What exactly is charge?
Origin of electric charge
How can a file size be zero? Just something I ran into and couldn't think of a proper explanation. If I create an empty *.txt file on my PC and then look at its size, it shows 0. But how is that possible? I mean even if the file itself is empty, it still must have some size, just to store its own name. How can this be explained? (Non OS specific) Why can I put an electrolytic capacitor on AC? I have seen here and there schematics with electrolytic capacitors put on AC. And this sounds weird to me.  Electrolytic capacitors have a polarity, right? If we invert the polarities on DC, bad things happen. As far as I understand, AC inverts polarity every now and then (commonly 50Hz). Why can we put such capacitors on AC without damaging them?  Example:    from demonstration here: Side effects of changing blackboard bold font with \setmathfont I'm  trying to create a FPS game in blender game engine. When I do KeyBoard[Spacebar] sensor and hook it up to a simple motion of z +1 or something like that, the guys pops up really fast as if he didn't have any motion between being on the ground and being in the air. Is there a way to make a more realistic, more slow jump in BGE? Why did JJ Abrams recreate the Star Trek Timeline? In the 2009 version of Star Trek, Nero and the Romulans come back through time via Blackhole and kill Kirk's dad and the original Star Trek timeline is altered  creating a whole new reality.  Why would Abrams not just stick to the timeline given by Roddenberry instead of altering it? Show that $\mathbb{Z}[i]/(2+3i)$ is a finite field.    By applying some isomorphisms and realizing $\mathbb{Z}[i]$ as the quotient ring of $\mathbb{Z}[x]/(x^{2}+1)$, I found that:  $\mathbb{Z}[i]/(2+3i) \cong \mathbb{Z}[x]/(x^2+1,2+3x)$  However, I run into trouble as I can't divide $x^{2}+1$ by $2+3x$ in $\mathbb{Z}[x]$. What would be another way to approach the problem?  Thanks for the help. I am currently using a macbook pro but the operating system is debian 8.2 not os x  I am using a macbook pro but there's this one device that's just draining a lot of power. This USB device: Card Reader (Apple)   32.5 W    100.0%                      Device         USB device: Card Reader (Apple)   2.60 W    100.0%                      Device         Radio device: btusb   209 mW     19.7 pkts/s                Device         Network interface: wlan0 (wl)   This device really drains my battery, I noticed that my laptop wouldn't even get 2 hours when not plugged in, I installed powertop. Made all the recommended changes but this device, I can't find out how to disable it. I don't even use that reader and forgot that it was there.  I configured all my devices to use limited power in my rc.local  echo '0' &gt; '/proc/sys/kernel/nmi_watchdog' echo 'min_power' &gt; '/sys/class/scsi_host/host0/link_power_management_policy' echo '1' &gt; '/sys/module/snd_hda_intel/parameters/power_save' echo '1500' &gt; '/proc/sys/vm/dirty_writeback_centisecs' echo 'auto' &gt; '/sys/bus/usb/devices/2-4/power/control' echo 'auto' &gt; '/sys/bus/usb/devices/1-12/power/control' echo 'auto' &gt; '/sys/bus/usb/devices/1-8.3/power/control' echo 'auto' &gt; '/sys/bus/pci/devices/0000:00:1f.3/power/control' echo 'auto' &gt; '/sys/bus/pci/devices/0000:01:00.1/power/control' echo 'auto' &gt; '/sys/bus/pci/devices/0000:00:1c.3/power/control' echo 'auto' &gt; '/sys/bus/pci/devices/0000:00:1c.4/power/control' echo 'auto' &gt; '/sys/bus/pci/devices/0000:00:1c.2/power/control' echo 'auto' &gt; '/sys/bus/pci/devices/0000:00:1c.0/power/control' echo 'auto' &gt; '/sys/bus/pci/devices/0000:00:1b.0/power/control' echo 'auto' &gt; '/sys/bus/pci/devices/0000:00:16.0/power/control' echo 'auto' &gt; '/sys/bus/pci/devices/0000:00:14.0/power/control' echo 'auto' &gt; '/sys/bus/pci/devices/0000:00:01.1/power/control' echo 'auto' &gt; '/sys/bus/pci/devices/0000:00:01.0/power/control' echo 'auto' &gt; '/sys/bus/pci/devices/0000:00:00.0/power/control' echo 'auto' &gt; '/sys/bus/pci/devices/0000:01:00.0/power/control' echo 'auto' &gt; '/sys/bus/pci/devices/0000:08:00.0/power/control' echo 'auto' &gt; '/sys/bus/pci/devices/0000:07:06.0/power/control' echo 'auto' &gt; '/sys/bus/pci/devices/0000:07:05.0/power/control' echo 'auto' &gt; '/sys/bus/pci/devices/0000:07:04.0/power/control' echo 'auto' &gt; '/sys/bus/pci/devices/0000:07:03.0/power/control' echo 'auto' &gt; '/sys/bus/pci/devices/0000:07:00.0/power/control' echo 'auto' &gt; '/sys/bus/pci/devices/0000:06:00.0/power/control' echo 'auto' &gt; '/sys/bus/pci/devices/0000:05:00.0/power/control' echo 'auto' &gt; '/sys/bus/pci/devices/0000:04:00.0/power/control' echo 'auto' &gt; '/sys/bus/pci/devices/0000:03:00.0/power/control' echo 'auto' &gt; '/sys/bus/pci/devices/0000:00:1f.0/power/control'   How can I get this device to act sanely or disable it completely because I  haven't used it. "who" vs. "whom" and when to omit the relative pronoun This is the boy who was asked by the teacher to stay back after school. This is the boy whom the teacher asked to stay back after school. This is the boy the teacher asked to stay back after school.   