Relativity in brief... or in detail..

Module 2 Electromagnetism and Galilean relativity

But what about electricity and magnetism? Could Galileo's sailor or our train travellers use electromagnetism to tell whether they are moving? Do the laws of electromagnetism work for both observers?

    Here is an example in which the multimedia module has advantages over this html version: it has film clips showing the relevant principles of electricity and magnetism, as well as animations. You might consider downloading it in a new window, while you continue reading here, or consulting the supporting sites.

Let's do what Einstein called a thought experiment.

Imagine that these two balls each carry positive electric charge. What is the force between them?
Like charges repel so you can calculate the electric force.

However, moving charges create magnetic fields and, in this case, the magnetic force is attractive.

So you see an electric repulsion, and also a magnetic attraction

But now look at it from my point of view.

With respect to me, the charges are not moving, so from my point of view there is no magnetic force. You see electric repulsion, reduced by the magnetic force. I see only the electric force. So your answer is less than mine.


This difference is very important. If electromagnetism works in different frames, then different observers get different answers for the force, solely due to relative motion.

In this case, the magnetic term is zero if the relative speed is zero. It increases with speed, so you can think of magnetism as a correction to the electric force that applies when the charges move. Follow this link to find out more.

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