source IP address of the IP packet which carries this label. And when this label information is programmed into the ILM, it records the Nexthop IP address as well in the NHLFE pointed by that ILM. But, I'm not sure if my assumption is correct.
Answer: When a router receives a label via LDP, it records the layer 2 information in the LFIB.  This allows the router to construct a layer 2 packet quickly because all the required information (destination MAC address) is there.


Remap TAB key to another key

Question: I am using Ubuntu 12.04, my TAB key is not working
and I want to use auto complete function.
Is there any way I could assign other key for the
same function in Ubuntu manually?
I looked into the keyboard layout but I couldn't able
to figure it out, Is it possible ?
Answer: For assign certain key to other, you need two tools, already available in ubuntu: xev and xmodmap
With xev you can output some information about the pressed key, or with
xmodmap -pke

you can read all the configured key, keycode and action.
With xmodmap you can change key map.
For example, this command assign tab action to backspace key (keycode 22):
xmodmap -e "keycode 22 = Tab"

to reassign to backspace is natural function, type:
xmodmap -e "keycode 22 = BackSpace"

These change will be lost after reboot. if you want that the changes are always loaded, you must create a file containing the keyboard mapping and load when the session start:
xmodmap -pke > ~/.Xmodmap (create file configuration .Xmodmap in your home directory)

then create file .xinitrc in your home directory (if not exists) and add this line (or append if file exists):
xmodmap .Xmodmap

this command loads the configuration at the start of each session.
You can create keyboard shortcuts by installing AutoKey from software center.
After install, run autokey and create new phrase like this:

the key combination  Alt+a  will emulate the tab key.
Comment: Sorry If i am asking the wrong question, Do I have to place the "Key" in place of TAB to assign the same function ?
Comment: if I understand it, you want to assign the "TAB" key to another: if this is so, you must use the command `xmodmap -e "keycode X = Tab"` with X the keycode of the key you want it to work like the TAB key
Comment: I want a combination of key like Alt+A , so can you tell me how to do it?
Comment: Just now I found out that pressing Esc key for a second gives me the same thing, why is that?, moreover it is also suggesting the possibilities.
Comment: i edited my answer.

Remap TAB key to another key


Does Visual Studio's built-in web server support HTTP PUT?

Question: Does the web server built into Visual Studio 2010 (and earlier, but 2010 in particular), WebDev.WebServer.exe, support the HTTP verb PUT? If so, does it support it by default or does it require configuration? If it requires configuration, please describe how to configure it.
Comment: Visual Studio's built-in web server is also known as the Visual Studio Development Server.
Answer: In Visual Studio 2010 at least, the Visual Studio Development Server web server does seem to support the HTTP PUT method.
I previously answered that it does not, but the URL for which I was submitting requests with the PUT method was incorrect. Once I corrected the URL the request was accepted without requiring any other changes to Visual Studio or the built-in web server settings.


Is there is a reason for Pauli's Exclusion Principle?

Question: As a starting quantum physicist I am very interested in reasons why does Pauli's Exclusion Principle works. I mean standard explanations are not quite satisfying. Of course we can say that is because of fermionic nature of electrons - but it is just the different way to say the same thing. We can say that we need to antisymmetrize the quantum wavefunction for many electrons - well, another different way to say the same. We can say that it is because spin 1/2 of electron - but the hell, fermions has by the definition half-integral spin so it doesn't explain anything. Is the Exclusion Principle something deeper, for example in Dirac's Equation, like spin of the electron? I think it would be satisfying.
Comment: Pauli's principle is less restrictive than symmetrization principle, they are not the same
Answer: I think that while these "explanations" are all dancing around the same pole, they aren't created equal. I think the meat is in the fact that nature has a local Lorentz symmetry, so we expect to be able to decompose things into representations of the group $SO(3,1)$. It's a mathematical fact that this group (or it's algebra, rather) has integer and half-integer representations.
Once you have this structure, then a few meagre assumptions about causality and unitarity lead to the Spin-statistics theorem. In order to understand the proof you'll need to first dig deeper into the representations of the Lorentz group, and how they label single-particle states.
Comment: @CheshireCat Perhaps add that the last step is that the spin-statistics theorem shows that for half integer spin representations the quantum state for two particles with quantum numbers $\vec{x}$ and $\vec{y}$ (I include "position" in the quantum number vector) is antisymmetric wrt swap of arguments $\psi(\vec{x}, \vec{y}) = -\psi(\vec{x}, \vec{y})$ so that now if two particles have the same quantum numbers $\psi(\vec{x}, \vec{x}) = - \psi(\vec{x}, \vec{x})$. A further piece of trivia which I like to dwell on here: when we represent the algebra by half integer representations, we're ...
Comment: ...actually representing the *double cover* $PSL(2,\mathbb{C})$ of the Lorentz group $SO(3,1)$, so you could say, with a slight strech, that the Dirac belt trick "proves" there are only bosons and fermions in the world.
Comment: Can we get a translation for those of us who aren't already experts in the subject?
Comment: @StevenSagona It's a tough call. There are many gnarly math concepts.  For example, in lionelbrits answer, in the link to spin-stats, if you scroll down to "Relation to representation theory of the Lorentz group" you will see that the Lorentz group has not finite-dimension unitary non-trivial representations. So seeing that, and since angular momentum is usually dealt with as a derivative, it is quite a job to get to the answer.
Answer: Under the influence of rising magnetic fields the spectroscopic lines of atoms get splitted more and more. At the end for atoms it was observed that the electrons have two possible states. This states there called spin up and spin down in analogy to the (now outdated) orbital motion of electrons around the nucleus.
The understanding of the distribution of electrons around the nucleus was made more difficult for two reasons:

