Where do motions generated by magnetism & gravitation derive their energy from?

[the Interceptor]

Lately discussing the scientific riddles of todays world with my workmate, we arrived at the following question:

Take two magnets. Put them close to each other, so that their magnetic fields start interacting. If you let them go, they will obviously move toward each other (or away, depending on the orientation). But where does the energy for that movement come from?
You put up a static system. This system suddenly develops kinetic energy. So there must have been another form of energy that was transformed into kinetic energy. Obviously, the energy comes from the magnetic field. But as we know, energy can not be generated or abolished, it can only be transformed. So when moving, the magnets must use up energy from somewhere. But does magnetism dissipate? Does a magnet use up its magnetism though magnetising things?

This thought experiment actually derived from a different, yet kind of similar thought. Let's say I could move planets in space. So it take two planets and put them close to each other (there's no rotation and no movement through space). Obviously, gravity will make them gravitate toward one another. But where do they get the energy for that movement from?
Gravity directly depends on mass. But the mass of the planets is not decreased when they gravitate. So the energy for generating velocity isn't taken away from the mass, and therefore not from the gravity. But if it isn't, where does it come from?

I'm a bit unsure about this whole thinking. I feel like I have some major flaws in understanding the principle of these energies and how they mesh together, but I really can't come up with the answer for these questions. Can you?

 

[Kylehnat]

So there must have been another form of energy that was transformed into kinetic energy.

There is: potential energy. Magnets create a magnetic field which has a magnetic potential. It's the same thing as if you hold a ball in the air, then drop it. The ball is initially at rest. You put no energy into it yourself (you don't push, pull, or blow on it), yet it falls anyway...because of gravity potential, a well-defined force that acts on any object that has mass. Magnets act much in the same way.

Obviously, gravity will make them gravitate toward one another. But where do they get the energy for that movement from?

That's a million-dollar question. We have no idea what causes gravity, or why it works.

 

[Philly]

Isn't it some kind of electrical attraction between the electrons depending on their arrangement in the sub-atomic orbitals? I also thought there might be a little flow of electrons in there, but that might just be electricity.

 

[Kylehnat]

Yes, magnetism is caused by polarization of electron-spin.

 

[Loon]

There is: potential energy. Magnets create a magnetic field which has a magnetic potential. It's the same thing as if you hold a ball in the air, then drop it. The ball is initially at rest. You put no energy into it yourself (you don't push, pull, or blow on it), yet it falls anyway...because of gravity potential, a well-defined force that acts on any object that has mass. Magnets act much in the same way.

That makes sense, the ball had potential energy, because of gravity when it was let go, it was transformed into kinetic. But where is the magnet getting its energy? The ball got it because of gravity, right? What is causing the magnet to be able to convert its potential into kinetic?

 

[Kylehnat]

Gravitational potential = magnetic potential = electric potential. They all create a force, though not in the same way. With electricity, there is an imbalance in charge. With magnetism, there is an "imbalance" in polarity, or electron orientation.

 

[Sage]

That makes sense, the ball had potential energy, because of gravity when it was let go, it was transformed into kinetic. But where is the magnet getting its energy? The ball got it because of gravity, right? What is causing the magnet to be able to convert its potential into kinetic?

Electromagnetism is like gravity – it’s a fundamental force that is just sort of there, that we know of because of empirical data, but not something we can really truly explain, yet.

Notice in your question how you just take gravity, its existence, and its effects for granted – you could easily switch your last question to “What is causing gravity to be able to convert potential into kinetic?”

 

[Loon]

Electromagnetism is like gravity – it’s a fundamental force that is just sort of there, that we know of because of empirical data, but not something we can really truly explain, yet.

Notice in your question how you just take gravity, its existence, and its effects for granted – you could easily switch your last question to “What is causing gravity to be able to convert potential into kinetic?”

Interesting...This will be something to discuss with my Physics teacher because it sounds intriguing, and I'd like to know more.

 

[the Interceptor]

There is: potential energy. Magnets create a magnetic field which has a magnetic potential. It's the same thing as if you hold a ball in the air, then drop it. The ball is initially at rest. You put no energy into it yourself (you don't push, pull, or blow on it), yet it falls anyway...because of gravity potential, a well-defined force that acts on any object that has mass. Magnets act much in the same way.

I understand what you're saying, but I think there is a difference. If I take up a ball and hold it at a height of 1 meter, I have loaded it with a specific amount of potential energy. This energy is transformed into kinetic energy when I drop it. The movement derives its energy from the work I put into the ball earlier.

So let's get back to the planets. Yes, I have moved the planets, but that didn't load them up with potential energy, because I didn't have to overcome gravity for that. And if I would have done, they wouldn't gravitate towards each other, rather than where I took them from. So in opposition to the ball experiment, there was no potential energy added. Still, they move...

Electromagnetism is like gravity – it’s a fundamental force that is just sort of there, that we know of because of empirical data, but not something we can really truly explain, yet.

Notice in your question how you just take gravity, its existence, and its effects for granted – you could easily switch your last question to “What is causing gravity to be able to convert potential into kinetic?”