Are the sentences above correct ? How do that charge gets transferred while rubbing? NSXMLParser chokes on ampersand & Handling registration ID changes in Google Cloud Messaging on Android Calculate limiting distribution of $\frac{\sum_{i=1}^n X_i}{\sum_{i=1}^n Y_i}$ I am not asking about the two factor authentication process where the user needs to enter both a password and a one-time-password.  Gmail recently (I don't know from when exactly) changed their login process which is depicted in the following screenshot:     So, the new steps are as follows:   Enter email   Click Next  Enter password   Click Sign In    as opposed to the previous and usual way of logging in:   Enter email and password   Click Sign In    Isn't the new process non-user friendly requiring extra user interactions?  Since, Google does not usually make UX blunders can you explain the reasoning behind the new process? I'm looking for details like user interaction simplicity or any other hidden advantages.  Note: The question  doesn't cover the reason behind the change while Mervin Johnsingh's answer cites Google's reason. Solve $x^2+5x+6 \equiv 0 \pmod{\!11\cdot 17}$ Solve $x^2+5x+6 \equiv 187 \mod 187$ Solution $$x^2+5x+6 \equiv 187 \mod 187$$ $$ (x+\frac{5}{2})^2 \equiv \frac{1}{4}$$ $$ 4(x+\frac{5}{2})^2 \equiv 1$$ $$ y:= x+\frac{5}{2} $$ $$ 4y^2 \equiv 1 \mod 11 \wedge 4y^2 \equiv 1 \mod 17  $$ $$ ( 2y \equiv 1 \mod 11 \vee 2y \equiv 10 \mod 11 ) \wedge ( 2y \equiv 1 \mod 17  \vee 2y \equiv 13 \mod 17)  $$ $$ ( y \equiv 6 \mod 11 \vee y \equiv 5 \mod 11 ) \wedge ( y \equiv 9 \mod 17  \vee y \equiv 15 \mod 17)  $$ Combining that from CRT I got: $$ y \in \left\{49, 60,83,94 \right\} $$ and for example: $$ x+\frac{5}{2}  \equiv 94 \mod 187$$ $$ 2x  \equiv 183 \mod 187$$ some calculus and get... $$x \equiv 185 $$ And the same thing for each other case. Question Is there any faster (or smarter) way to solve equations like that? What is the alternative for Teamviewer? Is there any Ubuntu alternative for team-viewer software for Windows? Centrifugal force acting on ring Take for example the case of a rod rotating about an axis passing through its centre of mass and perpendicular to it. It has a ring hung from one of its sides. The rotation of the rod causes the ring to move out wards and ultimately fall off the rod.   I asked my teacher what causes the ring to move out wards and he said the centrifugal force acting in the frame of reference of the ring causes motion. What exactly is the ring reference frame? Is it a rotating frame? How would u visualize motion in this frame of reference? Quoted vs unquoted string expansion Fibonacci divisibilty properties $ F_n\mid F_{kn},\,$ $\, \gcd(F_n,F_m) = F_{\gcd(n,m)}$ Can any one give a generalization of the following properties in a single proof? I have checked the results, which I have given below by trial and error method. I am looking for a general proof, which will cover the all my results below:   Every third Fibonacci number is even. 3 divides every 4th Fibonacci number. 5 divides every 5th Fibonacci number. 4 divides every 6th Fibonacci number. 13 divides every 7th Fibonacci number. 7 divides every 8th Fibonacci number. 17 divides every 9th Fibonacci number. 11 divides every 10th Fibonacci number. 6, 9, 12 and 16 divides every 12th Fibonacci number. 29 divides every 14th Fibonacci number. 10 and 61 divides every 15th Fibonacci number. 15 divides every 20th Fibonacci number. How to make a pattern follow a stroke, path or line? Here is my MS Paint job on a real bad attempt at what I'm trying to achieve:    I have access to Photoshop and Illustrator CS6, and I want to be able to draw a thick line (that is not straight) and have a pattern within the line show arrowheads within the line to indicate the direction of the line. Correlations between continuous and categorical (nominal) variables I would like to find the correlation between a continuous (dependent variable) and a categorical (nominal: gender, independent variable) variable. Continuous data is not normally distributed. Before, I had computed it using the Spearman's $\rho$. However, I have been told that it is not right.  While searching on the internet, I found that the boxplot can provide an idea about how much they are associated; however, I was looking for a quantified value such as Pearson's product moment coefficient or Spearman's $\rho$. Can you please help me on how to do this? Or, inform on which method would be appropriate?  Would Point Biserial Coefficient be the right option?