instead of spin the founded property has to be called magnetic dipole moment
instead of the used spherical harmonics with symmetry axis in analogy to the Cartesian coordinates it would be better to use the following spherical harmonics with eight sectors

https://en.m.wikipedia.org/wiki/Spherical_harmonics
The plus on the surface stands for electrons with magnetic dipole moments in one direction (say North to the nucleus and south pointing outwards). The minus than stands for electrons with south to the nucleus north pointing outwards). This is a perfect equlibration and could explain the stability of Neon and Argon.
Comment: All the answers explain Pauli's principle by mathematics, which is fine with me, but isn't there physics involved that lies behind the principle? You can say the math **is** the physics, but can't it be that on the physics of the principle (i don't know how the principle was found: by using math or by experiment) a mathematical net is thrown, making the "flesh" less visible?  I know a lot of physical stuff is predicted by math before the stuff is found, but that doesn't mean the stuff is a mathematical thing. I suppose it is how you want to look at the stuff.  To know **how** , for example,
Comment: two electrons (not too far apart of course) can't be in the same state is for some (the most, I guess) physicists mathematical knowledge, while others want to know the physical mechanism behind this principle.
Answer: This is a question about quantum physics but a "reason" is also asked for and there are some basic important properties associated with the PEP that nobody has mentioned in much detail. First of all the PEP serves the function of allowing objects composed of matter to remain separate. Without it we most likely would not be having this experience of using Physics Stack Exchange to great benefit.
Secondly as was touched upon in the question for two bound electrons in an atom to occupy the first shell one of them must adopt the opposite spin to the other. If the atom has three bound electrons one must jump to the second shell since there are only two spin states. As electrons fill out shells matter acquires stability and volume.
A neutron star shows us the extreme case of the PEP allowing matter an incredible resistance against gravitational forces. As far as I know the PEP is not even violated in this example and it is the Heisenberg Uncertainty Principle which allows momentum space to expand as position space (radius) decreases until time-dilation becomes infinite at the surface.
Answer: It's an observed phenomenon, aka a "Law of Nature."  You can't prove it, but you can show that the underlying math & "description" of the particle's behaviour is consistent with this law.


How to connect NetBeans to MySQL database?

Question: I have just installed NetBeans 7.0  and I am a newbie in NetBeans' world. can any one tell me how to connect my application to MySQl / Postgres ? I work on Windows XP.
Comment: Is your question about using the built in NetBeans SQL editor to access your database, or are you asking how to connect the program that you've written to a MySQL database?
Answer: In the Services window you can right click on the Databases tab and select New Connection.

Select MySQL(Connector/J Driver) from the drop down list. The driver file should be listed in the window. If not, you can download the file, then click add and select it from your hard drive.

Finally enter your database details such as servername, username and password and click finish.
Answer: If you meant you want to use IDE's GUI tool then

windows > services >database > driver
Get the driver jar
install the jar
right click on driver installed > connect using > and then provide the information

Or otherwise use JDBC
Comment: @jigar u don't need to install jar. just select connetor/J dirver
Comment: @ravi I meant installation by  that
Comment: @ravi and @Jigar : I have done all that... but still Class.forName() is throwing ClassNotFoundException. When I tried System.setProperty("jdbc.driver","driver info"); still it is not working.....
Comment: buddy u have to attache mysql.jar to run your project library.
attache with naetbean is just like workbench,querybrowser, toad etc.
Comment: You need to provide driver jar  into classpath
Answer: One way to connect to such databases is to use a JDBC (Java Database Connectivity) driver.  You can find more about JDBC at the Oracle FAQ.
Answer: click on window and then select services

then click on databases and select new connection

select driver which you want

then click finish
if you want to check the connection is successful or not
<code> try
    {
         con=DriverManager.getConnection("jdbc:mysql://localhost/student_result","root","");

JOptionPane.showMessageDialog(null,"connected with "+con.toString());

    }
    catch(Exception e)
    {
        JOptionPane.showMessageDialog(null,"not connect to server and message is"+e.getMessage());
    }
</code>
Answer: see mysql .jar for u project properties if  u waant to run or for netbeans shown by @Vincent Ramdhanie
Answer: Follow the steps:-
1.Create a new application
2.In projects section of netbeans right click on libraries
3.Check the picture(i)Libraries java mysql connector
Thus you successfully setup the connection with mysql.


Impact of bar velocity in rep for hypertrophy

Question: When doing barbell exercises is there an effect of the speed that a rep is completed on hypertrophy?
Comment: My impression is that slower reps are better for sarcoplasmic hypertrophy since they increase time under tension, and faster reps are better for myofibrillar hypertrophy and strength performance (particularly because faster reps recruit fast twitch fibers too).
Answer: Specifically for hypertrophy, researcher Chris Beardsley states:

... we observe hypertrophy after any workout in which effort is high (since effort determines motor unit recruitment) and bar speed is slow (since slow speeds permit muscle fibers to produce high forces, which produce the high levels of mechanical tension).

In other words, the weight needs to be heavy enough for the muscle to exert full force in order to move the resistance. You need high motor unit recruitment.
Moving a weight slowly with increased time under tension won't necessarily stimulate muscle growth, and moving a weight really fast won't either.
He gives the example of walking.  Walking technically is a constant tension resistance exercise which you can do for hours. The effort for every step is extremely low, but the speed is also really low. You could walk for hours every day and not see an ounce of muscle growth in the areas that are pushing your body.
Consequently, jumping may be pushing the resistance really fast, but the tension is really low. So box jumps are not really good for hypertrophy (at least up to a point).
So he puts it simply, the "stimulating reps" of a set is when the weight starts to move slowly; not because you purposely slow it down, but because you have no choice.
Now there are other reasons besides hypertrophy to slow down the weight. It's safer. It helps prevent form breakdown. You don't rely on momentum which increases muscular tension on the muscle you're trying to work. Those on itself will not cause hypertrophy.


