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Homework Help: Current in inductor used to measure EMF.

  1. May 18, 2013 #1
    1. The problem statement, all variables and given/known data
    This is not a formal HW question, yet I was wondering whether one of you might be willing to answer it nevertheless.
    According to Fraday's law, ε = -∂[itex]\phi[/itex]/∂t. If I reversed the direction of the current in an inductor used to measure ε, would that have any effect on ε?

    2. Relevant equations

    3. The attempt at a solution
    I would think it wouldn't, but am not certain. Wouldn't the flux remain the same once the current is reversed? Or, rather, the rate of change of flux?
  2. jcsd
  3. May 18, 2013 #2


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    Flux even it is a scalar, did some direction is associated with it (at a point)?
  4. May 19, 2013 #3
    No, but could the rate of change of that scalar once the current is reversed change?
  5. May 19, 2013 #4
    Relate the flux linking the turns of the inductor, ϕ, to the current through it, i, by:
    ϕ = L*i

    where L is the self-inductance of the inductor and is considered a constant.

    You would have to calculate the surface integral that gives you ϕ such that it has the same sign as i for some choice of reference direction of i. You have then:

    ε = -dϕ/dt = -L di/dt

    As an example, if the current i was increasing with time, -i would be decreasing, e.g. the sign of di/dt and ε would flip.
  6. May 19, 2013 #5

    rude man

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    emf = L di/dt. So if i is increasing and positive, emf is positive.

    If i is negative and increasing (in magnitude), di/dt is negative and so is emf.
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