# Homework Help: Magnitude of changing magnetic field

1. Mar 13, 2012

### likthem

1. The problem statement, all variables and given/known data
A square loop consists of a single turn with a resistance of 4.00 Ω. The loop has an area of 600 cm^2, and has a uniform magnetic field passing through it that is directed out of the page. The loop contains a 12-volt battery, connected as shown in the figure below.
C:\Users\Michael\Pictures\20-p-044.gif
At the instant shown in the figure, there is no net current in the loop. At what rate is the magnitude of the magnetic field changing?

2. Relevant equations
Φ=BAcosθ
ε=-NΔΦ/Δt

3. The attempt at a solution
ε=-NΔΦ/Δt
ε=-NΔBAcosθ/Δt, A is constant so
ε=-NAΔB/Δt
ΔB/Δt=ε/NA, negative sign is removed because only magnitude is asked for.
ΔB/Δt=12V/(1)(0.06m^2)=200T/s

I worked this out and it turned out to be wrong. I put in 12V as the emf as I thought that the total voltage should be the same as there is no net current in the loop, however that is the only part of the problem that I am uncertain about. The answer must be in T/s.
Thank you in advance for the help.

2. Mar 13, 2012

### likthem

Never mind, I got it. It wasn't asking for what was the magnitude of the change in magnetic field but the rate of change. Since the induced current from the field will be opposite to the that of that in the loop, the answer should be negative. So I did it correctly, just misread what the problem was asking for.

Thank you!