Switch from View Controller to Tab Bar Controller in a Storyboard xCode

Question: I am working on an app in xCode 5.  This is a first at using the Storyboard. My app starts with a simple username/password login screen. Upon successful login, I want to programmatically switch from this login View to my Tab Bar Controller with the tab index set at 1.
I do not have a custom class for my UITabBarController. I can build one if need be.  Can someone help get me started or point me in the right direction?
Answer: Funny how typing out the question can help you solve it.  Here is the code I used in case someone has the same issue in the future
 UIStoryboard *storyboard = [UIStoryboard storyboardWithName:@"Main_iPhone" bundle:nil];
    UITabBarController *vc = (UITabBarController *)[storyboard instantiateViewControllerWithIdentifier:@"UITabBarController"];

    [self presentViewController:vc animated:YES completion:nil];
    [vc release];

Answer: If you have only one storyBoard in your project then you can also use simply
self.storyboard

Answer: The best way to do this is to set your rootviewController to tabBarcontroller as soon as you authenticate the user. Here is how you can do this in swift.
let tabBarController = self.storyboard?.instantiateViewControllerWithIdentifier("TabBarController") as! UITabBarController
let appDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
appDelegate.window?.rootViewController = tabBarController

Where TabBarController is the storyboard id of your tab bar controller. It could be anything whatever name you have given it.
Answer: In storyboard; drag a segue from your ViewControllers button (or File's Owner to connect a method) to your TabBarVC. Choose modal style and animation if you wish.
Add a new custom class for UITabBarController and assign it to TabBarVC in storyboard.
In it's implementation file; put self.selectedIndex = 1;


London into Australia in 90 minutes

Question: Me and my friend are having a debate on whether it would be possible for a human to travel at 15,000 miles an hour from London to Australia in the matter of 90 minutes. Would a human  be able to  survive  travel in such at fast speeds  knowing  he will  have to overcome immense  amount of g's.  Basically  is it possible for a human? or will he suffer  death in the process?
Comment: Think ICBM. Think the Mercury launches. Or as Heinlein called it "semi-ballistic".
Comment: G-forces have nothing to do with high speeds.  G-forces have to do with acceleration--with changing speeds.  I don't think this question is really about how fast someone's going. Rather, it's about how they get going that fast.  What kind of acceleration mechanism do you have in mind?
Comment: Concerning _radial_ acceleration, you needn't worry about leaving your seat. Only at speeds of roughly 17,686 mph along the earth's (assumed smooth) surface will you become "weightless". And only at 25,012 mph you will experience -1 G while stuck to the roof of your vehicle. Above 30,633 mph you might want to sit upside down, because otherwise you will potentially experience redout and death.
Answer: The distance from London to Australia is about 17,000km. If you wanted to minimise the acceleration you'd feel during the trip you'd accelerate continuously for the first half of the journey (8,500km) then decelerate at the same rate for the second half. To work out what acceleration is required you use the SUVAT equation:
$$ s = ut + \frac{1}{2}at^2 $$
For half the journey the distance $s$ is 8,500km and the time $t$ is 45 minutes (2700 seconds), so using the above equation the acceleration required is about 2.33m/s$^2$, which is only about a quarter of a $g$. The only trouble is that your speed at the halfway point would be about 6,300m/s (about Mach 18), so you'd need a rocket rather than a plane to do it.
Answer: Gagarin flew around the world in 90 minutes 50 years ago, apparently without serious health problems.EDIT: OK, it was 108 minutes.
Comment: @Eduardo Guerras Valera: It is my understanding that intercontinental ballistic transport would be much more expensive with current technology - the costs would not be much lower than those for orbital flights (see the latter costs in the following relatively old source: http://www.futron.com/upload/wysiwyg/Resources/Whitepapers/Space_Transportation_Costs_Trends_0902.pdf ). However, some future technologies might change this situation (e.g., http://www.good.is/posts/can-nasa-launch-a-rocket-with-a-laser/ ).
Comment: I always wonder wether it would be much more expensive (in terms of fuel) to have sort of a ballistic civil transport between continents... It would be awesome: some minutes quickly rising, then one hour or so (depending upon your destination) of absolute silence with an amazing sight of the outer space and feeling weigthless (some passengers vomiting...) and eventually re-entering the troposphere with clouds and blue sky before landing...
Comment: Nice link! (+1), I was just talking this morning with my father about hypothetical wireless power transmission possibilities. That lasers/masers in your link were one of the possibilities, although it has thermodynamic limitations, since the rocket is nearly isolated (but in your link they overcome this by using the laser power to vaporize a metal, so the temperature would not rise as long as there were some solid metal left). Very interesing...

London into Australia in 90 minutes


Ficus religiosa leaves turning brown and yellow

Question: Ficus religiosa leaves are falling off, some by turning yellow, and some by turning brown. I tried treating with fungicide thrice with 3 days gap in between, which is of no use. I use vermicompost as fertiliser, I added vermicompost to top layer soil. Still I notice the leaves turning brown. Kindly help me out.
The plant gets sunlight till 11a.m in the morning. And i water once in two days.
Im from India.
Comment: There are what appear to be either tiny mites or tiny white eggs on the leaf surface - I can't tell what they are - if you have a magnifying glass, inspect the underside of the leaves closely. Is the plant in a pot or in the ground? Are you able to add a photo of the whole plant?
Answer: Wash your plant, Get a damp rag and wipe the leaves.
Also, It grows in sandy loam that's well drained. Poke your soil to check for dryness... it may be underwatering, but as a tropical it needs plenty of light


Why does ubuntu support a2dp but windows doesn't?

