The wire is at 50hz, and is an aluminum alloy (AAAC 1120). Resistance in the wire is at 0.0897 ohm/km. So for a length of conductor 135km long, the resultant resistance will be 12.1095 ohm. It's AC current at about 220 KV. Other data about the wire can be found . The line will be carrying roughly 15 MW of current.   How fast (in any measurement, but in an actual number) would the current move through the wire? I know about electron drift already, what I need to know is how fast it would travel through a wire with those specs, so if a pulse starts/stops how long it takes for this change to reach the other point 135 km downstream.   Edit: have checked possible duplicates, and while they do cover the same matter, i'm looking for a number or a fraction. While I know it's a fairly high portion of the speed of light, it's not an actual number. Some say 2/3, some say 1/10, some say 8/10, and when you deal with almost 300000km/s it's a large difference. They all say it depends on the wire, and aluminum alloy wire for large amounts of current is no doubt very different to standard copper.
42056
On a systems level, I understand that as electrons are pushed into a wire, there is a net field and a net electron velocity.  And I've read that the net electron drift is slow.  But electricity travels through the wire, essentially at c, and I want to understand that mechanism.  My apologies if my question is poorly stated, I know bare bones undergraduate quantum theory circa 1990s but it doesn't explain the motion of electricity in detail.  Here's my conception, I'm hoping someone will fill in the holes, ha ha.  An electron moves into the wire.  It's got a kinetic energy.  After travelling a short distance, it spontaneously emits a photon, which hits another electron in a valence shell.  That electron then presumably does the same.  If this conception is simply wrong, please enlighten me.  The questions that arise:   Presumably at this level, electrons are acting more like waves and less like particles, but is there any classical component in the picture, ie are electrons coming in imparting other electrons with kinetic energy through repulsion, or does it not work that way? If electrons momentarily have energy, then pass it on by a photon, what determines when that photon is emitted, and what frequency it will be?  I assume that electrons in this cloud are not limited by any kind of exclusion principal, and that any frequencies are possible? Why should a photon emitted by an electron be in the direction of travel?  Conservation of momentum tells me that if an electron is moving, the photon should be emitted in that direction, slowing the electron, but could an electron emit a photon in the opposite direction?  If it did, I assume it would somehow have had to absorb energy from elsewhere? That sounds possible by analogy with quantum tunneling. What is the mechanism by which electrons propagating increase the temperature of the material?  Are they transmitting energy to the electrons in the valence shell, which tug at the nucleus, do some photons hit the nuclei directly, or is there some other way?  Presumably, electricity travels slower than light, because there is some time in each exchange, and some time when electrons are moving at sublight speeds before emitting a photon.  By how much is this slower than light, and what is the speed of each interaction?
117626
This is a question that I've had for a while but have somehow managed to go without asking. Looking at all the wire gauge ampacity charts online, some of them appear to provide vastly different ratings for the same wire gauge. I understand this may be due to different standards, temperature, conductor types, strand count, etc.   However, lets looking for the current rating associated with 10AWG copper conductor at a low operating temperature (60c):      I've only cited two sources, however, I've come across many other charts that provide this 15 to 30 amp difference. What gives?
83500
How does alternating current get anywhere if it travels "back and forth" in the wire?. I understand that Direct Current is a flow of electrons through a wire, but I have always been confused with how Alternating Current works. I expect I am missing something simple but could anyone point me to a resource that does a good job of explaining AC, or perhaps offer an explanation themselves? Thanks!
100149
In the following image:   Why can't current flow across the following wire? It's a simple question, but I've kind off always wondered.  Thanks!
76621
Well, I have the following circuit:         &ndash; Schematic created using   When the switch is closed for a long time, the current that is flowing trough the resistor \$R_1\$ and the coil is given by:  $$\text{I}_\infty=\frac{\text{V}}{\text{R}_1}=\frac{10}{100}=\frac{1}{10}\tag1$$  With that given I can solve what happens when I open the switch. The current trough the coil will stay in the same direction as when the switch is closed. Now, at the moment when I open the switch the voltage across the coil \$\text{V}_{\text{Y}_1}\$, is given as:  $$\text{V}_{\text{Y}_1}=-\text{V}\cdot\frac{\text{R}_1+\text{R}_2}{\text{R}_1}\cdot\exp\left(-\frac{\text{R}_1+\text{R}_2}{\text{L}}\cdot t\right)\tag2$$     So, the voltage over the coil will start (when I open the switch) at \$-\text{V}\cdot\frac{\text{R}_1+\text{R}_2}{\text{R}_1}=-10\cdot\frac{100+100}{100}=-20 V\$, which is (absolutely seen) higher than the supply voltage. How is that possible? Or is the formula \$(2)\$ wrong?
