Tell me more ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.
up vote 2 down vote favorite

If electromagnetic radiation represents a transfer of energy, then doesn't a permanent magnet represent unlimited energy, and if so, why can't magnets be used for perpetual motion? Even if permanent magnets aren't permanent and rather just have a really long usable lifetime, don't they at least represent a large resource of energy?

share|improve this question
1  
umm.. yeah we pretty much generate all our electricity by spinning magnets around copper wire and vice versa... so it's not the source of the energy but it is used to generate electricity. e.g. water fall hydroelectric plants... remember it takes energy to move charged particles or conductors through a magnetic field and currents are generated in the process – Timtam Jan 31 '12 at 9:24
   
-1: because the total magnetic energy is finite. – Ron Maimon Aug 24 '12 at 20:15
@Ron - so if you have a finite energy output and it lasts forever... that's finite? What about a candle that burns forever. -1 on all your questions :) – orokusaki Aug 25 '12 at 1:49
@orokusaki: Sorry, I was in a bad mood, and I thought this was too simple a question. Why do you think there an infinite energy output from a permanent magnet? It's a finite amount of energy to magnetize, and the total field energy is finite. – Ron Maimon Aug 25 '12 at 1:57

2 Answers

up vote 6 down vote accepted

A (non-accelerated) permanent magnet doesn't produce electromagnetic radiation. It is surrounded by a magnetic field (just like a charge is surrounded by an electric field), but it does not radiate. Especially there's no energy leaving the magnet.

Note that for energy flow (as in radiation) you need both an electric and magnetic field, because only then you'll get a non-vanishing Pointing vector $\vec S=\vec E\times\vec B$. However note that even if you have energy flow, you don't necessarily have radiation (e.g. if you have a constantly moving charge, the field energy is flowing with the charge, but it doesn't radiate).

share|improve this answer
Would you mind commenting on the new answer and on my reply to it? – orokusaki Aug 24 '12 at 22:11
up vote 1 down vote

An object has potential energy if it has the capacity to do work. Work is done if an object is moved by a force in the direction of the said force. A permanent magnet can via it's force fields move a ferromagnetic material. Energy is said to be transfered whenever work is done. Permanent magnets emit force fields which are identical to the force fields that are emited from electromagnets. Permanent magnets can do work on an object that can be measured (force X distance). Correction with ref: celtschk with thanks.

share|improve this answer
Imagine a row of permanent magnets, 1,000,000 miles long, each's N facing downstream in the line, and then you accelerate 1 magnet with its S facing upstream against the N magnets. It's on a track and locked from turning. Couldn't you accelerate the magnet down track at a certain speed SP and have it remain at speed SP until the end of the line (assuming you set up everything just right and there was low friction) – orokusaki Aug 24 '12 at 22:10
Meant to say "N facing upstream against..." – orokusaki Aug 25 '12 at 1:56
You cannot radiade a force. You radiate waves, or you radiate particles. The magnet is surrounded by a magnetic field, but it doesn't radiate it. Note that also an electromagnet doesn't radiate (except when you switch it on or off, and ignoring the heat radiation because of the wire's resistance). And it is true that you do work (and thus change energy) if you move a magnetic object in the magnetic field, that's something different from radiating energy. When radiating, you lose energy even without objects moving. Also note that when you move objects in the field of another object, … – celtschk Aug 25 '12 at 8:44
you don't take energy from the other object, but from the field. A nice example where you can see it quite directly is the electric field of a (charged, but not connected) plate capacitor. The two plates attract each other, therefore you need energy to move them apart. There's a homogeneous field in between then, whose energy you can calculate before and after moving the plates. If you do so, you'll find that the total energy in the field has increased exactly by the amount of work needed to move the plate against the field. So the plate's potential energy actually is the energy of the field. – celtschk Aug 25 '12 at 8:51
@orokusaki: If I understand your setup correctly, then yes, that should work. You are moving the magnets orthogonal to the force, therefore you're not doing any work, and thus the magnet should be moving forever. Or in field energy terms: The fielf always looks exactly the same, just at a different position, and since you don't accelerate, there's also no radiation. However, that's no perpetuum mobile, because you don't gain energy; it's no different than the magnet moving with constant speed where there's no other magnet (that is, no magnetic field). – celtschk Aug 25 '12 at 9:11
show 1 more comment

protected by Qmechanic♦ Dec 19 '12 at 12:55

This question is protected to prevent "thanks!", "me too!", or spam answers by new users. To answer it, you must have earned at least 10 reputation on this site.

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