Question: I have an Intel Proset/Wireless Bluetooth 4.0 integrated card,
In ubuntu it detects my bluetooth headphones (Philips SHB4000) and I can stream audio to them, but Windows can't, I can't find the a2dp profile, I installed the newest drivers from Intel, tried installing the broadcom driver which includes the profiles but when I select the profiles to update my headphones it gives me an error, (10)
I just cannot make them work!
I also tried this program:
http://www.broadcom.com/support/bluetooth/update.php
But it cannot detect my integrated bluetooth card nor install the profiles.
So, any help would be appreciated..
Comment: Welcome to Super User! I would perhaps change the question a bit to put emphasis on the fact that you have a problem with getting it to work on Windows, and not a question of theory as to why it doesn't work on Windows compared to Ubuntu. *The short answer could be that the driver functionality isn't available by default, and now you're having some issues getting it to work.*
Comment: Last time I dealt with Windows 7 its bluetooth stack didn't support a2dp. Broadcom made a replacement attack you might try.
Answer: Bluetooth profiles like A2DP are implemented in software, and usually don't require any special functionality in the Bluetooth transceiver.  Which Bluetooth profiles are available can be totally different from one operating system to another (e.g. Windows vs. Linux) since the whole Bluetooth software stack (including the basic OS Bluetooth stack, the drivers for your specific Bluetooth transceiver, and application support for Bluetooth features) is different.


paint a fiberglass tub

Question: We're going to be redoing our bathroom and, unfortunately, there's no room in the budget for a new tub.  Is there a way we could paint the tub and, if so, how would I go about doing that?
Answer: There are numerous DIY epoxy products for refinishing fiberglass tubs. The quality of the resulting finish can vary widely, with best results when properly preparing the existing surface and following the manufacturer's' instructions exactly (and using only top quality application tools).
This is one brand (not an endorsement): Klasskote
Answer: Any finish on a tub has to be designed to be a non-slip surface. I don' t think you just paint a tub. There are special resurfacing preparations for fiberglass tubs that incorporate a non-slip agent.
There are epoxy resurfacing finishes for lavatories which make a slippery surface which is totally unacceptable for a tub and are in fact dangerous.
EDIT
I once tested a tub in rental apartment where someone had slipped getting into the tub for a shower--resulting in a broken humerus (upper arm bone) and a concussion.
I took a shower in it and it was virtually impossible for me to keep my footing. It was the most slippery surface I had ever been on. I managed to give the surface acceptable wet friction by going over it with with abrasive scrubbing powder barely wet and very firm pressure on a rag.
Before I did that even a rubber suction mat would slide underfoot. I surmised that the tub had been refinished with the wrong product, but I didn't question the management. It's possible this was a new tub that had a protective finish on it that had not been cleaned off, but I think it was finished with an epoxy designed for lavatories.

paint a fiberglass tub


Why is the Standard Entropy of a Proton Zero?

Question: It was my assumption that the standard entropy was the absolute entropy of a substance at standard state; however, my textbook states various standard entropies ranging from $-144.77\ \mathrm{J/(mol\ K)}$ for $\ce{Al^3+}$ to $0\ \mathrm{J/(mol\ K)}$ for $\ce{H+}$ to $213\ \mathrm{J/(mol\ K)}$ for $\ce{CO2}$. It is clear that these values are not absolute entropies, as the only substance that can have an absolute entropy of $0$ is a perfect crystal at $0\ \mathrm K$ according to the third law of thermodynamics. Why are standard entropies notated like this instead of just stating their absolute form? My assumption is that this is just a way to make calculations involving entropy easier, since they merely involve changes in standard entropies; however, why is the standard entropy still notated $S^\circ$ and not instead $\Delta S^\circ$ (since it is not absolute)?
Answer: Indeed, it is clear that these values are not... Wait! What "these values"?
You are putting two entirely different things in the same basket. (If your textbook does so, then it does a poor job.) It is much like treating dogs and cupboards similarly, on the basis of both having four legs. Then, however, many generalizations will fail quite miserably. You feed them meat, but the results are not always great. You call in the carpenter to fix them, but...
Same thing here. See, ions and molecules are very, very different things. You totally can have a mole of $\ce{CO2}$. It is a thing. You can work with it, can measure its heat capacity and stuff, and eventually work out the entropy. Indeed, the textbook value of $\ce{CO2}$ entropy is absolute.
On the other hand, you totally can't have a mole of $\ce{Al^3+}$. It is not a thing. Unless you have some counterions, that is, but in that case you can't tell how much of the entropy comes from the counterion and how much from $\ce{Al^3+}$.
How can we assign entropies to ions, then? Why, it is as simple as that: we assign one ion arbitrarily, then work out the entropies of all its counterions from there, and then turn to their counterions. Just what one ion could that be? Why, let's say it is $\ce{H+}$, because so much of our chemistry is dominated by the acid/base equilibria. It is kind of natural to consider its entropy $0$, much like the absolute elevation of the sea level is $0$. Now there you have them, the textbook entropies of ions, which are not quite absolute.
So it goes.
Comment: So basically, it is impossible to determine the absolute entropy of an ion, so instead we determine its entropy relative to a H+. In the case of performing calculations (such as Gibbs free energy) it would not matter whether or not the value is absolute, just if there is a change in entropy values. This makes sense; however, if we ran a reaction in which ions were produced, such as the reaction between sodium and chlorine to form NaCl, these values would no longer be relative to each other (with one set absolute and the other relative to H+), so how do we compare these changes in entropy?
Comment: Well, the values for cations can all be increased by the same arbitrary constant, but then the values for anions will be decreased by the same constant. See, these things are not quite absolute, but not quite arbitrary either.


