# Homework Help: Find the induced magnetic current on the inner of two rings

1. Apr 28, 2013

### rocapp

1. The problem statement, all variables and given/known data
A small, 1.60-mm-diameter circular loop with R = 1.10×10−2Ω is at the center of a large 120-mm-diameter circular loop. Both loops lie in the same plane. The current in the outer loop changes from + 1A to -1A in 8.00×10^−2s .

What is the induced current in the inner loop? in nA

2. Relevant equations

I=ε/R
ε=Phi/t
Phi=A*B
3. The attempt at a solution

I=ε/R

Phi=(Area)*(Magnitude of field)
Phi = (PI*(8x10^-4^2 m^2)*??)

2. Apr 28, 2013

### rocapp

Ok, I tried this:

B=mu*I/2R

B1=4PIx10^-7 * +1 / 2*(1.1x10^-2)= 0.000785 T

B2=4PIx10^-7 * -1 / 2*(1.1x10^-2)= -0.000785 T

I=V/R

V=PHI1-PHI2/t

PHI1=A*B1

PHI1=PI*(8x10^-4)^2 * 0.000785 =1.58x10^-9

PHI2=A*B2=PI*(8x10^-4)^2 * - 0.000785=-1.58x10^-9

V=(1.58x10^-9)*2/(8x10^-2)
V=3.95x10^-8

I=V/R

I=(3.95x10^-8)/(1.1x10^-2)
I=3.59x10^-6 A =3590 nA

Yes?

I submitted, but this was not correct. The correct answer was 4.79 nA. Can anyone please explain how?

Last edited: Apr 28, 2013