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Theory of faraday's law

  1. Feb 4, 2007 #1
    According to faraday's law of induction "Whenever a conductor is moving in a magnetic field a current flows through it"

    the question is "How the magnetic field get converted into current ? "
    How the MMF get converted into EMF ?

    No mathematical eqn's plz.

    your explanation should be full of theory.

    Advance thanks to your answers.
  2. jcsd
  3. Feb 4, 2007 #2
    Well, I'll let someone else give you the full answer, but your definition:

    "Whenever a conductor is moving in a magnetic field a current flows through it""

    is not technically correct. A conductor moving in a magnetic field will have a potential (voltage) induced across it; current will only flow if there is a circuit for it to flow through (i.e if the conductor forms a loop).

    I'm sorry to use equations, but live with it. It helps explain the answer.

    Faraday's Law says:

    Induced Voltage = Number of Turns of Coil * rate of change of magnetic flux

    So, as long as you have a change in flux with respect to time, you get an induced voltage in the conductor.

    So, you can either vary the magnetic flux around the conductor to induce a voltage (how a transformer works) or vary the position of the conductor wrt time (i.e. how a generator induced voltage works, by spinning the rotor in a static magnetic field.)

    For example, if your conductor is 1 single loop, you can just use Ohm's Law to get the current once you calculate the induced voltage (in this case, the number of turns = 1).
  4. Feb 5, 2007 #3
    MMF to EMF

  5. Feb 5, 2007 #4
    I'll have to leave the full "how" you want to someone else who can explain it better...

    "why" in physics is sometimes a philosophical question.

    Why is Vemf=N*d/dt(B) (Faraday's Law)? That's because nature works like that. ;)

    How can we get to Faraday's Law from other concepts?

    "How an magnetic field get converted into electric current ?"

    Magnetic fields , are , by definition, the result of electrons moving with a velocity (which is current).

    Hence, if you have a wire with current flowing through it, there is, by definition of a magnetic field, a magnetic field around the wire caused by the current flow.

    Conversely, if you have an external magnetic field, and you place a wire in it, current can be produced. Remember, however, that magnetic fields only act on moving electrons (current). So, if there's current in the wire, a force will act on it in this case (Ampere's Law).

    If the conductor moves, there's moving electrons, so the external magnetic field will induce an EMF on the conductor. However, if there's no conductor movement, there's no moving electrons, so the magnetic field won't effect the conductor (remember, magnetic fields are caused by and only influence MOVING electrons).

    Note: this is a gross oversimplification. There is of course a natural drift of electrons within the conductor, but there's no net movement.

    I hope this helps.
  6. Feb 7, 2007 #5
    Sorry, but the original statement is indeed correct. Both current and voltage will be induced regardless of whether the path is open or closed. A prime example is a simple dipole antenna which is merely a parallel wire transmission line separated and bent into a "T", terminating in mid air. Current flows even in the absence of a "return path" or "closed loop". This is "displacement current" which must be understood in order to grasp induction.

    Likewise, if the conductor is a superconducting closed loop, with zero ohms of resistance, both current and voltage are induced. In the shorted case, inductance is still present, with its associated reactance. A small induced voltage accompanies the induced current. In the open circuit case, capacitance is present, with an associated susceptance. A small induced current accompanies the induced voltage.

    Ampere's law describes induced currents and the time-changing (or relative motion) magnetic field, whereas Faraday's law describes induced voltage and the field. Both I and V are induced. It is impossible to get one without the other, and there is no pecking order. Attempting to resolve this further is an endless vicious circle, a chicken-egg scenario. Best regards.

  7. Feb 7, 2007 #6
    Whenever a charge moves in a magnetic field it experiences a force mutually perpendicular to direction of motion and the magnetic field. A change in flux associated with a condcutor is equivalent to relative motion of the conductor. Dude to this motion the free electrons in the conductor starts moving and that's how current flows (in a closed circuit). However if the circuit is not closed, the charge separation occurs across the length of the conductor until force dude to magnetic field on a free electron balances the force due to the electric field (from the charge separation in the conductor).

    Hope its clear.
  8. Feb 8, 2007 #7
    Thanks for the clarification guys.
  9. Feb 8, 2007 #8
    I forgot about my friend the "displacement current".

    A "closed loop" is a tricky word itself.

    We think a closed electrical loop being one connected on both ends by a conductor (ie. wire.)

    Well, we consider a circuit with a resistor, capacitor, and voltage source "closed". However, there is an air gap between the parallel plates of the capacitor :)
  10. Feb 11, 2007 #9
    Yes but that's how a capacitor works. It can only be charged up to a particular point for a particular voltage.
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