[chris (OP)]

This interesting question surfaced on the radio today (thanks Albert!) and I'd be interested to hear how people here would explain it...

Chris

[Soul Surfer]

Presumably your electromagnet was energised using an alternating current which means that the magnetic field is changing constantly and reversing in direction cyclically.

Steel is magnetic and will be polarised by the field and produce an amplified magnetic attraction. This will be attractive either way.  Just like iron nails are attracted to either pole of a permanent magnet.

Aluminium is non magnetic and this does not happen.  However it is a good conductor of electricity and the changing magnetic field will induce "eddy currents" in a block of conducting material.  the net effect of these eddy currents is to produce a weak magnetic field that opposes the field of the electromagnet so the aluminium is repelled by the AC electromagnet.

You can see this effect happening with a permanent magnet and a superconductor which generates enough opposing magnetic field as the superconductor approaches the magnet to suspend it over the magnet.  In normal non superconductors the electrical resistance causes the eddy currents to die away quite quickly.

Why then is the steel not repelled because it too is a conductor of electricity?  Firstly steel is a much worse conductor of electricity than aluminium so the eddy currents are smaller and die away quicker and secondly the ferromagnetic induction polarising the electrons in the steel produces a much stronger field.

[RD]

Aluminium isn’t diamagnetic (it's paramagnetic) but can appear to behave like a diamagnet, i.e. repel a magnet, if moved towards one,  but this is due to induced currents (lenz law).

The same repulsive force between aluminium and a magnet also occurs if the magnet is an electromagnet powered by an alternating current, (alternating field), e.g.

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