# Homework Help: Change in flux

1. May 2, 2010

### jimbkob

1. The problem statement, all variables and given/known data

A flexible, circular conducting loop of radius 0.15m and resistance 4 ohms lies in a uniform magnetic field of .25 T. the loop is pulled on opposite sides by equal forces and stretched until its enclosed area is essentially zero m^2. it takes .30s to close the loop. what is the change in flux?

2. Relevant equations

3. The attempt at a solution

2. May 2, 2010

### xcvxcvvc

since there are no details on the change in area with respect to time (i.e. you don't know exactly how the area changes as it bends from the 2 forces) it seems like a simple estimate using final - initial

$$B\frac{A_F-A_i}{t_F-t_i}$$

3. May 2, 2010

### ideasrule

The change in flux doesn't depend on time. It's just the final BA minus the initial BA.

4. May 3, 2010

### jimbkob

Thanks for the quick response!

I'm not say either of you guys are wrong but I tried both equations and I got two different answers.

The first equation got me .01767 but when i plug this into Faraday's equation I get -0.0589 volts

The second equation got me -0.0589 Wb.

Which one is correct?

5. May 3, 2010

### xcvxcvvc

i accidentally told you how to find the change in flux with respect to time (I saw you mention faraday's law and it set me on that track)

If the problem is only asking for the change in flux, do what ideasrule said:
B(A_f - A_i)

sorry about setting you off on the wrong track.

6. May 3, 2010

### jimbkob

haha no it's ok no big deal. Thanks again!!