Can anyone explain what a dominant scale is?

Question: I wanted to sign up for some guitar lessons, the prerequisites for joining it are in the image attached. I know and understand most of the things mentioned except for the "dominant" scale mentioned. I was hoping if anyone here can explain what it is and provide a link to a lesson about it
Comment: There's a comma there so I'm pretty sure they're not asking for you to know dominant scales, but what dominants are instead. Dominant scales are a thing such as the [Phrygian dominant scale](https://en.wikipedia.org/wiki/Phrygian_dominant_scale).
Comment: As it comes after '4 shapes of major, minor...', I would guess they mean dominant seventh chords. But who knows? Also, which '4 shapes' do they mean, when they later say 'respective chords in two shapes'? To be honest it doesn't seem well-written; I hope the person's actual teaching is clearer!
Comment: @topomorto: "To be honest it doesn't seem well-written", that was my first impression, too, as soon as I saw that "understanding and ability to play quarter notes"... Is there a secret lurking there that I should know? I'm a beginner, so, as you said: "who knows"... Thx. (BTW, landed here by googling for  "dominant scale" mentioned in [this video about Spanish Phrygian](https://www.youtube.com/watch?v=yRggh1XXZZI).)
Comment: Probably it means "any 4," since it is simply a gate-keeping requirement. Probably one needs to "audition" for such a teacher. I will say though: having taken no lessons and generally not caring about theory, I do not actually know what a dominant is aside from knowing that dominant is contextual. Having been playing for 30 years, I *probably* play dominants all the time. This teacher would reject me (probably with good reason)
Answer: There isn't one "dominant scale", but they do exist. Dominant scales are an interesting phenomena in music because they almost always have other names and the requirements are simple: be able to build a dominant 7th chord off the tonic so the scale needs to have a Major 3rd, Perfect 5th, and minor 7th in it.
The most popular dominant scale is the Phrygian dominant scale as it is a mode of the harmonic minor scale, but there are others such as the Aeolian dominant scale,Mixolydian mode which can be looked at as a dominant scale, and Lydian Dominant.
Comment: Plus 1 for answering the question that the OP thought they were asking and future readers might  stumble upon.  Future readers do need to know that the interpretation of the pasted ad is wrong as well however.  Also - did you mean "The most popular "dominant" scale is the Phrygian dominant scale?  Or did you actually mean to say that the Phrygian dominant scale is the most popular scale period (which is the way your answer is worded)?
Comment: What about Lydian Dominant? thats my favourite
Comment: @RockinCowboy it's the dominant scale you'll hear most often (as it is a mode of the harmonic minor scale it shows it's head pretty often). I'll update for clarity.
Comment: Thank you, Dom! BTW, the linked Wikipedia page says "the Phrygian dominant scale is the fifth mode of the harmonic minor scale, the fifth being the dominant". Since the very definition of the "dominant" is that it's the fifth grade, why do they explicitly specify that there? Or is that a confusing way to say "the scale is called dominant because of its root being the fifth grade, also called the dominant"? Which is confusing, too, as it doesn't seem to be enough to be called a "dominant scale", as per your answer. So, I'm confused on multiple levels. (But as a beginner, I probably should be.)
Answer: Major, minor and dominant (seventh) chords.  Scales in all keys.
We hope this person's teaching is better organised than his writing!
Comment: He asked for information on 'the "dominant" scale mentioned.'   Such a scale wasn't mentioned.  He misread an admittedly sloppy bit of writing.

He's had his useful answer.  The question is of little use as stated.  Shall we erase the whole thing?
Comment: While this addresses the OP's request (due to a bad description by a teacher) this is extremely unlikely to help anyone who is actually curious about dominant scales which is what the question is about.
Comment: Having re-read the middle para of the advert, it makes even less sense now. 'Chords' in 4 shapes, followed by chords in 2 shapes. I want to go for a lesson!
Answer: The prospective teacher either can't string a sentence together properly in English or is being pretentious. What should be there is — 'major, minor, dominant chords.
Can't think of a better reason for avoiding this teacher, or going along just to burst a bubble or two. Do you have a contact point?
Comment: While this addresses the OP's request (due to a bad description by a teacher) this is extremely unlikely to help anyone who is actually curious about dominant scales which is what the question is about.
Comment: @Dom - whilst you are technically spot on, I very very much doubt that the teacher actually knew what dominant scales are, so I responded in this way. Leaving the technicalities of the dominant scale to be explained by someone far more qualified than I...Hence the bottom line. Maybe I should be far more serious in future. Maybe...And also the actual true answer to this actual question is - 'Probably, yes'.
Comment: And I understand and this answer helps the OP which is great. I'm slightly concerned about what happens a year from now when someone actually wants to know about the dominant scale and finds this question in response. Maybe this should be a meta post as this isn't the first time what the OP wants to know is drastically different then the question actually asked,
Comment: @Dom - they read the other answers, like yours, and are enlightened. Meanwhile maybe they have a little chuckle. Maybe...


concrete drive heaves where it meets garage floor

Question: This past winter, my concrete driveway heaved 1.5" above where it meets the garage floor. It is three years old. Also, we dug along the sides of the drive to try to guage if a 4" slab was actually poured- discovered many crevices on the side. Could this cause a problem later? Specifically cracking?
Answer: Frost heaves are caused by water in the ground freezing and pushing up. The thickness of the slab has nothing to do with it. The force caused by ice expanding when it freezes is huge, and it's almost certainly not practical to resist it. Better to remove the force in the first place.
There are only two ways to prevent frost heaving, and neither is perfect:

Dig down below the frost line. This is obviously not practical for a driveway. (Also, even a foundation whose bottom is below the frost line may be susceptible to heaving if water can freeze around the sizes and pull it up.)
Reduce the amount of water underneath the structure with grading, drains, etc. A properly-prepared subsurface can also help here by reducing the water content of the soil

What is the drainage situation like around your driveway?


