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Electro-Magnetic induction

  1. Feb 8, 2009 #1
    I know that current can be induced in a coil, thanks to changing magnetic field, but can it be also induced in a straight conductor?

    Is there a current induced in a conductor, moving upright on the field lines, in a homogenous magnetic field? Will the result be different when moving in non-homogenous magnetic field?
  2. jcsd
  3. Feb 9, 2009 #2
    Yes, current can be induced in a straight conductor.

    You get the most current for your effort when the conductor cuts the flux lines at a right angle. The further you get from cutting the flux lines at 90 degrees the less current you get for your effort. Moving the conductor exactly parallel to the flux lines will get you no current at all. (That motion is all relative, of course: you can just as well hold the conductor still and move the magnet; the effect is the same.)
  4. Feb 10, 2009 #3
    Thank for clearing that up
    But I still have some questiones. If I put strong current discharge (from one charged plate of a capacitor to another) to the straight conductor in a homogenous magnetic field for a fraction of a second, would such fast change in its magnetic field consume part of the energy of the discharge? Would the conductor move after that (because the magnetic force has been applied on it for a fraction of a second) and so produce current of the opposite direction? Wouldn't this violate the law of conservation of energy?
  5. Feb 10, 2009 #4
    The wire will get hot and if it cuts the magnetic field lines there will be some movement which induces a back emf.Energy is dissipated as heat and as work done against the back emf and the sums add up-the conservation of energy is not violated.
  6. Feb 11, 2009 #5
    Let me just tackle this question, for the moment. Part of the discharge will, obviously, be "lost" as heat due to resistance of the wire. There is an additional resistance to the change in the magnetic field called inductance:

    "Electrical inertia or inductance * The inertia exhibited by an electric circuit in opposing the creation, destruction, or variation of its magnetic field is known as the property of inductance. Since every conductor produces a magnetic field when current is flowing through it, it follows that every conductor necessarily possesses some degree of inductance. The property of inductance is always associated with magnetic fields, and specifically, with any change that may take place in the field. This last fact, that of a changing field, is to be particularly noted, as it is under such conditions of change that effects of inductance must be considered.

    A magnetic field is formed when current flows. Any action, therefore, that opposes the field will directly affect the current that produced the field. This being the case, it is also possible to define inductance as that property of a circuit which opposes any change in the current flowing through a circuit.

    A full definition of inductance may now be stated as follows: Inductance is that property which opposes any change in the current flow or in the magnetic field of an electric circuit.

    The magnetic field around a coil or solenoid is far stronger than that existing in a straight wire. The property of inductance in such a device is, therefore, more pronounced, and because of this, coils and solenoids are often called inductors and inductances."

    -Basic Electrical and Electronic Principles
    Maurice Grayle Suffern
    McGraw-Hill, USA, 1949, 1962 edition p. 183

    So, back to your question: "...would such fast change in its magnetic field consume part of the energy of the discharge?" This might better be restated as "Does overcoming inductance consume energy"?

    The author I quoted compares electrical inertia, inductance, to physical inertia. The answer, therefore, to your (restated) question should be the same answer to the same question about physical inertia: "Does overcoming inertia consume energy?"
  7. Feb 12, 2009 #6
    We were taught at school, that in a circuit made by capacitor and coil, known as high frequency EM oscilator, the energy consumed while creating magnetic field around the coil is negligible and also the heat losses, if we assume this beeing done in extremely low temperatures. It is deducible from this that producing mag field produces only "imaginary resistance" called inductance. The only reason, why you need to charge the capacitor, from time to time, is that some electrons are staying on the other plate of capacitor.

    If this is right, than producing a magnetic field causes only the "imaginary resistance" and small (almost negligible losses). So if I turn on the switch on a circuit powered by capacitor (2 plates), the electrons would try to get to the positively charged plate as fast as possible. Thus prodicing massive current flow. If the conductor is in a homogenous magnetic field, as it is stated in one of the previous posts, it would produce this "imaginary resistance" and most of the electrons (almost all of them) would get to the positively charged plate. But massive current causes a "massive" magnetic field and in a homogenous magnetic field also magnetic force. This force would accelerate the conductor (assuming it is hanged on something so it can move) and thus induce current of opposite direction. With a help of diodes, tree circuit, and another capacitor, we could charge the other capacitor by the induced current. And if I am calculating correctly, we have two charged capacitors instead of one, even though both are a bit less charged than the one at the biggining. Isn't this violation of the law of conservation of energy?
    Or have I assumed it incorrectly?
  8. Feb 13, 2009 #7
    I think "imaginary" is incorrect. It can be negligible, as in a short, straight conductor, but to say it's "imaginary" is to make a different, incorrect claim.

    How massive the current flow is depends on the capacitance of the capacitor, how fully charged it is, and the resistance of the wire, doesn't it? Negligible inductance doesn't guarantee "massive" current flow.

    I think that the main reason you even remotely suspect there might be some violation of a conservation law is because you keep jumping around to different circuits and mix them all up in your head. The original circuit was a capacitor shorted through a straight conductor, and now you're suddenly talking about a tank circuit, diodes, etc.

    You really have to pick one set up at a time and thoroughly follow all the effects through.
  9. Feb 15, 2009 #8
    OK, then we can stick to negligible.

    Yes, I agree, but compared to regular battery, you can charge these plates on much higher voltages and the wire can be superconductor so the resistance would be minimal. Somehow or other the "massive" current flow can be reached.

    I am not thinking about mixed circuits, I have introduced the tree circuit and diodes only as a mean to capture the induced current.
    You have said that inductance is not imaginary, does it depend also on external magnetic field? How much?
    If yes, than the induced current could be equal to losses while inducing mag field arround the conductor, so there would not be any violation of conservation law.
    But on the other hand, if we put current X to the conductor in that mag field and it creates magnetic force, shouldn't the current, induced by induction caused by the magnetic force, be equal to the current X minus the "negligible" losses on creating the mag field around the conductor?
    So to conserve the law of conservation, the "negligible" losses would have to be half of the "primary" current which causes the creation of mag field around the conductor.
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