# Homework Help: E & M

1. Dec 1, 2004

### Physicsisfun2005

Problem:
A hundred turns of insulated copper wire are wrapped around an iron cylinder of cross-sectional area 1x10^-3 sqr. m and are connected to a resistor. The total resistance in the circuit is 10 ohms. If the longitudinal magnetic field in the iron changes from 1.0 T in one direction to 1.0 T in the opposite direction, how much charge flows through the circuit.

I was wonderin why the direction will affect the magnitude of the charge?

2. Dec 1, 2004

### Diane_

I suspect they're talking about the total charge. The system as you describe it is not connected to a battery, so in steady-state there is no current in the circuit. If you change the magnetic field in the core, you're going to induce a voltage, with the corresponding current flow. This will continue until a new steady-state of no current is achieved. It looks to me like the question is asking how much charge will flow through the circuit between one state and the other.

3. Dec 1, 2004

### Physicsisfun2005

Is a emf emitted?.......Faraday's Law or Lenz's Law I think but how do I apply it?....

4. Dec 1, 2004

### Diane_

You have a conducting coil with a magnetic field passing through it, right? Any change in that field will induce a voltage in the coil, just as any change in the current through the coil will induce a magnetic field.

Methinks you need to run back over the section on electromagnetic induction in your text.

5. Dec 1, 2004

### eridanus

$$E = N\Delta\Phi/\Delta t$$
or induced voltage equals the number of loops times the change in flux over change in time
direction matters because flux is a vector. so the change in flux would be (1.0 T)(1x10^-3 sqr. m)-(-1.0 T)(1x10^-3 sqr. m)
change in flux is proportional to emf and emf to current

6. Dec 1, 2004

### Diane_

Not to get all Forum Police-y, but it is better to let him work it out on his own. Yes, I have problems doing that, too. :)

7. Dec 2, 2004

### Physicsisfun2005

thanx ;).......i figured i'd use that equation.