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Homework Help: Applications of Faraday's Law

  1. Oct 2, 2008 #1
    1. The problem statement, all variables and given/known data
    A magnet is suspended so that it is free to swing above a coil that is connected to a power supply. A pointer has been attached to the magnet so that a small swing of the magnet will result in a large deflection of the pointer. When there is no current through the coil, the magnet is horizontal and the pointer is vertical.

    What is the deflection of the pointer (if any) when the switch on the power supply is closed?

    What happens to the pivot as the current through the coil is varied?

    2. Relevant equations
    Faraday's law = E = -d(magflux)/dt
    magnet flux = BAcos(theta)

    3. The attempt at a solution
    When the switch on the power supply is closed, this means that there is no current. This means that there will be no deflection of the pointer.

    Is this correct? What does it mean by the switch is closed?
  2. jcsd
  3. Oct 2, 2008 #2
    When the switch is closed, the circuit is complete. You have interpreted it incorrectly.
  4. Oct 2, 2008 #3
    okay, thanks!

    so if the switch is closed, this means that the circuit is complete. right after the switched is closed, no current should flow but the rate of change shouldn't be zero (this is because inductor current can't change instantaneously).

    but after some time has passed, the inductor current, resistor current and resistor voltage will rise from zero. because the power supply emf should be constant, the magnitude of the inductor emf should drop... eventually the whole circuit should reach a steady state (the rate of change approaches zero) & the inductor emf should be zero...

    is this right? how does this affect the pointer in this device? should the pointer go horizontal?
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