# Homework Help: Magnitude of the non-coulomb electric field around a solenoid.

1. Nov 10, 2012

### Pepjag

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

A metal ring of radius r = 24 cm surrounds a
solenoid of radius R = 2 cm and 10000 turns.
The magnetic field inside the solenoid changes from 5 T to 3 T in
0.1 s. Find the magnitude of the non-Coulomb
electric field along the ring.

2. Relevant equations

emf = -N*d(flux)/dt = -N*db/dt*A
emf = integral of E*dl

3. The attempt at a solution

Ok, so i pretty much just plugged in numbers. for the emf:

emf = -10000*(3-5)/0.1*pi*0.02^2 = 251.3 volts.

For the electric field:

E = emf/L = 251.3/2*pi*0.24 = 166.7 V/m

Unfortunately, the computer keeps saying i'm wrong. Am I really doing something wrong? This seemed pretty straight forward.

2. Nov 10, 2012

### aralbrec

You're looking at the emf induced in the metal ring. How many turns of wire does a metal ring have?

3. Nov 11, 2012

### Pepjag

Wait, so you have to use the emf equation from the ring's perspective?

emf = (3-5)/0.1*pi*0.24^2 = 3.62 volts.

E = emf/L = 251.3/2*pi*0.24 = 2.40 V/m

Or do I still use the solenoid's area for the emf?

Last edited: Nov 11, 2012
4. Nov 11, 2012

### Pepjag

I have until 11:30 ct to finish this one problem, can anyone please give input.