1
$\begingroup$

How to explain the fact that magnet can attract an object (apply a work W) without losing a (significant) part of its internal energy?

How to apply the energy conservation principle?

Please think to a purely permanent magnet motor (without any electricity supply).

$\endgroup$
2

1 Answer 1

1
$\begingroup$

Magnet's energy doesn't go anywhere. It always stays $\frac{B^2}{2\mu}$ (in some conditions). So where does the work come from, the energy to pull metal to the magnet?

If you put piece of iron, or better a wire with flowing current $j$, on distance $d$ from source of magnetic field $B$, then there will be force between two: $\vec F=\vec j \times \vec B(d)$ (see Lorentz law). If you hold this wire in place, then there will be potential energy $U=-\int_{0}^{d} Fdx$. If you let this wire go, potential energy will turn into kinetic energy of movement $E=\frac {p^2}{2m}$.

That is how you apply conservation of energy: there is potential energy of B-field, and there is potential energy in system magnet-wire. No wire -- no second part of potential energy.

Now, what does piece of wire with current has to do with real-life piece of iron? See page on ferromagnetism for more, but briefly, there are currents in piece of iron due to electrons' spins, as in any other material. But because in iron currents from many electrons are oriented in same direction, cumulative effect is very strong.

P.s.: I come from/practice experimental physics, so might not care too much about details. That is a flaw. Please comment, so I can refine this answer.

$\endgroup$
2
  • $\begingroup$ The user who place the iron piece in mag. field gives the initial PE that transforms to KE. The final KE will vanish and transform into strain (on both the magnet and the iron piece). Right? But I'm still unable to find the explication of failure of the mag. perpetual rotary machine: [youtube.com/watch?v=zqG-TL0WnjE] It's not possible but I try to understand the reasons. thank you very much $\endgroup$
    – aayyachi
    Mar 16, 2015 at 11:10
  • $\begingroup$ I think these youtube perpetual motion machines are magician tricks. There could be an electric motor and batteries inside the wood posts. $\endgroup$ Oct 27, 2015 at 3:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.