# Can electromagnetic momentum be introduced at pre-university level as for electromagnetic energy?

Electromagnetic energy is introduced at pre-university level, starting with static electric energy followed by static magnetic energy. But the introduction of electromagnetic momentum usually has to wait until university, usually after Poynting's theorem has been derived from Maxwell's equations.

Is there a way of introducing electromagnetic momentum at the same time as electromagnetic energy, or must is always wait until after Maxwell's equations have been introduced?

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## 1 Answer

Einstein developed a simple though experiment to show electromagnetic radiation must carry momentum:

Suppose you have a rocket floating in space. A photon is emitted from one end and absorbed at the other. This reduces the mass at the emitting end by $h\nu/c^2$ and increases the mass at the other end by the same amount, so the centre of mass of the rocket moves. However the centre of mass cannot move unless a force is applied, therefore there must have been a force exerted on the rocket when the photon was emitted and again when it was absorbed. Hence the photon must carry momentum.

As I recall it's possible to work out the momentum and you end up with $E^2 = p^2c^2$ i.e. just the usual relativistic expression for the energy with $m_0$ set to zero.

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You mean $E^2=p^2c^2$? –  Robert Mastragostino Mar 1 '13 at 17:28
Eek! Yes, thanks :-) –  John Rennie Mar 1 '13 at 18:06
Well this assumes that the electromagnetic interaction is mediated by photons. This may not be that obvious at this level. –  alexarvanitakis Mar 1 '13 at 19:43
I would guess every school physics student knows that light is made up of photons. It's weaning them off the concept that can be the problem! –  John Rennie Mar 2 '13 at 10:04