Q & A: relative motion and electromagnetism

Q:
I know what relative motion is. I know what frames of motion are. I have garnered this knowledge from dozens of sources who sought to answer the question "explain what is meant by relative motion" by using examples... many, many examples. But when I put pen to paper, I was at a loss for words. please Explain what relative motion is in a concise, simple paragraph. and while we are at it, how is it relevant to Faraday's electromagnetic induction experiment with relative motion b/w a conductor and magnet?
- Philip (age 17)
Australia
A:

Let's say that you look in a frame where a magnet is at rest and a wire is in motion at right angles to the magnetic field. Then there's a "Lorentz force" on the charges moving through the field, and that force causes the charges to move along the wire.

In the frame where the wire is at rest and the magnet is moving, the moving magnet makes an electric field. That field exerts a force that causes the charges to move.

So the description changes between the frames, but the basic physical effects remain the same. We know that some features of the physical situation are "invariant"- the same in eaach reference frame. Special Relativity amounts to making a consistent acount of what those features are.

As for what "relative motion" is, I can't think of anything to say except that it's the motion of one object as seen from the other's point of view.

Mike W.

(published on 04/01/2014)