13637
Drift velocity (explained to me as how fast the electrons are moving) is really slow. My book says the electrons move at around 10 mm/ s.  If electrons move so slowly how do circuits work so fast? If you make a basic circuit with just a light bulb, the bulb lights up almost immediately after you connect the wire to the positive terminal.
112974
It's been happening to me for years. I finally decided to ask users who are better with "practical physics" when I was told that my experience – that I am going to describe momentarily – prove that I am a diviner, a psychic, a "sensibil" as we call it. The right explanation clearly needs some electrodynamics although it's "everyday electrodynamics" and theoretical physicists are not trained to quickly answer such questions although each of us has probably solved many exercises that depend on the same principles.    When I am biking under the power lines – which probably have a high voltage in them – I feel a clear tingling shocks near my buttocks and related parts of the body for a second or so when I am under a critical point of the power lines. It is a strong feeling, not a marginal one: it feels like a dozen of ants that are stinging me at the same moment. It seems almost clear that some currents are running through my skins at 50 Hz. I would like to know the estimate (and calculation or justification) of the voltage, currents etc. that are going through my skin and some comparison with the shock one gets when he touches the power outlet.  Now,    my bike that makes this effect particularly strong is a ; the speed is about 20 km/h and the velocity is perpendicular to the direction of the current in the power line; the seat has a hole in it and there is some metal – probably a conducting one – just a few centimeters away from the center of my buttocks. It's plausible that I am in touch with the metal – or near touch; my skin is kind of sweating during these events and the liquid isn't pure water so it's probably much more conductive than pure water; the temperature was 22 °C today, the humidity around 35%, clear skies, 10 km/h wind;  may be between 22 kV and 1 MV and at 50 Hz, the altitude is tens of meters but I don't really know exactly.   What kind of approximation for the electromagnetic waves are relevant? What is the strength? How high currents one needs?   Does one need some amplification from interference etc. (special places) to make the effect detectable? (I only remember experiencing this effect at two places around Pilsen; the most frequent place where I feel it is near Druztová, Greater Pilsen, Czechia.)   Is the motion of the wheels or even its frequency important? Is there some resonance?  Does the hole in the seat and the metal play any role? Just if you think that I am crazy, other people are experience the effect (although with different body parts), see e.g.  and .  seems to suggest that the metals and electromagnetic induction is essential for the effect but the presentation looks neither particularly comprehensive nor impartial enough.  An extra blog discussion on this topic is here:
18346
I am a licensed radio amateur, and find bewildering the many different explanations, which range from folksy urban myth to Maxwell-Heaviside Equations, of what happens at the termination of a transmission line or feeder. I realise that they all come to the same thing in the end (or should do, pun perfect), but none of them give me a gut feeling for what is going on.  I like diagrams, so an answer in terms of (graphical) phasors for the currents and voltages at the load would suit me best. How, for instance, does a step pulse down the line cause twice the voltage at an open circuit termination? Similarly for current at a short circuit. And how is the reflected step generated by the inductance and capacitance of the line?  Can anyone help, without getting all mathematical, and not telling any "lies to children"?
1365
I understand all the concepts of what voltage is using all the analogies but some things related to the drop of voltage across a circuit confuses me.   If I had a short circuit and attached a voltmeter I would get a potential difference reading of 0 volts. How is current then going through the wire if it is 'X' volts at any point in the wire? Let's say I have a DC 9V battery with a load in the middle. I read somewhere (probably mistaken here) that the voltage drop in this situation must be 9V or rather, that the sum of load resistances must be equal to voltage of the source. I mean, there is a variable amount of resistance to each load so at the base of the load I might have 9V and at the end of the load I might have 5V with a 4V potential difference.
43576
It took me a long time to understand that current is constant throughout the circuit. And now I am stuck at one last thing: The speed of charge. Here is what I am thinking:  Current is defined as quantity of charge passing through a point at a given time, and from this we can deduce that current is constant throughout a circuit. But, if this is true, then wouldn't the speed of charge through a resistor be increased? Let us think about an analogy, water flowing through a pipe. Suppose water is flowing through a broader pipe(analogous to a less-resistance wire), and  a greater quantity of water can flow past a point because of larger cross-sectional area. Now, suddenly water encounters a smaller pipe(analogous to the resistor). So, less quantity of charge can flow through it because of less cross-sectional area. But, if the volume of water flowing in a time $T$(analogous to current) is same, won't the rate of flow of water in the narrower pipe(the resistor) be more?  I am asking this question because while seeing some answers for the question that "Why is current constant?" , I saw some people saying that speed of charge slows down a bit through a resistor, which seems incorrect to me.
