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How electricity moves through wires

  1. Sep 7, 2012 #1
    Can electrical currents in a wire affect eachother? Imagine racecars and how they affect eachother in a race for this. Reply in simple english please.
     
    Last edited: Sep 7, 2012
  2. jcsd
  3. Sep 7, 2012 #2
    When you are talking about "electrical signals" I assume you mean the movement of electrons (which creates current). Electrons move through a wire in a very randomized motion and are always interacting with other electrons and the atoms inside the wire. The electrons in reality move forward, back, up, down and in all sorts of "random" motion. However, if there is a voltage between the ends of the wire, the electrons will in the end have an average velocity in one direction. So yes, the electrons are constantly being pushed around by other electrons and atoms, but ultimately have an average velocity (also called drift velocity) in one direction.
    Here are some good links that can help you understand [some details I left out]-
    http://pfnicholls.com/physics/current.html
     
    Last edited by a moderator: Sep 25, 2014
  4. Sep 8, 2012 #3
    I changed it to electrical currents because that's what I meant, sorry. I'll look at the links soon.
     
    Last edited by a moderator: Sep 25, 2014
  5. Sep 8, 2012 #4
    I looked at the links and I know where to go if I want to learn about drift velocity, I appreciate it. What I meant was the electrical current but I can use the information about drift velocity too, thanks!
     
    Last edited by a moderator: Sep 25, 2014
  6. Sep 8, 2012 #5
    I don't know of any easy way to explain this, it is not the movement of electrons in a pure transporting sense. If it is from moving of electrons, it will take a long time to go from one end of the wire to the other end. Drift velocity is very slow, the better the conductor, the slower electrons move.

    If you apply a voltage at one end of the wire, you'll see the voltage at the other end of the wire traveling at almost light speed from the applied end to the measuring end. It is the EM wave that travel. Remember to every source, there must be a return path of electricity. The forward wire and the return wire form a wave guide that EM wave propagate in between. The voltage and current is the consequence of the boundary condition of the EM wave..........


    But of cause I can just tell you that when you apply a voltage at one end, the current travel from one end of the wire to the other end. That is what most people thing about and how most of the books' explanation. For the most part, it is good enough and they developed most of the formula using this concept. Just remember my first paragraph, if it is really electron movement, you can inject an electron at one end of the short cable, you'll have time to get a cup of coffee, then come back and wait for that electron to come out from the other end.
     
  7. Sep 8, 2012 #6
    When I wrote 'electrical current I meant the one that's almost the speed of light not the drift velocity. What's a more proper name for it? What's meant by forward wire and return wire, isn't an antenna only one wire? I don't know very much about this but I appreciate all the help!
     
  8. Sep 8, 2012 #7
    People just use current!!! Most electronics are written in current and voltage, it is no point to make it complicate by introducing EM theory every time. I just want to bring it to your attention what's really happen. You want it in English, you got it!!! :surprised :rofl: But electronic engineering work with current and voltage and most for the equations are based on that.

    Antenna is not a single piece of wire, it is always two wires, be it a ground plane, dipole, loop etc.
     
  9. Sep 8, 2012 #8
    That would explain a lot, the electrons would have nowhere to go otherwise, I was wondering about that yesterday. I don't know where I wrote about EM theory though, thanks for the info!
     
  10. Nov 28, 2012 #9
    Actually never mind what I wrote there is wrong, So I'm trying to find out more about how it moves through wires. Say there's a wire that separates into 2 wires, how can I predict how much electricity goes either way? I bet there could be a lot of variables but I want an example or some way I can study it more on my own if it isn't too much of a problem to give.
     
  11. Nov 29, 2012 #10
    webberfolds,

    There is only one electrical current in a wire. There is no "each other". I don't know what racecars have to do with your question.

    There is no electrical current that travels at the speed of light. Current travels at its drift velocity in a conductor, as was pointed out by others. The voltage that defines the electric field and drives the electrons establishes itself at the speed of light. The specific charge carriers (electrons) that enter a wire are not the same carriers that almost immediately exit the other end of the wire. Depending on the length of the wire, it might take all day for the first electrons to finally exit the wire.