List of "essential prayers"?

Question: If someone is learning how to pray and wants to start with the essentials - to not be overwhelmed by too much at once - or if someone is limited in time available to pray, what are the core (most essential) prayers?
Answer: This is a great question and I am surprised we don't have it already.
Prayer is called the "service of the heart", meaning that kavana (intention) and understanding of the words are truly essential. As such, you are correct not to want to rush through but rather go-slow and understand what you say.
Experience shows that one should learn one prayer well, and as one grows in practice, it becomes possible to add more within the same time frame. One Rav experienced with baalei tshuva advises to pick a time frame for prayer that matches your abilities (say a quarter of an hour in the morning), and gradually add prayers as you go faster over those you know.
The core of morning prayers is in order of priority (based on R Eliezer Krohn's A woman's guide to practical halacha, incidentally a very recommended book)

the morning blessings (birkot haTorah and birkot haShachar at the very beginning of the prayer order)
Shema Israel (at least the first two verses, ideally all three paragraphs) - if the choice is between Shmonei Esrei and 3 paragraphs, then prefer the former
the Shmonei Esrei (or Amida)
Psukei d'Zimra (a series of psalms before Shema, the core minimum being Baruch Sheamar, Ashrei, Yishtabach)
Aleinu

Artscroll's women siddur gives a slightly different order of priority: Morning amida, afternoon amida, first two verses of Shema, blessings of shema OR morning blessings. Then Psukei d'Zimra with same priorities as above, then Aleinu. This siddur has a very nice introduction on women's prayers incl. general guidelines as well as women-specific prayers.
Depending on your level of understanding of Hebrew, you might find an interlinear siddur to be helpful, it has the words in English below the Hebrew. After a bit of practice, it is a very helpful way to understand prayers as one says them.
Any beginner is always well advised to choose a Rabbi (or Rabbanit) she feels comfortable with and who can advise on all sorts of questions that will pop up. Prayer is no different.
Comment: Would this list be different for a man?
Comment: @JoelK surely and, according to most authorities, women too. See my answer below for a more comprehensive picture.
Comment: @joelk - List of priorities for women may not be the same for men. For example, Krias Shema is biblical obligation for men but not for women, and therefore it would be highest on the list.
Comment: @Chrotkov krias shema isn't a prayer. For prayers there is no recorded distinction between men and women (at least in all extant Jewish literature till the 17th century or so, when a few rabbis first suggested as a possibility there maybe should be)
Answer: Classically, Jewish prayer was entirely free-form, spoken from the heart in the pattern of praise-request-thanks. That's still what someone does in an emergency, such as soldiers on a battlefield (Berakhot 29b). Due to practical compositional difficulties, our Sages formalized the process into what we call "the amida". In traditional rabbinic terminology, "prayer" is actually just the name of what we often call "the amida".
Prayer is thus defined as a set of topical blessings, preceded by Psalms 51:17 and followed by certain free-form supplications (commonly, "Elohai Netzor" and, on non-festive days, "Tachanun"). Some customarily add other opening verses and customs vary widely which preset nice supplications are appropriate in the closing on which occasions. This prayer is said standing
at attention (until the final supplications), kempt, and facing the site of the Temple. That is the essential Jewish prayer.
That's what you use whenever you want to pray, but our Sages enacted praying at least 3-5 times daily: once in the morning, (once additionally on festivals focused on that festival's sacrifices,) once in the afternoon, (once at the very end of special public fast days,) and once at night. (Technically, the night one can be skipped if too busy with another mitzva, but traditionally Jews have tried to do it even in that uncommon case.) It's generally assumed that nowadays our capacity for proper prayer has weakened such that we shouldn't seek more times than that.
Throughout the day there are various short blessings on foods and sights and aspects of life. In a sense they are all "mini-prayers" and everyone should also say all of them as relevant.
(In the early modern period, some rabbis bemoaned that the largely illiterate women of the time had stopped following these guidelines, and tried with great difficulty to defend the change in practice. Thankfully most women nowadays don't have this issue.)
Men are obligated to recite Shema in the first half of the morning and at night. Certain blessings were enacted to precede and follow the Shema, complimenting its themes. A woman voluntarily reciting these would be a beautiful thing too.
That's essentially it. R' Yosef Eliyahu Henkin in his introduction to the synagogue prayer guide which defined the standard practice in America writes those in a rush to work can stop here.