95275
This is a summary from Physics World of the paper: L J Wang et al. 2000 Nature 406 277-- "Wang and colleagues begin by using a third continuous-wave laser to confirm that there are two peaks in the gain spectrum and that the refractive index does indeed change rapidly with wavelength in between. Next they send a 3.7-microsecond long laser pulse into the caesium cell, which is 6 centimetres long, and show that, at the correct wavelength, it emerges from the cell 62 nanoseconds sooner than would be expected if it had travelled at the speed of light. 62 nanoseconds might not sound like much, but since it should only take 0.2 nanoseconds for the pulse to pass through the cell, this means that the pulse has been travelling at 310 times the speed of light. Moreover, unlike previous superluminal experiments, the input and output pulse shapes are essentially the same."   I realise that the velocity of light in a medium is comprised of the phase velocity, the group velocity, and the front velocity. While the group velocity can exceed the value of c in a vacuum, the front velocity is not supposed to. The way this sounds, the front velocity is exceeding c in a vacuum by 310 times.    blue dot=phase velocity, green=group, red=front (wiki)  The wavepacket seems to exit the cell well before it enters, but the negative refractive index "forward shifts" the leading edge of the pulse. It is argued that although this is superluminal, information cannot be transmitted faster than c.  introduced a jump discontinuity into the waveform.Its max speed was c Here is a popular account from  . What if you used one photon? What if the photon itself were the information and not a carrier?
62984
I understand that this is a really basic question, but, I've been unable to really understand electrical terminology for some reason. What exactly is an amp? How does it correspond to a watt?  And: How does a watt correspond to a volt? What does the frequency of electricity refer to? (Hz, 50/60)  Please be as specific, and laymeny as possible :).  Thank you!  Edit: Since I didn't get anyone yelling at me for talking about adding a few more questions, here goes:  What is a volt? My understanding of it is the difference between the neutral and phase, but, this doesn't make it any clearer. If anyone can help out with more information, with an analogy or two, plus some technical information, I'd appreciate it very much!  What does the "earth"/ground portion of the circuit do? How does it help safety? If it helps safety, why aren't all plugs 3-pin instead of 2-pin?  Hypothetical:  If I plugged in two wires to a socket, and put the other end into something conductive (for instance, a bucket of water), assuming the breaker doesn't go off (if it would, why would it?) after I remove said wires(or turn off the socket), does the water "hold" the charge? Is it in any way unsafe?  For what it's worth, I'm assuming no. The current flows from phase-neutral, the water simply facilitates this. Right? If I'm right, I assume this means that I am in some way "using" the electricity? How does this differ if I for instance put the wires up against a block of wood? How does it differ if I stick the wires on myself?  I apologize if some of these questions show a fundamental mis-understanding of electricity. If they do, smack me across the head and point me to a resource that will educate me and help me answer such questions myself. These questions have plagued the back of my mind for a while, but I've never found the right place to ask.  I have more such questions, both hypothetical, and real (how does a stabilizer work, why is it required? Why do motors require 2x as much electricity to start up vs to run? Why can't this be optimized/why hasn't it been fixed yet?). I'm hoping I'll be able to answer the hypothetical ones myself once I gain more of an understanding, with your help.  I hope questions at these level (and their answers) are useful to at least one other person!  Again, Thanks to whoever helps me out!
41980
If you have a circuit of a battery, a Resistance and a light connected with a wire by example: in the middle the wire is cutted in two pieces. if you connect each piece of wire with a metal object and put those twee metal objects together so the current can flow to the resistance and the light. How is it possible that the electrons can flow from the wire to one of the metal objects into the other metal object and again in the wire? I do understand how electrons move in metals and wires, but i don't know how the transaction of electrons between those components goes. Is it really that simple that electrons just can flow from one object into another of is the (kinetic) energy passed through when an electron clashes with the edge and that energie is passed through to the other object?
89181
I have just finished learning the basics of magnetism, and it should be noted that I am not very familiar with Maxwell's equations.  Note:    In the question, when I say , I am referring to the equation without Maxwell's correction. Also, when I say , I am referring to the equation: $\mathrm dB= (\mu_0/4\pi)(I)(\mathrm dL~ X~\hat r)/r^2$     Consider an infinitely long straight wire, carrying a time varying current I(t) such that dI(t)/dt is non-zero. Also consider a point P which is at a distance r from the wire. Using Biot Savart Law, we find out that the magnetic field is $\mu_0\cdot I(t)/2\pi \cdot r$, at any instant t.   Now, I have read that Ampere's Circuital law is NOT valid for cases in which the currents are time varying. However, if we consider an Amperian loop along a circle of radius r and centre at the perpendicular from P to the wire, using symmetry arguments, we obtain the same value of field: $\mu_0 I(t)/2\pi\cdot r$. Since Ampere's law is invalid for such a current, the expression mentioned for the magnetic field must be incorrect.  So, can Biot Savart Law also NOT be used for time varying currents? Also, just out of curiosity, what would be the actual value of the magnetic field at time t?  My book (Halliday and Resnick) derives the equation for the magnetic field created due to a moving point charge. However, after the derivation, it states that the result obtained is not really valid, since "a point charge cannot be assumed as a steady current by any stretch of imagination". This makes me believe that even Biot-Savart Law is only true for non time varying currents. Am I right or wrong?
