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Homework Help: LC circuit problem

  1. Mar 29, 2016 #1
    1. The problem statement, all variables and given/known data
    When the current in the portion of the circuit shown in the figure below is 4.00 A and increases at a rate of 0.500 A/s, the measured voltage is ∆ = 15.0 V. When the current is 4.00 A and decreases at the rate of 0.500 A/s, the measured voltage is ∆ = 10.0 V. Calculate the values of (a) L and (b) R.

    2. Relevant equations
    Given formula sheet.


    3. The attempt at a solution
    My attempt:
    I was given an equation:
    adding flux formula:
    Γ=L/R ---> Γ=L/(V/I)----> Γ=LI/V

    Am I on the right track?
  2. jcsd
  3. Mar 29, 2016 #2


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    Homework Helper
    Gold Member

    Voltage between points a and b is sum of inductor voltage and resistor voltage. Inductor voltage is Ldi/dt..
  4. Mar 30, 2016 #3
    So what I do is:
    Vr= I*R
    VL= -L(di/st)

    So put in the variables given:
    15v=(4A*R)+(-L(0.5A/s)) increasing rate
    10v=(4A*R)+(-L(-0.5A/s)) decreasing rate
    Then solve for L & R like?
  5. Mar 31, 2016 #4


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    Staff: Mentor

    Hi Am_0. smiley_sign_welcome.gif

    That's the right idea. Except there should not be a negative sign in the expression for VL.

    Also, where you are asked to provide all relevant equations in your homework post, please only include those relevant to the problem at hand.
    Last edited by a moderator: Apr 18, 2017
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