# Homework Help: Question about changing magnetic flux

1. Apr 28, 2012

### Dany7777777

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

A 200-loop coil of cross sectional area 8.5 cm2 lies in the plane of the page. An external magnetic field of 0.060 T is directed out of the plane of the page. The external field decreases to 0.020 T in 12 milliseconds.
What is the magnitude of the change in the external magnetic flux enclosed by the coil?

2. Relevant equations

$\Phi$m = A * B

3. The attempt at a solution
Ive tried solving for both values of magnetic flux at each time and finding their difference. However, I still seem to get it wrong. I dont know If I am missing something, any help or tips would be greatly appreciated.

2. Apr 28, 2012

### bdh2991

i believe it would just be the initial flux - the final flux...

the full equation you want is $\Phi$= N*B*A*cos($\phi$)
(N = the number of turns of the coil)

however the direction of the area and the magnetic field are the same so cos(phi) = 1
just convert 8.5 cm2 to m2 which is 8.5 * 10-4

so.... N(.06T)(A) - N(.02T)(A), just plug in your numbers...i believe this is right anyway, someone should check my work