# Change in magnetic flux

1. Feb 18, 2008

### rcmango

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

A loop of a wire has the shape shown in the drawing. The top part of the wire is bent into a semicircle of radius r = 0.24 m.

The normal to the plane of the loop is parallel to a constant magnetic field ( = 0°) of magnitude 0.75 T. What is the change in the magnetic flux that passes through the loop when, starting with the position shown in the drawing, the semicircle is rotated through half a revolution?
Wb in weber

here is a pic: http://img213.imageshack.us/img213/4917/75104403bx0.png

2. Relevant equations

3. The attempt at a solution

not sure where the time comes from, how to figure that out from the diagram.

also, are A0 = x0*L

that is the area swept out in time t.

need help finding the variables for this one.

also, i know that I need to find the angle A and B make at time t.

I need help on how to do this.

2. Feb 19, 2008

### Mindscrape

So what you probably want to do in your imagination is rotate the loop and figure out how the area vector's normal changes. After all, flux will only go through the parts of the loop that are parallel to the magnetic field. How do the components of the area normal vary in time if we call the angular velocity of rotation omega?

The math formalism you want is essentially

$$\Phi = AB\matbf{\hat{n}}(t)$$

Last edited: Feb 19, 2008