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Homework Help: Physics problem : Oscillating circuit

  1. Jun 6, 2012 #1
    1. The problem statement, all variables and given/known data

    In an oscillating circuit, the capacitance is 2 microfarads and the maximum voltage at the clamps is 5V.
    Find out
    1:the maximum energy of the magnetic field.
    2:energy of the mag field when the voltage of the capacitor is 3V.

    2. Relevant equations

    3. The attempt at a solution

    First I transformed microfarads in farads.
    2 microfarads= 2*10^-6 farads.

    There isn't any mention of resistance, and it is said that it's an oscillating circuit, so i assumed that resistance is 0.
    If R=0 => (CU^2)/2 = (LI^2)/2 => 2 * 10^-6 * 9
    so the energy of the mag field when the voltage is 3V is 10^-6 * 9

    The max energy of the magnetic field is 1/2 * CU^2 =10^-6 * 25

    I managed to come to these results by using various bits from others problems in my notebook, but i'm pretty sure they're wrong.
  2. jcsd
  3. Jun 6, 2012 #2


    User Avatar

    Staff: Mentor

    Hi Voidbane, Welcome to Physics Forums.

    When you have an LC oscillator, energy is being traded back and forth between the capacitor's ELECTRIC field and the inductors MAGNETIC field. It doesn't have to be all in one place at the same time...

    When the capacitor's potential difference is at a peak, then all of the energy is in the capacitor's field. When the capacitor's P.D. is zero, then current is maximum and the energy is all in the inductor's field. In between there will be energy in both places. If you know the capacitor's P.D. for a given instant, then you should be able to find the energy in the capacitor for that instant, and the rest should be in the inductor's field.
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