1. Mar 29, 2006

### pkossak

I was wondering about the following problem:

You are looking down on a single coil in a constant magnetic field B = 0.9 T which points directly into of the screen. The dimensions of the coil go from a = 6 cm and b = 15 cm, to a* = 20 cm and b* = 19 cm in t=0.028 seconds. If the coil has resistance that remains constant at 1.7 ohms, what would be the magnitude of the induced current in amperes?

Now, I have the answer, and I was told how to get it. I used the formula I = (delta A*B)/(delta t*R)

What I was wondering was if someone could tell me what rule or law this formula came from? I can't figure out how to derive it from any of the formulas given in this chapter. Thanks a lot.

2. Mar 29, 2006

### Hootenanny

Staff Emeritus
This is an application of Faraday's law which is defined as the negative change is magnetic flux over time mulitplied by the number of turns on a coil and is defined mathematically thus;

$$emf = -N\frac{\Delta(BA)}{\Delta t}$$

You will also need Ohm's law;

$$V = IR$$

Can you go from here?

-Hoot

If you need a derivation of Faraday's law, you can do a search on the net or I'm happy to guide you through it here.