# Magnetic and electric forces

1. May 6, 2006

### Random Variable

There are two long horizontal wires. The currents in both wires are traveling in the same direction. The current in the top wire is much larger than the current in the bottom wire. A weight is suspended from the bottom wire. Since magnetic forces can't do work, what force is doing work on the weight to lift it upwards? It's probably an electric force, but how was it generated?

2. May 6, 2006

### JohnPhys

You're right, since magnetic forces can't do work, it's an electric force. The electric force is from the fact that the electrons (free charges in the current) in the bottom wire are constrained to be in the wire!

For example, imagine you have a stream of electrons NOT confined to a wire. Then you switch on a constant external magnetic field. All of the electrons will then travel in a circle.

They are NOT allowed to do this in a wire! As a somewhat crude model, you can think of the electrons in the wire curving upward and "hitting" the "wall" of the wire, and the collision raises the wire up a tiny bit (while repelling the electrons down). The magnetic field tells the electrons to curve upward again, and the process is repeated. So, it's the electric force between the electrons in the wire and the rest of the wire that is responsible for the wire rising up.

3. May 6, 2006

### ptabor

The work is being done by the battery, or generator.