    A current must have a circuit to exist. That simply means that a conduction path must exist between two different voltage points.

    I think you better stick to simple circuits at present, and not worry about electrical wave propagation. That gets into EM theory.

    Depending on the electric field polarity, charge carriers are attracted or repelled by an electric field caused by a voltage. If there is a conduction path like a metal wire between two different voltages, an electric field will move the charge carriers along the conduction path (wire).

    Don't say "electricity". That is a generic term that means everything electrical, and says nothing about what specific thing you want to know about. You really mean current. Study Kirchoff's current law and Kirchoff's voltage law. The sum of the currents in the two separated branches has to equal the current in the main wire. The voltage across each parallel branch has to be the same. The resistance of each branch and the common voltage are all you need to know to calculate the current. I think you need to study basic circuits more.

    Ratch
     
  12. Nov 29, 2012 #11
    I reply to Ratch: "Current travels at its drift velocity in a conductor", "Don't say "electricity". That is a generic term that means everything electrical, and says nothing about what specific thing you want to know about. You really mean current". I want a way of saying the one that's almost the speed of light and when most people say electricity that's what they usually mean so I use it, that's the problem with English sadly. "There is only one electrical current in a wire. There is no "each other"." I'm a beginner and the only way I can think of "electricity" in wires is sort of like a lightning except in wires not air and since there would be so many "bolts" it would be like a stream. Know what I mean? So I would love to know how the reality is, that's why I started this thread because I don't get it at all. Never mind the race car thing, it's hard to explain what I meant by it. I know about drift velocity. I do study simple circuits but I don't see an explanation of how the "electricity" moves through them. Thanks.
     
  13. Nov 29, 2012 #12
    Sorry! I made an error, when I wrote "electrical currents" I meant "electricity" or the one that's the fastest. Kirchhoff's laws seem important for what I'm trying to learn, thanks.
     
    Last edited: Nov 30, 2012
  14. Nov 30, 2012 #13
    Read my post #5 again. The reason why you see current and voltage is because of the interaction between the EM wave inside the dielectric ( the material between the two conductors) and the surface of the metal conductor. This is called BOUNDARY CONDITION in electromagnetics. This is a very very import concept that you need to realize, but as I said, don't need to think about everyday when you design circuits.

    Without getting into the detail, when EM wave hit the metal surface while traveling down the two conductors, surface current and charges developed on the surface of the conductors, that's the physical current you are measuring. As EM wave travel at almost the speed of light, the current also seems like it is traveling at almost the speed of light.

    Here is a drawing of TEM wave travel down a parallel plate transmission line that I drew using a diagram from the book Field and Wave Electromagnetics by David K Cheng. Obviously, it is a cosine wave where max is at z=0 and propagate from left to right.

    1) The EM wave is travel from left to right.
    2) Electric E field is in orange color as arrow indicates. The longer the arrow, the stronger the field. The strength vary as a cosine wave in the diagram.
    3) Magnetic field H is going in and out of the page in green color where the circle with a DOT is coming straight out of the page towards you, circle with a CROSS is going into the page. The stronger the field, the more circles there are along the direction of propagation as shown.
    4)Surface current Js is in purple and the direction as indicated by the arrow inside the top and bottom conductor plate. The higher the current, the longer the arrow. Notice the arrow change direction depend on the polarity of the wave? That's AC current!!!
    5) Surface charge is in + or - in red on the surface of the top and bottom plates. The higher the charge density, the more + or - are drawn.

    If you look at any point along the line on z direction, the EM wave moving at almost the speed of light, so one incident, the wave is at it's peak, a moment later, it's at 0 point. The surface current change accordingly, so if you measure the voltage at one fixed point, you see the change in the E field which give the voltage where V=d*E in this case. d is the separation between the bottom and the top plate as shown in the diagram. So when you measure the voltage at any point down the line, it is as if the voltage and current are traveling at close to light speed. We don't talk electricity, we only worry about current and voltage.

