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I bought a 4 W Electromagnet to collect aluminium cans laying on the floor. A can weighs around 0.015 kg. I read aluminium is only paramagnetic. And playing manually with the current magnet, I cannot appreciate any force being exerted to the aluminium can.

How powerful should a magnet be to be able to pick the cans from the floor? (let's say, from 10 cm away?)

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    $\begingroup$ Wood is also para-magnetic, as is air. Are you planning on picking them up with a magnet too? I suspect that by the time your magnet is strong enough to pick up a can it'll also be strong enough to pull out any iron nails holding the floor together. $\endgroup$
    – Andrew
    Jan 11, 2017 at 9:38
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    $\begingroup$ Strong electromagnets are used at recycling plant to remove magnetic metals like iron & steel that may contaminate collections aluminum cans $\endgroup$
    – Fred
    Jan 11, 2017 at 11:11
  • $\begingroup$ Things to think about that may help to formulate an answer: How much iron does the relevant grade of aluminium contain as an impurity? Is that iron in a solid solution within the aluminium crystals, or is the iron segregated into its own crystals? If the iron is segregated into its own crystals, are those crystals big enough to be ferromagnetic? $\endgroup$
    – Daniel Hatton
    Oct 9, 2020 at 21:46
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    $\begingroup$ Strong electromagnets are also used to separate non-magnetic metals like aluminum from plastic. But they don't act on principle of magnetic attraction, just on inducing eddy currents - aluminum and plastic mix is shot at high speed through a strong magnetic field, which inducing current in the quickly moving conductor slows it down and makes it fall into a different bin than the plastic. $\endgroup$
    – SF.
    Feb 4, 2021 at 11:55

2 Answers 2

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A magnet will never be able to "pick up" an aluminum object, no matter how strong.

To move aluminum with a magnet, you have to subject it to a changing magnetic field. This field will produce eddy currents in the aluminum, which will in turn create a magnetic field around the can, which will then oppose (repel) the magnetic field which induced the eddy currents.

Practically speaking that means if set an empty can on a table, and swung a very powerful magnet past it, you might move the can a little.

If the can and magnet were stationary relative to each other, nothing would happen.

If you used an electromagnet instead and brought it near the can then turned it on, the can would be briefly repulsed.

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It turns out THERE IS a magnetic way to separate aluminium.

When a piece of nonferrous metal, such as aluminum, passes over the separator, the magnets inside the shell rotate past the aluminum at high speed. This forms eddy currents in the aluminum, which in turn create a magnetic field around the piece of aluminum.

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  • $\begingroup$ And then what? Can we use that to attract the cans to the same rotating magnets or do we need others? $\endgroup$
    – Transistor
    Sep 7, 2020 at 11:08
  • $\begingroup$ @Transistor well, a large enough eddy current can cause the aluminum to melt then it can be collected in a container by gravity... :) most efficient? $\endgroup$
    – Solar Mike
    Sep 7, 2020 at 14:43
  • $\begingroup$ @Solar, Yes, that might work but it's unlikely to be efficient compared with picking up steel with a simple magnet or electromagnet. I'm not being critical - just thinking ... $\endgroup$
    – Transistor
    Sep 7, 2020 at 14:54
  • $\begingroup$ @Transistor I'm not getting at you, sorry - just trying to get the answerer to think a bit... $\endgroup$
    – Solar Mike
    Sep 7, 2020 at 14:56
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    $\begingroup$ At best you can cause a deflection. $\endgroup$
    – StainlessSteelRat
    Sep 7, 2020 at 19:19

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