# LC circuit problem

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1. Mar 29, 2016

### Am_o

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:
i=Imax(1-e^-t/Γ)
Γ=L/R ---> Γ=L/(V/I)----> Γ=LI/V
R=V/I

Am I on the right track?

2. Mar 29, 2016

### cnh1995

Voltage between points a and b is sum of inductor voltage and resistor voltage. Inductor voltage is Ldi/dt..

3. Mar 30, 2016

### Am_o

So what I do is:
Vt=Vr+VL
Vr= I*R
VL= -L(di/st)
Vt=(I*R)+(-Ldi/dt)

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?

4. Mar 31, 2016

### Staff: Mentor

Hi Am_0.

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