    If you want to look into this deeper, read the BOUNDARY CONDITION of EM wave, that's where all the magic happens. This is not easy to understand, but since you kept asking, there is really no easy way to explain this. Again, read post #5 again, just keep in mind this is the mechanics, don't worry too much until the time comes when you learn EM.
     

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    Last edited: Nov 30, 2012
  15. Nov 30, 2012 #14
    Thanks for explaining, I'll keep on finding out more about it until I understand it, but there was a lot of good info in that post, that diagram says a lot too, I can't find a better one on the web, thanks a lot :).
     
    Last edited: Dec 1, 2012
  16. Nov 30, 2012 #15
    Electrons in a conductor are kind of like a gas inside a container. A conductor is made of atoms packed into a 3-d lattice structure. Most of the electrons are bound to the nucleii and do not move at all, but a conductor has the property that some of the electrons are not bound to an individual nucleus and can move anywhere within the boundaries of the conductor. Those are the electrons that "drift" around and create current. They can't leave the boundaries without being given an extra boost of energy from some source such as light or heat or really high voltage. Check out the Photoelectric Affect.

    If you consider a gas of electrons in a conductor, then pressure is kind of like voltage. On average, particles move from high pressure to low pressure, even though the path of each particle is irregular and bounces all over the place. On average electrons move from high potential to low potential, but they bounce all over the place.

    If you ring a bell in a volume of gas you can detect pressure waves on the other side even though no gas particles are moving from the bell to your ear. The wave propagates through the gas with a speed related to the average speed of the gas particles, which is the speed of sound. In a wire, a voltage wave propagates at about 2/3 the speed of light, and if you want you can imagine that this is the average speed of the electrons bouncing around all over the place. This similar to AC voltage waves. This is the part that travels at nearly the speed of light. I would call it the "Voltage Signal" or "Electromagnetic Wavefront" instead of current.

    If you put a smelly object on one side of a volume of gas, it will take some time before you can smell it on the other side because the particles have to physically travel from one side to the other along a path that zigzags randomly all over the place. If you were able to track an individual electron moving through a wire(you can't because all electrons are identical), it would appear to be moving slowly because its just bouncing all around randomly. This is how individual electrons move in a DC current.

    Since electrons are indistinguishable from eachother, nobody can really say for sure how much time it takes for an electron to travel down a wire when you are putting a current through it, but the number of electrons total in the wire (~1023) is large compared to the number of electrons passing through it (~1019/second for 1 amp) so if they all moved as a blob (they don't) it would take thousands of seconds for the blob to traverse the wire.

    Instead of picturing racecars on a track, think of bumper cars.



    yungman: that diagram is brilliant
     
  17. Dec 1, 2012 #16
    I understood all of that, thanks as well. :)
     
  18. Dec 3, 2012 #17
    webberfolds, moving racecars affect each mostly through the Bernoulli's principle. When moving side by side they tend to get closer together and will do so until they crash with each other. The drivers need to constantly steer away from each other.

    This is because the air between them moves faster than the air at their outer sides. When air or any other fluid moves faster, its pressure decreases. So there is a lower air pressure between the cars and the higher pressure from the outside of the cars is pushing them together.

    A safety rule forbids large ships from cruising too close together for the same reason - they would bump into each other eventually. It is a major topic in naval battle theory.

    It is also why planes fly, the wing is thicker on the top and the air has to travel faster there which lowers the pressure above the wing so it gets pushed up by the air below the wing.

    Now whether Bernoulli's principle can be applied to electrical current, well is it a fluid?
     
  19. Dec 16, 2012 #18
    yes I know about that principal. Electrical current behaves like a fluid but it depends on what is meant by fluid, thanks. I wasn't comparing electrical current to race cars and if i said electrical current i meant the one that's almost the speed of light. What's a good word for it? I know now that I shouldn't be calling it electrical current but electricity seems to be confusing. Thanks.
     
  20. Jan 24, 2013 #19
    So the electric field causes a lot more current than the magnetic field in this picture? What if the electric and magnetic fields swapped places? So do the purple arrows represent the velocity of the current? V=d*E, V is the voltage between what points? "E" is the strength of the electric field where? Thanks, I'm understanding more and more of it.
     
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