# Why is the magnetic field stronger at this specific point?

1. Nov 17, 2008

### selishaphysic

A rod (shown in the picture on the link) rotates around the point O in a magnetic field. It is said for sure that the induced EMF in the rod is larger at point R compared to point P. Why's that?

The picture: http://img58.imageshack.us/my.php?image=physicskx4.png

2. Nov 17, 2008

### Nick89

The EMF is larger because the area of the enclosed circuit is larger. The magnetic field is NOT larger, it is constant.

3. Nov 17, 2008

### selishaphysic

Oh I meant the EMF of course. But can you please explain your answer more thoroughly I don't fully understand the situation.

4. Nov 17, 2008

### Nick89

The EMF is given by:
$$\epsilon = \oint \left( \textbf{v} \times \textbf{B}\right) \cdot d\textbf{l}$$
Here, B is the magnetic field, dl is a small segment of a circuit, and v is the velocity of a charge in that circuit.

If you think of the electrons in the rotating rod as charges going around in a circular circuit, then you can easily see why the EMF is larger when you get further from the origin.
First of all, the velocity v is larger, secondly the path of integration (the length of the loop) is larger.