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Homework Help: Archived Lenz' Law Scenarios for two wires with varying distance between them

  1. Aug 3, 2014 #1
    1. The problem statement, all variables and given/known data

    For each of the following scenarios, predict the direction of the induced current ( c-to-d;
    d-to-c; NO INDUCED CURRENT) in the second set of coils, N2.

    1. The current, I1, direction ‘a-to-b’, is increasing rapidly. The coils remain stationary.
    2. The current, I1, direction ‘a-to-b’, is increasing slowly. The coils remain stationary.
    3. The current, I1, direction ‘b-to-a’ is constant; the coils, N2, are moved coaxially and slowly toward the stationary coils, N1 .
    4. The current, I1, direction ‘a-to-b’ is constant; the coils, N2, are moved coaxially and rapidly away from the stationary coils, N1 .
    5. The current, I1, direction ‘b-to-a’ is constant ; the coils, N2, are moved slowly and laterally, maintaining the separation, L, between the planes of the coils .

    2. Relevant equations


    3. The attempt at a solution

    1. When I increases in one direction on one wire, Lenz' law implies that the other wire will have an increase in current in the opposite direction in order to maintain the net flux. Therefore a rapid increase in current from a to b will induce a current from d to c rapidly.

    However the question does not ask for the magnitude, only the direction, so the fact that the change is rapid or slow does not affect our answer, right?

    2. Rate of change in flux will affect magnitude but not direction so the answer is once again, D to C.

    3. Using the equations in the pictures above, I explained magnetic flux is proportional to (1/L) with L being the distance between wires... So If L decreases, flux increases. Lenz' law says an increase in flux will result in an induced emf in the opposite direction.

    Therefore if the current is moving from B to A while L decreases, the induced emf will be from C to D.

    Increases in L will result in decrease in flux and induced emf in the same direction. Since current is fro A to B this will be C to D

    The separation L is maintained so there should be NO change in flux, right?
  2. jcsd
  3. Feb 28, 2016 #2


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    Homework Helper
    Gold Member

    1-4 seem comprehensively answered and I agree.
    5 although L is fixed, the separation of the coils is increasing, so the flux linkage is decreasing and the induced voltage is D to C.
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