# Coil placed in a time-varying magnetic field

1. Jun 5, 2014

### PKay

1. The problem statement, all variables and given/known data
A coil 3.75cm radius, containing 500 turns, is placed in a uniform magnetic field that varies with time according to B = (1.20 x 10-2 T/s)t + (2.70 x 10-5x4)t4. The coil is connected to a 610-Ω resistor, and its plane is perpendicular to the magnetic field. You can ignore the resistance of the coil. What is the current in the resistor at time t0 = 5.50s ?

2. Relevant equations
ε = N d/dt(BA)
I = ε/R

3. The attempt at a solution
First I looked for the emf using the first equation and when I put it into mastering physics, I got it right (0.0265 +0.000265 t3. Then, I used the second equation to get the induced current. I plugged 5.50s into the emf equation and then divided by R. I got 1.16 x 10-4. I can't figure out why it's saying my answer is wrong. I also put in 1.15 x 10-4 also just in case masteringphysics likes just truncating the answer. However, that was also wrong. Does it have something to do with t being the initial time and not the final time? If so, I'm not really sure how to proceed from there.

2. Jun 6, 2014

### Staff: Mentor

Hi PKay. I'm just wondering what is x in your equation?

3. Jun 6, 2014

### rude man

Assuming the second term in B is 2.7e-5 t^4 I don't get 0.000265 t^3 for the second term in the emf.