For those with more time/ability, there are a host of other beautiful Psalms and petitions that have ages of Jewish tradition behind them, and I'll discuss some common ones below, but for the essentials you can just stop here.
Shema and prayer compliment each other thematically and thus it is recommended to follow the shema service with a prayer.
The Talmud (Pesachim 118a) praises one who reads Psalms every day. Accordingly, as a beautiful way to set the tone for the obligatory prayer, morning services can be preceded with a set of Psalms, especially 145[-150]. Short blessings surround these.
A short study session was instituted after morning prayer to allow everyone to learn some Torah daily. Topics would vary but nowadays it is common to study Psalms 145 and 20, followed by a short petition for success.
The "Alenu" section from Rosh Hashana was seen as a beautiful way to close out the service before heading out to work, and it became widely used as a conclusion.
As each of those became more formalized, new ideas of various timely psalms and/or readings got tacked on to the standard set in appendices which themselves became formalized leading to new ideas being put in new appendices, etc.
All that can be wonderful and meaningful for those who can handle it. As Rabbi Yochanan said (Berakhot 21a) "If only one could pray all day!"
Beware though biting off more than you can chew. The earliest record we have of widespread neglect of prayers (Meshivat Nefesh (Beshallach) about 15th century German women) writes they gave it up because they thought the siddur ("from adon olam to alenu") was too long and didn't know how to focus on the essential parts.
Answer: In addition to the answers already given, I would say a good road map is to look at the shulchan aruch Siman 52 where he discusses the order of prayers for one who comes late to shul. That provides a hierarchy of importance for prayers.

List of “essential prayers”?


Does using a focal reducer shrink depth of field?

Question: When I use a 50mm f/2 lens on an APS-C camera with a focal reducer (speed booster), the angle of view is similar to a full-frame camera.  Will I also get shallower Depth of Field (more background blur), or does it remain the same?
Answer: The combined lens (50mm f/2 and a 0.71x speedbooster/telecompressor) will act like a 35mm/f1.4 lens in all regards (Same applies to teleconverters, in the other direction. lens designs exist that consist of a teleconverter or telecompressor behind a main lens, eg many 100mm macro lenses are 50mm macro lenses with a teleconverter in the back...).
DOF will be in the end dependent on aperture and magnification (subject to sensor). That truth is often arrogantly misinterpreted to say focal length doesn't matter re: background blur. It does, read on.
At THE SAME FRAMING, the actual DOF (in metres of depth) of the in-focus subject will, counterintuitively, be a bit shallower since you gained an f-stop of aperture.
A smaller acceptable circle of confusion for a smaller sensor will counteract this lightly, but in practice not by a whole stop.
The beginning and end of the DOF area around the focal plane might shift due to a change of camera to subject distance, which can also introduce apparent changes of DOF depending on what areas behind or before the focal plane matter in the picture.
Apparent background blur will still be less, since the change in distance needed for the same framing will decrease the magnification of the background.
Answer: Focal reducers convert both the focal length and aperture of the lens. You can think of it as multiplying by 1 = 0.7/0.7. Your 50/2 lens will behave like a 35/1.4 (= 50×0.7 / 2×0.7) lens when used with the focal reducer.
Although DOF and background blur tend to be inversely related (minimizing DOF will maximize background blur), they are different concepts. The formulas used to calculate them are different.

Depth of field is based on focal length, aperture, distance, and a predefined acceptable sharpness level. It is concerned with what parts of the image are expected to be sharp, not what parts of the image will be blurry, or how blurry unsharp portions of the image will be.
What happens to DOF depends not only on focal length and aperture, but also what else you do to accommodate the lens change.

DOF = 2 u2 N C / f2  (from Wikipedia)
N = aperture F-number
C = circle of confusion
u = distance to subject
f = focal length

Background blur – How blurry is the background? How big are bokeh balls a given distance from the lens? Different lenses with the same focal lengths, apertures, and distances can create different, though similar, amounts of blur because of different amounts of distortion, aberration, and field curvature. (There's also foreground blur, but people tend to be less interested.)

b = f ms xd / (N (s + xd))
b = blur
f = focal length
N = aperture F-number
ms = subject magnification
xd = distance between subject and background
s = subject distance

As the subject-background distance increases, the formula simplifies to: b = f2 / N s.  If the changes in focal length (f) and subject distance (s) are proportional, to maintain subject size within the frame, background blur is proportional to f/N (the physical aperture size).

Other related concepts are Bokeh and Subject-background isolation.

Bokeh is a qualitative description of the blur that is produced. Are the bokeh balls round? Do they vary in shape throughout the frame? Are they smooth? Do they have edge highlights? Are they smeared? Some people refer to how lenses "render" images.

Subject-background isolation refers to (subjectively) how well the subject stands out from the background. This can be achieved with depth of field and background blur, as well as appropriate lighting (such as rim lighting and creative use of "glow"). The common formula is to try to use narrow depth of field with high background blur. However, some types of bokeh can achieve good subject isolation with high depth of field and low background blur. For instance, Sonnar lenses create bokeh balls with an edge highlight toward the center of the frame, but a smear toward the edge of the frame. This tends to emphasize the sharpness of the subject toward the center, while also emphasizing the blurriness of the background toward the periphery.
Answer: We measure the potential "speed" of a camera lens using the f-number system. The f-number is derived by dividing the focal length of the lens by the working diameter of the lens aperture (iris).
As an example, a 50mm focal length lens with its aperture set to 25mm is operating at f/2.
If we wish this lens was faster, we can mount a "speed buster". What happens is, this supplemental lens shortens the focal length of the primary lens. Most "speed buster" lenses reduce the focal length about 70%. Thus, a 50mm camera lens is converted 50mm X .7 = 35mm.
Now on a full frame "FF" 50mm on a full frame is considered "normal". By "normal", we mean it will deliver an image with a perspective that closely replicates what we see with our unaided eyes.
If we mount a 35mm focal length lens on a full frame the resulting view is now classified as wide-angle. Please note: a 35mm mounted on a full frame delivers a moderate wide-angle. The shorter the focal length the wider the view.
The point is, the "speed buster" makes an adjustment, it shortens the focal length, and this widens the angle of view. Because the working focal length has been shortened, we must recalculate the f-number. Should the working diameter of iris remain unchanged, the math is 35 ÷ 25 = 1.4. The key point is the original and unmodified 50mm lens with a 25mm working iris diameter is operating at f/2. With the "speed booster" in place it becomes a 35mm operating at f/1.4. This is a gain of 1 f-stop in speed. A 1-f-stop gain doubles the amount of light that traverses the lens. This the "speed boost".
The angle of view of a camera lens is a function of focal length and format size.
If a 50mm is mounted on a FF (full frame 24mm by 36mm format), the angles of view will be 27° height, 40° length, 47° diagonal (the diagonal is the value most often published.
Mount a 35mm on a FF and the angles of view are 38° height, 55° length, 63° diagonal.
Mount a 50mm on a APS 16mm height by 24mm length the angles are 18° height, 38° length, 32° diagonal
Mount a 35mm on a APS angles are 26° height, 38 length, 45° diagonal
As to depth-of-field – As a general rule-of-thumb, shorter focal length lenses ill exhibit a greater span of depth-of-field. Thus the "speed booster" causes the lens to gain speed while at the same time reducing the focal length which grants increased depth-of-field.
Comment: There is no change in DOF (lens gives same amount of background bloor) when using speed booster, only angle of view change?
Answer: I. Short and step by step

