# Homework Help: Electromagnetism physics question

1. Oct 27, 2006

### L89

Hi, I have a question concerning the attached diagram:
The question which accompanied the diagram is: Why does the aluminium rod (the one that lies across the other two) slow down when a current is produced?

The official answer is: "A current is produced in the rod which experiences a force in an opposite direction to the original motion, therefore slowing down the motion."

However, I do not really understand this and tried to formulate my answer. This is my attempt:

As the rod moves towards the right , its velocity is at 90° to the magnetic field. Thus the electrons in it will have the same velocity. Hence, according to the Left Hand Slap Rule [opposite of RightHSR for positive particles], a magnetic force is exerted on them and they move to the further end of the roller. The closer end becomes positively charged. Meanwhile, this potential difference induces a voltage across the rod thus causing electrons to flow anticlockwise. The two electron motions are opposite, thus the total current is small. Hence the magnetic force exerted on the rod (as current is at 90° to field) is small, and a smaller speed is produced.

MY QUESTION IS THIS: Is my answer correct? If it's not can someone enlighten me?

Help will be appreciated
L89

2. Oct 27, 2006

### OlderDan

There is no diagram, but this is a standard problem, so I think I get the geometry. The official answer is a more consice way of saying a lot of what you are saying. The part about the motions being opposite is not accurate. When the wire moves the electrons are forced in the direction of the wire. This is perpendicular to, not oopposite, the direction of motion of the wire. The part of their motion in the direction of the wire results in a force that is opposite the direction the wire is moving, so there is a force that opposes the motion of the wire.

The motion of the electrons in the direction of the wire is conveniently described as a current, and the force perpendicular to this current is well known. That is why the official answer uses current to describe the effect.

3. Oct 27, 2006

### L89

Thanks very much. Yeah, it clicked. Stupid me :)