Agreed. But despite we don't know what happens in detail, we can calculate precisely what happens to a ball that's dropped. Such calculations also take gravity as granted, as a simple constant which is just there. On that basis, I am not sure if we need to know how gravity works when we ask these questions. Or do we? I don't know...

 

[Famine]

I understand what you're saying, but I think there is a difference. If I take up a ball and hold it at a height of 1 meter, I have loaded it with a specific amount of GRAVITATIONAL potential energy. This energy is transformed into kinetic energy when I drop it. The movement derives its energy from the work AGAINST GRAVITY I put into the ball earlier.

Fixed.

 

[Sage]

So let's get back to the planets. Yes, I have moved the planets, but that didn't load them up with potential energy, because I didn't have to overcome gravity for that. And if I would have done, they wouldn't gravitate towards each other, rather than where I took them from. So in opposition to the ball experiment, there was no potential energy added. Still, they move...

And a similar thing can happen in magnetism. If a positively-charged particle moves at right angles to a uniform magnetic field, the particle will travel in a circle.

Remember, forces accelerate objects. An orbit is just an instantaneous acceleration.

Agreed. But despite we don't know what happens in detail, we can calculate precisely what happens to a ball that's dropped. Such calculations also take gravity as granted, as a simple constant which is just there. On that basis, I am not sure if we need to know how gravity works when we ask these questions. Or do we? I don't know...

You can say the exact same thing about electromagnetism.

Here, maybe a few field diagrams will help:

 

[wfooshee]

So let's get back to the planets. Yes, I have moved the planets, but that didn't load them up with potential energy, because I didn't have to overcome gravity for that. And if I would have done, they wouldn't gravitate towards each other, rather than where I took them from. So in opposition to the ball experiment, there was no potential energy added. Still, they move...

The planets are exceedingly full of both potential and kinetic energy. Their orbits (i.e. their kinetic energy along the curve of their orbits) counteract the gravitiational potential. Anything in orbit is perpetually falling, but its speed keeps it falling out at the same rate it falls in, thus weightlessness for astronauts and satellites. That's all weightlessness is, is falling. Slow something in orbit down, it falls. Speed it up, it rises outward. You've changed the balance.

As for your magnetism question, where does the energy come from, the answer is that the magnetism in the magnet IS the energy. They repulse or attract each other, and unless restrained (held in place, nailed down, glued to a frame, whatever) they will move towards or away from each other. You said you worked against gravity to put the ball 1 meter off the ground. Well, you worked against the magnetic fields to put them in proximity to each other. While you were moving them, the fields were interacting.

Magnetism and gravity are a bit different from what we like to refer to as energy, meaning electricity and light. We don't seem to have found a basic particle that carries magnetisim or gravity, the way we have electrons for electricity and photons for light. Perhaps this is why you have trouble dealing with them as "energy."

 

[Danoff]

It's tempting to boil these questions down to "how does gravity work" and "how does magnetism work", but I don't like the way the questions are put.

Take two magnets. Put them close to each other, so that their magnetic fields start interacting. If you let them go, they will obviously move toward each other (or away, depending on the orientation). But where does the energy for that movement come from?

Let's say I could move planets in space. So it take two planets and put them close to each other (there's no rotation and no movement through space). Obviously, gravity will make them gravitate toward one another. But where do they get the energy for that movement from?

As you take something from its stable state and move it into an unstable state, you'll see a reaction as it relocates to a stable dynamical scenario. The reason you didn't get that reaction before is because you hadn't put force into your environment yet. As soon as you move planets around, or move magnets around, you're going to FORCE and change in a system that has found a balanced state. And when you do that, you're going to see something you didn't see before as a DIRECT RESULT OF YOUR FORCE. How did they get there in the first place? The force involved in the origin of the universe.

That's why I don't like the way these questions are worded. Let me attempt to give you a wording that you'll enjoy better.

The Earth is traveling around the sun. If the sun weren't there, the Earth would shoot off in a straight line with it's current tangential velocity, but it doesn't. The presence of the sun keeps the Earth in orbit by accelerating it. But that force doesn't require any energy to act on the objects around it. What's more, no matter how many objects are in orbit around the sun, the sun will exert an acceleration on each one. If you add the force on all of those bodies up, you get an infinite amount of force exerted by the sun, and it does not deplete the sun whatsoever.

So where does this force come from? We don't know. But you shouldn't think of it quite as much like a force as much as you do a shaped landscape (shaped by gravitational potential) in which these objects travel in a stable, low energy state.

 

[the Interceptor]

So where does this force come from? We don't know. But you shouldn't think of it quite as much like a force as much as you do a shaped landscape (shaped by gravitational potential) in which these objects travel in a stable, low energy state.

Well, Einstein did find quite a nice visualization of spacetime and how objects with a mass (like planets) curve it:

This explains gravity on a level we can understand, as it ulitizes simple geometry. And maybe it's the missing knowledge of how gravity actually works that generates the trouble I have with these questions. It just bugs me that gravity seems to prove the conservation-of-energy principle wrong, to me at least.

