How much energy does a magnet contain?

[Gozu]

If magnets turn out to be viable replacements to batteries (storing energy magnetically rather than electrochemically), how much energy could we store in one?

[Q.E.D]

None. Magnets don’t store energy, they transfer it.

[Squink]

This story?
Inventor Doesn’t Dare Say ‘Perpetual Motion Machine’

In Heins’ machine, he explains that magnetic friction somehow gets turned into a magnetic boost. Working with an electric motor, he attached the drive shaft to a steel rotor with small round magnets lining its outer edges. In this set-up of a simple generator, the rotor would spin so that the magnets passed by a wire coil just in front of them, generating electrical energy.

Then Heins did an experiment: he overloaded the generator to get a current, which typically causes the wire coil to build up a large electromagnetic field. Usually, this kind of electromagnetic field creates an effect called “Back EMF” due to the so-called Lenz’s law. The effect should repel the spinning magnets on the rotor, and slow them down until the motor stops completely, in accordance with the law of conservation.

But instead of stopping, the rotor began to accelerate. Heins recounts that the first time it happened, the magnets starting flying off and hitting the walls, as he ducked for cover.

[Strain_of_Thought]

@Squink

This story?
Inventor Doesn’t Dare Say ‘Perpetual Motion Machine’

A Perpetual Motion Machine? Pfft. It’s obviously just some form of cold fusion.

[wolf_meister]

Just to expand on Q.E.D.'s answer, magnetism is a force and not energy.
If you put a stationary magnet inside a coil of wire how much energy do you get? Nothing.
Moving the magnet produces the energy.
Energy = force • distance
No motion = no energy

[Crafter_Man]

An inductor with current flowing through it is storing energy in a magnetic field. But if you want to go that route, I would suggest capacitors, as they tend to be more efficient (high Q, low d, or whatever) than inductors.

[ZenBeam]

There is energy in the magnetic field of a magnet as well (yeah, the link talks about inductors, but the energy density formula is the same). But realistically, it’s a very tiny amount, negligible compared with batteries.

[kanicbird]

There is a flywheel battery, but that’s storing energy mechanically just using magnetics to add or subtract energy.

@Crafter_Man

I would suggest capacitors, as they tend to be more efficient (high Q, low d, or whatever) than inductors.

In terms of efficiency we have made 100% efficient induction using superconductors, I don’t think we have come close to that in capacitors.

[Crafter_Man]

@kanicbird

In terms of efficiency we have made 100% efficient induction using superconductors, I don’t think we have come close to that in capacitors.

Do the inductors have to be cooled below ambient temperature?

[Q.E.D]

@kanicbird

In terms of efficiency we have made 100% efficient induction using superconductors, I don’t think we have come close to that in capacitors.

This is wrong. Superconductivity reduces resistive losses; however, the major loss factor for inductors is incomplete magnetic coupling. Using a core of a high-mu material like ferrite can improve this, but you can never achieve perfect coupling, if for no other reason than the copper windings cannot occupy 100% of the available space, no matter how tightly you wind them.

[Q.E.D]

@Crafter_Man

Do the inductors have to be cooled below ambient temperature?

Heh. Just a little.

[Crafter_Man]

@Q.E.D.

Heh. Just a little.

Exactly… and hence the overall efficiency goes out the window.

[Tim_T-Bonham.net]

@Gozu

How much energy does a magnet contain?

The mass of the magnet, times the speed of light squared.

Theorized by Albert Einstein, and proven by various others.

[kanicbird]

@Crafter_Man

Do the inductors have to be cooled below ambient temperature?

It depends on where you place them, in deep space out of the sun no cooling should be needed.

[Q.E.D]

@kanicbird

It depends on where you place them, in deep space out of the sun no cooling should be needed.

And what’s the energy cost of lifting a payload into deep space?

Also, of what practical value is a component which must be kept out of the Sun at all times?

[kanicbird]

@Q.E.D.

This is wrong. Superconductivity reduces resistive losses; however, the major loss factor for inductors is incomplete magnetic coupling. Using a core of a high-mu material like ferrite can improve this, but you can never achieve perfect coupling, if for no other reason than the copper windings cannot occupy 100% of the available space, no matter how tightly you wind them.

I don’t believe you are correct here, first:

Superconducting loops of wire have been shown to carry electrical currents for several years with no measurable loss.

Second:

Another property of a superconductor is that once the transition from the normal state to the superconducting state occurs, external magnetic fields can’t penetrate it.

So there should be no magnetic drag if I understand this correctly.

Either way still better then capacitors.

both cites from http://www.howstuffworks.com/question610.htm

[Chronos]

“Efficiency” can refer to a lot of different things, though. I’m reasonably sure that over a long term, you can store more energy in a capacitor than in an inductor, for devices of the same size. So in that sense (which is very important for many practical applications), the capacitor is more efficient.