When you put a 0.71 focal reducer (speed booster) on a a 50mm f/2 lens, the combo (lens + reducer) act as a 35mm f1.4 lens (1).

When you use a combo (lens + focal recuder) on an APS-C camera:

The angle of view is similar the one when the 50mm is used on a full-frame camera.

A gain of 1 stop in exposure is made:  if the lens as an aperture ring: f2 on ring is becomes 1.4 on the sensor(2)(3).
This point is often overlook but has a very practical consequence: you gain 1/stop for ISO so at the end regarding the noise, this one will be more or lens similar than the on on the full frame.

The depth of field is similar the one of the 50mm on a full-frame camera(4)(5)

II. On the depth of field
The depth of field on a digital camera is a delicate subject because there are two different approaches.
The first, which has no equivalent in the field of analog photography, corresponds to the best possible image taking into account the characteristics of the sensor, i.e. the optical image on a sensor of a point in the scene must be smaller than the size of a pixel on this sensor.
The second, which is the one that will be discussed next, s related to the perception of a person looking at the image, the observer, looking at the image on a print, or possibly a screen assuming that the definition of this screen is such that the observer does not perceive the pixels of that screen.
It is the first definition that is used here.
III. Formulas
Assuming that the distance to subject does not change
Values with a k index correspond to the case of a crop factor k;
With:

Focal (mm): f
Aperture: N
Entrance pupil diameter (mm): d
Circle of confusion (mm): c
Distance to the subject (mm): D

Aperture
From N = f / d, it follows that Nk = fk / dk = (f / k) / d = (f / d) / k = N / k.
Depth of field
The size of the spot of the real point is a function of d (see Depth of Field Outside the Box by Richard F. Lyon), to have the same blur effect it is thus necessary that dk = d.
To be convinced of this, one can make the calculation in the formulas for the limit sharp plans in standard cases, i.e. outside of macro photography:

Front Sharp Plan (mm) : FSP = D H / (H + D)
Back Sharp Plan (mm) : BSP = D H / (H - D)
and
Hyperfocal (mm) : H = f²/(c N)

In order to maintain the size of the object point during visualization (on a print), the relationship between the confusion circles c and ck must be such that ck = c / k
It follows that:
Hk = fk² / (ck Nk) = (f / k)² / ((c / k) (N / k)) = f² / (c N) = H

QED

Notes
(1) Obviously the size of the image is also reduce, but this point is not relevant when a smaller sensor is used.
(2) I do not know if specific reducers modify the data so that the aperture displayed by the camera is or not modified.
(3) With a small approximation as the simple presence of new glass elements will reduce very marginally the light
(4) With a small approximation because 0.7 is not exactly equal to 1/1.5.


What to do with duplicates, both are answered and accepted questions?

Question: What should I do if I saw two old questions being duplicates both having answers and both having an accepted answer? Should I still vote to close one of them? Both?
Answer: By all means vote to close.
It might be worth flagging one of the questions for moderator attention so that they can merge them, though the questions really have to be identical for this to work.
Answer: IMHO, the idea behind closing-as-duplicate is to help OP (and future googlers) to get quality answer without reinventing the wheel. From this point of view, closing n months old question already having adequate answer is useless.
Also, it was noted many times by the founders that some amount of duplication is acceptable.


Wave propagation through conductors

Question: During wave propagation through conductor, why is the current density out of phase with electric field ?
When the oscillating electric field applies a driving force on the free electrons, they will start oscillating. Then the current density should be in phase with electric field , isn't it ?
Answer: The effect is identical to that of a harmonic oscillator. The electric field accelerates the electrons similar to how the spring accelerates the mass at the end. In the harmonic oscillator, too, the velocity is the highest when the force is lowest, i.e. when the displacement is zero.
If I have to guess at the origin of your confusion, you are trying to apply Ohms law $J=\sigma E$, but this applies in a different situation than what you describe. The reason is as follows. Electrons cannot move freely though the conductor, but they will collide with the atoms, meaning they cannot accelerate indefinitely. How often they collide gives the value of $\sigma$ in Ohms law. If you create an electric field that oscillates very rapidly, the electrons will not accelerate to high speeds, and will not travel for great lengths, so these collision effects will not be large. In this case, the behaviour will be similar to the harmonic oscillator, rather than to what Ohms law describes.

Wave propagation through conductors