Relativity in brief... or in detail..

Electricity and magnetism in a moving frame: what would you expect?

If you accepted what we said in An introduction to the mechanics of Galileo and Newton, or better still if you did some experiments yourself to check what we said, then you will probably be convinced that doing mechanics experiments on a smoothly moving ship, train or plane will give you the same results that you get on the ground.

Now let's think about other experiments. Virtually all electronic equipment has capacitors, which are devices that store electric charge by creating a local electric field. Many also have inductors, which are coils that can exchange electrical and magnetic energy. Would you expect the operation of such equipment to be changed when you board the train? And if so, why? Would the same arrangement of electric charge give the same field, as measured by you on the train, as it would if you did the experiment on the platform? Would the same current in a coil give the same magnetic field. It is worth reflecting on these questions - Einstein did.

For those with some background in electricity, we can consider an example explicitly, which we do here.

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