Eddy Current Brakes: Explaining the Mechanics

[tamtam402]

Hey guys, our professor did a demo for us after the class since they just received new equipment, and unfortunately he didn't want to explain to us all the mechanics of what we witnessed because I'm currently following the mechanics course, but this was an E&M related demonstration. I know a little bit of E&M so I was trying to explain this stuff to my friend, but I'm not sure if my explanations are right.

Basically, he had a pendulum swing between an electromagnet creating a concentrated magnetic field. The pendulum was made of a metal that can conduct electricity. The pendulum was braked by the electromagnet.

Here is my explanation: I know about F = vq * B to explain the orientation of an electromagnetic force. The sheet of metal was moving perpendicular to the field (and so the electrons are moving perpendicular to the field), in a direction that applies a force downward to the electrons. The electrons in the back of the plate must move upward to balance the charges on the sheet, creating some sort of current. However, when these electrons in the back move upward, and they cross the magnetic field, F = vq * b shows that the resulting force goes against the movement of the pendulum. All this stuff is true no matter what side the pendulum is going; the current in the sheet will be in the opposite direction, but that's fine because the pendulum is also going in the opposite direction, meaning the force will always be opposite to it's movement.

Is this right?

 

[phyzguy]

Basically correct. There is a rule called Lenz's law which says that, "An induced current is always in such a direction as to oppose the motion or change causing it".

http://en.wikipedia.org/wiki/Lenz's_law