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Inductance and Energy

  1. Apr 1, 2007 #1
    1. The problem statement, all variables and given/known data

    After you measure the self-inductance of a coil, you unwind it and then rewind half the length of wire into a coil with the same diameter but half the number of turns. How does this change the self-inductance?

    a. It is the same.
    b. It is doubled.
    c. It is quadrupled.
    d. It is halved.
    e. It is quartered.

    2. Relevant equations

    See below.


    3. The attempt at a solution

    L = (N* magnetic flux)/i = (N*B*A*cos theta)/i = (n*l*A*B*cos theta)/(i), where N = n*l (l = wire length)

    L2 = [(l/2)*A*(n/2)*B]/i = (1/4)*[(L*A*n*B)/i] = (1/4)*L??



    1. The problem statement, all variables and given/known data
    How much does the energy stored in an inductor change if the current through the inductor is doubled?

    a. It remains the same.
    b. It is doubled.
    c. It is quadrupled.
    d. It is halved.
    e. It is quartered.




    2. Relevant equations

    See below.

    3. The attempt at a solution

    U = 0.5*L*I^2

    U2 = 0.5*L*(2I)^2 = 4*0.5*L*I^2 = 4*U??


    1. The problem statement, all variables and given/known data

    The magnetic flux through a loop is made to vary according to the relation magnetic flux = 6t^2 + 7t + 1, where the units are SI. The emf induced in the loop when t = 2 s is

    a. 38 V
    b. 39 V
    c. 40 V
    d. 31 V
    e. 19 V


    2. Relevant equations

    Emf_induced = -(d magnetic flux)/(dt)

    3. The attempt at a solution

    Taking the derivative, |emf| = 12t + 7, where t = 2 s

    |Emf| = 12*(2s) + 7 = 31 V??

    Thanks.
     
  2. jcsd
  3. Apr 2, 2007 #2

    Mentz114

    User Avatar
    Gold Member

    You don't seem to be having any problems with this. They all look correct.
     
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