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Are induced currents created by motion or change in flux?

  1. Jul 22, 2015 #1
    The book I am working through is beginning to dive into induced currents and Lenz's Law.

    When a conductor is moving perpendicular to a magnetic field, the charge carriers inside will experience a magnetic force; This causes the charges to move. Eventually, the electric field from the charge separation will be so strong that the electric forces on the charges will be equal and opposite to the magnetic force. The presence of an electric field implies an electric potential difference. The epd is commonly known as a motional emf for this specific type of scenario...Slap on some wires and you get a current.

    So now let's say I drop a magnet through a loop of wire. The relative motion of the field lines and the charges inside the conductor are parallel; The charges should not be experiencing a magnetic force, hence no motional emf. But nonetheless an induced current occurs.

    My book introduced induced currents by demonstrating motional emf. But now we have moved onto Lenz's Law and motion does not seem to matter. Only changes in magnetic flux seem to matter. Even by changing the strength of a stationary magnet, an induced current will occur in a stationary coil.

    I'm having a hard time identifying the why the current occurs simply because of a change in flux. It seems like motion can explain it, but only when the conductor is moving perpendicular to the magnetic field.
  2. jcsd
  3. Jul 22, 2015 #2


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    draw yourself a pic of your setup so you can see that the field lines and conductor are not always parallel
    come to some conclusions about that

    The change in flux is going to be caused by motion, either ...
    1) moving magnetic field or ...
    2) moving conductor in a magnetic field

    either way the conductor is going to experience a change in flux

  4. Jul 22, 2015 #3


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    A change in the magnetic flux or motion of a conductor in a magnetic field does not induce current, but emf. Say you have a transformer with varying flux through the iron-core (through the secondary windings) and the secondary windings are not loaded by anything, no current will flow through the secondary windings, but you can measure voltage across the windings. Ohms law then states that if you close a loop ( loading the secondary windings by some resistor ) a current will flow: I = V / R. Current flows in closed loops.

    Emf = dΨ/dt , where Ψ is the flux passing through a closed loop.

    Now, say you have a closed loop formed as a rectangle ABCD. AB and CD are arranged parrallel, and so is BC and AD. If you move this rectangle in a homogeneous magnetic field, the sum of induced emf's in the four sides of the rectangle will be zero, emfAB = -emfCD and so on. Thus the current through the rectangle will be zero.

    But if you move only one side of the rectangle perpendicular to the magnetic field, say AB, the sum of the emf's will not be zero and a current will flow. You may say this is due to the conductor AB being moved in the magnetic field, or you may say that when moving only one side (AB), the area of the rectangle must be increased/decreased and thus the flux through the rectangle.
    Last edited: Jul 22, 2015
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