 

[Danoff]

I think I need a better example of how you think gravity proves conservation of energy incorrect.

 

[Sage]

It just bugs me that gravity seems to prove the conservation-of-energy principle wrong, to me at least.

Huh?

[edit]: Six years here, and I still never learn to not sit on a reply for several minutes.

 

[the Interceptor]

I think I need a better example of how you think gravity proves conservation of energy incorrect.

Sure. Let's stay with Sages example of the Earth rotating around the sun. If we'd take the sun away, the Earth would fly straight on - so far, so good.

As we know, the sun gravitates the Earth, so the Earth won't fly away from the sun. In terms of physics, that means that we have to put energy into this system, just like a hammer thrower has to lean backwards to counter the force of the hammer as he brings it up to speed. According to the law of conservation of energy, the energy to keep the Earth on its path around the sun must be used up elsewhere. But gravity does not use up. So we have a system where we can use kinetic energy without putting potential energy into it. Sure, you can say that a mass is pre-loaded with potential energy, but if so, you would use it up when you swap it for kinetic energy.

I think I'm very slowly getting to the heart of the problem, I feel something growing inside me...

 

[Danoff]

Sure. Let's stay with Sages example of the Earth rotating around the sun.

The Earth is actually revolving around the sun. It rotates on its axis (and it was my example, not Sage's ).

As we know, the sun gravitates the Earth, so the Earth won't fly away from the sun. In terms of physics, that means that we have to put energy into this system, just like a hammer thrower has to lean backwards to counter the force of the hammer as he brings it up to speed. According to the law of conservation of energy, the energy to keep the Earth on its path around the sun must be used up elsewhere.

Actually, the energy of the Earth isn't altered by the Sun in a time-varying sense. Here's the equation for the energy of Earth.

http://en.wikipedia.org/wiki/Specific_orbital_energy

If you look at that equation (the part after the E is broken into Ek and Ep), you'll see that the energy is dependent only on the velocity of the earth, the distance of the Earth from the sun, and the gravitational parameter of the sun. There is no time varying component - that's because the Earth is in a constant energy state. All orbits are actually. Velocity doesn't change (much) through it's orbit, the sun's mass doesn't change. The distance of the Earth from the sun doesn't change (much) through it's orbit. The reason I say much is because the Earth isn't in a perfectly circular orbit, so you'd have to look at the last part of the equation (the one that starts with -1/2...) to get an equation who's parameters don't change at all at any point in the orbit. The bottom line, though, is that E is constant for Earth (until we start exploding nukes and launching rockets and stuff).

 

[the Interceptor]

The Earth is actually revolving around the sun. It rotates on its axis (and it was my example, not Sage's ).

Whoops, sorry mate!

I think I'm not getting any further in understanding this today, as it already is late (just midnight) and my head kinda hurts from this. Any contribution to this matter, may it even be a tiny one, is still very welcome though, I'll start looking into it again when I wake up.

 

[Kylehnat]

Okay, I think we've conquered gravity and magnetism. Let's try to understand strong nuclear force, shall we?

Just kidding...carry on!

 

[wfooshee]

The Earth is actually revolving around the sun. It rotates on its axis (and it was my example, not Sage's ).



Actually, the energy of the Earth isn't altered by the Sun in a time-varying sense. Here's the equation for the energy of Earth.

http://en.wikipedia.org/wiki/Specific_orbital_energy

If you look at that equation (the part after the E is broken into Ek and Ep), you'll see that the energy is dependent only on the velocity of the earth, the distance of the Earth from the sun, and the gravitational parameter of the sun. There is no time varying component - that's because the Earth is in a constant energy state. All orbits are actually. Velocity doesn't change (much) through it's orbit, the sun's mass doesn't change. The distance of the Earth from the sun doesn't change (much) through it's orbit. The reason I say much is because the Earth isn't in a perfectly circular orbit, so you'd have to look at the last part of the equation (the one that starts with -1/2...) to get an equation who's parameters don't change at all at any point in the orbit. The bottom line, though, is that E is constant for Earth (until we start exploding nukes and launching rockets and stuff).

Thanks for finding that, Danoff - it's where I was trying to go with my earlier post about "exceedingly full of both potential and kinetic energy" and my description of planets in orbit as an equilibrium state, not an energy consuming state. The Sun's gravity is not "energizing" the Earth's orbit. The Earth has potential energy as a result of its distance from the Sun, and it has kinetic energy as a result of its motion around the Sun. The Sun imparts no energy into the Earth as far as its orbit is concerned. (Obviously we get radiant energy as heat and light, but that's not what we're talking about.)

An orbit does not require energy input to be maintained. At least on the time scales we need to worry about. When the sun dies, the earth will still be in orbit around it.

And technically, I don't think it's correct to talk of gravity and magnetism as energy. They are forces. There is no energy until there is an interaction with matter. This may be the Interceptor's problem. He should separate the energy from the force that generates it. They can be a source of energy, as in a falling object develops kinetic energy, an object lifted has potential energy, etc. I'm not the science guru I could have been, because I decided to work for a company that does useful stuff and make money.

That didn't work out as well as it could have, either, but I digress.