Does a current produce an electric field?

  1. A current will produce a magnetic field but I don't think an electric field will be produced. Why is that? Why is a test charge affected by a static charge but not a moving charge?
     
  2. jcsd
  3. Doc Al

    Staff: Mentor

    Because a current-carrying wire (I assume that's what you are referring to) has no net charge (nor is there a changing magnetic field, assuming a constant current).

    If it were a current of just charge carriers without a wire--a beam of electrons in a vacuum chamber, for instance--then there would certainly be an associated electric field.

    Why do you think that?
     
  4. HallsofIvy

    HallsofIvy 40,767
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    Another way to look at it is that a varying magnetic field will produce an electric field but a (constant) currant only produces a static magnetic field.
     

  5. Why is this the case? Is it because the wire acts as an insulator so charges inside the wire will not show up on the outside of the material so anything outside will not know of any electric charges anywhere.
     
    Last edited: Jul 24, 2006
  6. Maybe I was wrong. A test charge should be affected by a moving charge just as much as it is affected by a static charge. Given that there is nothing else that could affect the two charges in the surrounding area.
     
  7. Hootenanny

    Hootenanny 9,677
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    :eek: :eek: :eek:
     
  8. Doc Al

    Staff: Mentor

    (Regarding the current-carrying wire)
    Realize that by definition an insulator is something that cannot conduct an electric current; the wire, of course, is a conductor not an insulator. But if you look at a section of the wire, what's the net charge on it? Zero! So the net electric field is also zero.

    Perhaps you are thinking that the current is "extra" charge added to the wire? Not so. It's just a forced movement of the electrons that already exist in the wire. (Any electrons added at one end are being removed at the other end.)

    (Regarding the test charge)
    Right.
     
  9. jtbell

    Staff: Mentor

    To make an analogy between wire and a water-pipe, a wire without any current flowing through it is not like an empty pipe. Rather, it's like a pipe that is full of stationary water.
     
  10. A current in a wire can be viewed as the two types of charges moving in opposite direction. Say in a horizontal wire, from left to right, negative charges moving to the left and positive charges moving to the right. So for a test test charge at rest outside the wire, the wire will be neutral.

    So remember that, there is always the two kinds of charges moving in the wire.
     
  11. Doc Al

    Staff: Mentor

    In an ordinary metal wire, the only charges that are moving are the negative electrons. The positive charges are fixed in a lattice and are not free to move. (This is not the case in many other kinds of materials that can conduct a current.)

    Don't confuse the direction that the electrons actually move with the direction of "conventional" current flow, which is opposite to the direction the electrons move. (The direction of conventional current was defined with respect to positive charges before it was understood that negative electrons carried the current.) So if the electrons moved from left to right, we say the current moves from right to left.
     
  12. Andrew Mason

    Andrew Mason 6,875
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    I think you mean the net electric field contributed by that section of wire is zero. Of course, there must be an electric field in the wire if current is flowing.

    AM
     
  13. Doc Al

    Staff: Mentor

    Right. Good catch!
     
  14. I was thinking of a (thick) wire used in the home such as the wire that connects the CPU to the main electrical socket (what do you call these?) These have an insulating material around the wire, otherwise you would be in great danger. So in that way a test charge outside would not sense the charges inside the wire whether it is moving or not.

    But we are considering a plain wire so I will stick to that. If we have a circuit with charges flowing, lets consider the whole circuit by enclosing a surface over it. The charges may be moving inside this surface but they will always be inside it hence a net charge exists and electric field lines will extend outside and a test charge will feel it. But this contradicts your explanation. Maybe your explanation works only when the wire is infinitely long?

    Why is it that a current of just charge carriers without a wire--a beam of electrons in a vacuum chamber, then there would be an associated electric field? Is it because we are not measuring the electric field with respect to a portion of space such as wire but always to the charges themselves. Hence the Electricfield is moving with the charge carriers?
     
  15. Andrew Mason

    Andrew Mason 6,875
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    You seem to be forgetting that the wire contains as many protons as electrons. The protons are fixed and don't move. The electrons move but for any given volume, the number leaving is equal to the number entering, so there is no change. The ratio of electrons and protons always remains at 1.00000000000...
    An electric field (created by the potential difference between cathode and anode) is required in order for electrons to leave the cathode and create a current. The electron flow does not affect the electric field between the anode and cathode. That field depends only on the potential difference that is applied.

    AM
     
  16. So you are saying that copper when in a block of solid must be electrically neutral? How can you tell?

    If it must be neutral than that explains why the electrons on a current carrying wire has no electric field associated with them because there will always a positive charge nearby. This is what Doc Al said. Now I understand.

    However, charges moving in a vacuum will have an electric field because there are no protons to cancel their charges.
     
  17. Andrew Mason

    Andrew Mason 6,875
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    It doesn't have to be electrically neutral. You can easily create a charged conductor. An airplane frame is often a charged conductor as it flies, so all of the electrical circuits connected to the frame will have a static charge. But that static electric charge that does not change when current flows or stops flowing in those circuits.

    And that is why a cathode ray oscilloscope works: the electron beam is deflected by electric charge on plates placed horizontally and vertically around the electron beam.

    AM
     
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