120006
So, for my science fair I had to test the gauge of a copper wire vs electrical resistance. What I did was I set up a series circuit involving a 6v battery, a light bulb, a multimeter, and a voltmeter. I would change out three different wires with three different gauges.   For the 4-gauge wire, I got 0.004V and 0.04A. For the 8-gauge wire, I got 0.003V and 0.04A For the 12-gauge wire, I got 0.001V and 0.04A.   In this experiment, what would the independent and dependent variables?  We also have to fill out a table like this:
100113
Remark: This is not a homework question...It is pure out of theoretical interest. I asked this the mathematics-community a couple days ago and got no answer, so I figured I'd try here.  Most standard physics textbooks compute the force two infinite wires exert on each other, but they remain silent about the case where the wires are finite. Let's say we have two parallel wires carrying a current of equal magnitude in the same direction, both of which have a length $d$ and also seperated by a distance $d$. I now want to find out the force one wire exerts on another, using the Biot-Savart Law.  Let the left wire be positioned at the origin of the $xy$-plane, going along the $y$-axis, and let the other wire be a distance $d$ to the right. We assume the currents are flowing in the positive $y$-direction. Then we first choose a source element (on the left wire) of infinitesimal length $dy$ described by the position vector $\mathbf{r_0} = y_0 \hat{j}$. This constitutes a current source of $I d\vec{l} = (Idy) \hat{j}$.  We then pick an arbitrary field point $P$ on the other wire with position vector $\mathbf{r_p} = x\hat{i} + y \hat{j}$. Then the position vector $\mathbf{r}$ pointing from the source point to the field point is given as  \begin{align*}  \mathbf{r} = \mathbf{r_p} - \mathbf{r_0} = x\hat{i} + (y - y_0) \hat{j}, \end{align*} with $\sqrt{x^2 + (y-y_0)^2}$ being the length of this vector. If we now calculate the crossproduct $d\vec{l} \times \mathbf{r}$, we can write it as \begin{align*} dy \hat{j} \times (x\hat{i} + (y - y_0)\hat{j}) = -dy x \hat{k} \end{align*} Now comes the tricky part. I think I need to setup a double integral, because we are working with infinitesimal force elements $d\mathbf{F}$, each which is given as  $d\mathbf{F} = I d\vec{l} \times \mathbf{B}$. But we also have that \begin{align*} d\mathbf{B} = \frac{\mu_0 I}{4 \pi} \frac{d \vec{l} \times \hat{r}}{r^2}  = \frac{\mu_0 I}{4 \pi} \frac{d \vec{l} \times \mathbf{r}}{r^3} = -\frac{\mu_0 I}{4 \pi} \frac{dyx}{\sqrt{x^2 + (y-y_0)^2}} \hat{k} \end{align*} Hence I need to somehow integrate over $d\mathbf{F}$ and $d\mathbf{B}$. Does anyone have an idea how to do this?
50914
I have 3 12VDC/40A automotive relays () that I want to use with my Arduino. Based on the tutorial I am following () I need transistor, resistor, and diode. I am not an electrical engineer therefore I am unsure about the parts and calculations I made. For the start, relay coil resistance is 90+-10% Ohm per datasheet. So I proceed by calculating the current flow.  Voltage=Resistance*Current Current=Voltage/Resistance Current=12V/90 Ohm Current = 133mA  For the transistor I can get 2N3904 or 2N4401. At this point I have to calculate resistance for the base of transistor. In tutorial its as following  hfe = Ic / Ib Ib = Ic / hfe Ib = 0.03 A / 75 Ib = 0.0004 A =&gt; 0.4 mA R1 = U / Ib R1 = 5V / 0.0004 A R1 = 12500 Ohm  2N3904 datasheet states the H(fe) is 30-300 when lc = 100mA (mine is 130mA) and Vce = 1V. At this point I have no clue what is going on, thus I need help. Edit: Here is what I ended up with. RLY1 in the picture is 12VDC/40A ()