Using magnets to generate electricity(?)

[trexmatt]

Hi,

Recently I've become interested in the possibility of using magnets to generate electricity, a "magnet motor" if you will. There are evidence of a lot of different attempts on the internet as well as a number of actual patents for these motors.

When I say "magnet motor" I mean a device that in some ways using multiple magnets and their interactions to spin or move something thus generating electricity.

Anyways, I have a couple questions that I was hoping someone could help me with and Physics Forum sounded like a good place to start. If this is off-topic or not a suitable forum topic for whatever reason, mods feel free to delete :)


Is a magnet motor even possible (according to the laws of physics)?

Is this different from perpetual motion?

If yes to both questions above, why aren't we using them?



Thanks for your time, any help is much appreciated...

- trexmatt

 

[cragar]

do you mean like moving a magnet in and out of a copper coil to generate electricity .

 

[trexmatt]

Ah, no sorry, should have been more specific. That is definitely possible!

I mean something along the lines of this

http://www.youtube.com/watch?v=4Y-AFQqZVBw&feature=related"

But that is an extremely simple design and the video might not even be true, it's just to illustrate the concept. There are a lot more sophisticated designs and that's more what I am interested in.

Basically, ignoring the specifics or design in that video, is the concept possible?

 

[cjl]

Do you mean a motor based exclusively on permanent magnets, which is what the one in that video appears to be?

If so, that would violate conservation of energy, as well as allowing for perpetual motion. It's not possible to make a motor with only permanent magnets (though many have tried).

 

[trexmatt]

I see, so it does violate conservation of energy.

But how so? The magnets themselves create a force (with the Earth), how is it that this force cannot be used to turn something, producing electricity?

 

[cjl]

It's because the field around a magnet is a conservative field. This means that the work done by a magnetic field is path independent.

In other words, if I had two magnets, and I moved one of them from point A to point B, then the field did some work on the magnet in that process (since there was a force acting on it due to the magnetic field). However, no matter which path I use to get from point A to point B, the work done will always be the same. Similarly, if I go from point B back to point A, regardless of the path, the work will always be exactly the negative of the work done from point A to point B. So, anytime you have a device that completes some sort of cycle (moving magnets around in some way or another such that they end up in the same place as they started after each cycle), no net work can be done. Work can be done at intermediate stages, of course, but by the time the machine completes its cycle, the net work done by the magnetic fields must be zero.

Of course, you could extract some energy by using the force to turn a generator. However, the only way this could be done is if in the process of turning the generator, the magnets got closer together (I'm assuming an attractive force here - for a repulsive force, all of the same physics apply, the forces are just turned around). If you continued this process, the magnets would eventually touch, and then you have effectively extracted all of the magnetic potential energy from the system. Before you could extract any more, you would have to move the magnets apart, and to move the magnets back to their starting positions would require just as much energy as you got out of the system when they were allowed to come together.

The same is true of gravity. Gravity is another conservative field, although it is one that most people are much more familiar with. Gravity exerts a force on objects, but you cannot produce electricity from gravity unless the system ends up in a different state than it started.

(Does that make sense?)

 

[Phrak]

It's because the field around a magnet is a conservative field. This means that the work done by a magnetic field is path independent.

I'd be hard pressed to prove that moving charges in the fields of moving magnets are subjected to conservative fields. Can you motivate this claim, or where you just guessing?

 

[mr magoo]

1.) Have the disk with magnets on the outer rim of the disk, and the disk can spin freely.
2.) Have large magnets on the far left and right of the disk that can spin. These are electromagnets that can be turned on and off via electricity.
3.) These large magnets on the far left and right of the disk that can spin have a shield that blocks them from attracting the magnets on the disk. This shield covers half of the magnet.
4.) When the disk spins the magnet on the far left or right attracts the magnets on the disk and the disk spins.
Then, the magnet is shielded and the other magnet on the other side is available as a magnet and attracts the magnets on the disk.
What turn the shield on the magnets is the shield is attached to the disk and spins when the disk spins.
This way the magnets on either side of the disk are always attracting the magnets on the spinning disk, perpetually.

 

[cjl]

I'd be hard pressed to prove that moving charges in the fields of moving magnets are subjected to conservative fields. Can you motivate this claim, or where you just guessing?

Well, I'll admit that I've never gone through any formal proof, but thinking about it, it seems to make sense.

Since the force on a moving charge in a magnetic field is always perpendicular to velocity, the dot product of force and an infinitesimal displacement will always be zero. Therefore, no work should ever be done, even though the force will change depending on the velocity of the particle (and the field). If I'm wrong, I'd love to hear about it, but I can't see any flaws in the basic argument.

(Also, I was referring to the force between several magnets, not the force on charges near several magnets, although admittedly it could still definitely be relevant)

 

[Phrak]

Well, I'll admit that I've never gone through any formal proof, but thinking about it, it seems to make sense.

Since the force on a moving charge in a magnetic field is always perpendicular to velocity, the dot product of force and an infinitesimal displacement will always be zero. Therefore, no work should ever be done, even though the force will change depending on the velocity of the particle (and the field). If I'm wrong, I'd love to hear about it, but I can't see any flaws in the basic argument.

(Also, I was referring to the force between several magnets, not the force on charges near several magnets, although admittedly it could still definitely be relevant)

Good thinking cjl. I had a clever answer to counter your argument--and discovered that it was not so clever, after all. I'll have to put a little more thought into it.