# Changing B through a coil

1. Feb 20, 2009

### JSapit

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

The component of the external magnetic field along the central axis of a 50 turn coil of radius 5.7 cm decreases from 1.8 T to 0 T in 3.3 s.

(a) If the resistance of the coil is 2.8 ohm, what is the magnitude of the induced current in the coil?

(b) What is the direction of the current if the axial component of the field points towards the viewer?

1. CCW
2. CW
2. Relevant equations

EMF=-(phi)/(time)

Phi=BASin(theta)

3. The attempt at a solution
I found the current using -N(B/t)*A

I found it to be 0.9442 A.

Now I'm just not sure on part B.

Last edited: Feb 20, 2009
2. Feb 20, 2009

### Delphi51

Well, there won't be any current unless the B is changing!
"A changing B causes an E to circulate around it."

3. Feb 20, 2009

### JSapit

B is changing, though. It said in the problem that "B decreases from 1.8T to 0T."

4. Feb 20, 2009

### Delphi51

Yes, B is changing so you will get an emf in the coil, causing a current.
I meant that any formula you find for the emf MUST have a changing B.
Faraday's Law is what you are looking for.