# Homework Help: Induced Magnetic Fields

1. Mar 28, 2005

### TickleMeElma

Hey all smartie-pants!

So this is the problem I have a tough time with:

The magnetic field perpendicular to a single 13.2-cm-diameter circular loop of copper wire decreases uniformly from 0.750T to zero. If the wire is 2.25mm in diameter, how much charge moves past a point in the coil during this operation?

This is what I was able to come up with:

I need to find the circumference, which will give me the length of the wire, which will help me find the resistance of the wire. Knowing that, I can find out the current, which they are looking for?

But what about time? Don't I need time to find out the flux??

Thanks so much for any help!!

2. Mar 28, 2005

### Gamma

Induced emf V = $$- d \phi /dt = -A dB/dt$$

But ohms law, V = I R = dQ/dt * R

From above two you have

$$A * dB/dt = - R * dQ/dt$$

or

A dB = R dQ

integrating,

$$Q = A/R * (change in B field)$$

A is the cross sectional area of the loop

R can be found using R = s L / A where s is the resistivity of copper, L circumferece and A is the cross sectional area of the wire.

Last edited: Mar 28, 2005
3. Mar 28, 2005

### plusaf

color me puzzled.....

isn't the strength of the magnetic field in a one-turn solenoid, as you've described, governed by the current in the wire and its dimensions alone?

you have the diameter of the loop; the strength of the magnetic field should be strongest in the middle of the loop, in the plane of the loop, and fall off in all directions, radially, right? so how much current will produce the specified magnetic field?

current = charge/second; you only need volts and ohms and resistivity and stuff like that if they're part of the problem OR solution, which, it looks to me, they're not.....

color me wrong?
+af
)