[Andhee]

Hey,

A week ago I thought of this really strange question during one of my physics lessons at college, and I thought I'd bring it here because its pretty strange.

How does a magnet exert a force on an object without using energy therefore doing no 'work'? Imagine a magnet on a smooth horizontal surface, attracting a metal object somewhere 10cm away. It pulls on the metal object, the closer the metal object to it, the stronger the force acting on the object. How is it attracting it without using any energy?

Please dont try to be a smart arse and copy and paste something off wiki or whatever (I havn't checked on there for an answer), and try put it in simple physics terms so everyone can get an idea for what we're on about.

Andy

[turtle777]

I'm not at all a physicist, but what law says that there needs to be energy consumed to attract other objects.

Think of gravity, which, in a sense, is attraction due to mass. No energy consumption there either.

-t

[OreoCookie]

There is energy stored in the electromagnetic field which is converted into motion. Ditto for conservation of momentum. Energy is converted from one form to another (field energy to kinetic energy), but if you leave the field energy out of the energy balance, it seems `energy is created out of nowhere.'

Am I making sense to you?

[Andhee]

Originally Posted by turtle777 

I'm not at all a physicist, but what law says that there needs to be energy consumed to attract other objects.

Think of gravity, which, in a sense, is attraction due to mass. No energy consumption there either.

-t

Right, say you jump from the ground, your kinetic energy will convert into potential energy. The point you have no kinetic energy (the place where you are furthest from the ground), you have maximum potential energy, which is then converted into kinetic energy for falling downwards. To sum up, you do have energy when you are stood on the earth, and you have to use the energy you have to move.

Hence the strangeness of my question, a block of metal (potential energy is at a constant), is attracted by a magnet and moves (so it gains kinetic energy). OreoCookie said there is energy in the magnetic field, so am I correct in saying a magnet would 'use up' all of its energy eventually even though there still is a north south on the magnet?

When a magnet is heated up, it loses its magnetivity. Does this mean, it suddenly releases loads of energy in the form of heat? I dont think so. So where is the energy that could have been used to attract objects just suddenly disappeared to?

[Dakar the Fourth]

I am quite intrigued by this thread.

[osiris]

Sure - magnets, forces, weird, physics... what's not to like.

[turtle777]

The problem is, that "work" in the strictly physical sense does not correspond to "work" in our common sense.

E.g., if you carry a 30 lbs suitcase on a flat surface for 100 yards, it certainly feels like work to you. But in a physical sense, unless the object changes altitude, you have NOT performed any work.

-t

[Andhee]

In reply to Turtle777:

Work = Force x distance

so work is done when moving a block through any kind of distance (doesnt matter whether it is vertically or horizontally).

And also, remember magnets can pick things up against the force of gravity.

[turtle777]

You are mistaken. Carrying a suitcase is NOT work in the physical sense.
You don't need any physical force to move it vertically.

The force you need to initially lift the suitcase IS work, but once you have it on a certain level, and you start walking, your are *NOT* performing physical work on the suitcase.

The "work" that you feel by having to hold the suitcase is gravity, but that "work" is not work in the physical sense.

-t

[MarkLT1]

Originally Posted by Andhee 

In reply to Turtle777:

Work = Force x distance

so work is done when moving a block through any kind of distance (doesnt matter whether it is vertically or horizontally).

And also, remember magnets can pick things up against the force of gravity.

No. Work = F dot d or, Fdcos(theta) where theta is the difference in angle between the force vector (F) and distance vector (d). So if your distance vector is perpendicular to the force vector, the amount of work done is zero.

That being said, the assumption of moving a mass horizontally being zero work, is the amount of work that is done against the gravitational force. If you measure the work done against frictional forces, you will do some "work" but it will be much much less than it seems.

[turtle777]

Thanks, MarkLT1. I couldn't put it in real physical terms, but I KNEW it was true.

This was a heavily discussed topic during my physicas classes. At first, I wouldn't believe my prof, because it sounds so counter-intuitive.

-t

[Andhee]

Fair enough, but there is an initial amount of work done in the horizontal direction. If it was accelerating it would have work done on it as Force = mass x acceleration (from Newtons II Law).

In an ideal world there would be no work done if the object is moving at a constant speed or not moving at all - BUT - we don't live in an ideal world, so once the object is moving, a constant force must be applied as the frictional force will oppose it. In my first post I said smooth surface, but lets open it out a bit, a rough surface as of now.

[turtle777]

Originally Posted by Andhee 

Fair enough, but there is an initial amount of work done in the horizontal direction.

Yes, that's what I said in my post above.

-t

[Andhee]

In reply to Mark:

Yeah, work does = force x displacement, but I didnt want to screw the thread around putting anything too technical in, but yeah, you are right about that.


Anyway - Work must be done on the object going towards the magnet as the force increases closer towards the magnet (so I assume), even if it stays the same, a constant force would provide an acceleration and hence work needs to be done.

Where does the energy come from to do the work?

[MarkLT1]

But to answer your original question...

In the context of your question, magnetic forces and gravitational forces act in the same way. Just like a body's mass will exert a gravitational field (and thus gravitational force) on another mass, a magnetic material will exert a magnetic field (and thus magnetic force) on another magnetic material. So when you hold two magnets near each other, you "feel" the potential energy between them (due to the magnetic field). If you let go of one magnet, it will accelerate its mass (due to the magnetic force) as this potential energy is turned into kinetic energy. Once the two magnets contact each other, the potential energy is zero.

[MarkLT1]

Another way to think about it- think VERY VERY big for mass. Imagine if out in space, you had two stationary bodies that had a huge amount of mass (think two Jupiters). Now, lets say you are a giant. You float out in space, and hold the two huge bodies away from each other, but well within their gravitational fields. What would this feel like? Most likely, it would be like holding two magnets near each other. Once you let go of the two bodies, they would move towards each other, and collide. Just like the two magnets.

Magnetism is simply another of the fundamental forces.

[Andhee]

Right, this is what I debated about with my teacher. He says magnetic fields are no way similar to gravitational fields (even though they appear to have the same characteristics). Nobody know hows gravity works, many theories suggest there being particles called gravitons which keep is in together, I don't know the full story, but its not like a north attracts to a south in a magnet.

Also, I found it strange how + and - works differently to a magnets poles, even though a magnet can repel a wire with a current going through it...

[OreoCookie]

Originally Posted by Andhee 

so work is done when moving a block through any kind of distance (doesnt matter whether it is vertically or horizontally).

No, turtle is entirely correct: unless there is friction, you will not do any work. The reason why you `feel like you're working' is because you have tiny muscle contractions and your suitcase is `oscillating.' Plus, there is friction. But so far, you don't have any friction in your model.

Even if you lift it up, there is no work done, because you gain all the energy you needed to put into the system lifting the suitcases up when you put them back down again.

[Andhee]

Originally Posted by MarkLT1 

Magnetism is simply another of the fundamental forces.

In my physics lessons, we have been told there are only four fundamental forces:
Gravitional force
Weak Nuclear
Strong Nuclear
Electromagnetic force

I'm guessing that magnets come under the 'electromagnetic force', but I am unsure.

[OreoCookie]

Originally Posted by Andhee 

Anyway - Work must be done on the object going towards the magnet as the force increases closer towards the magnet (so I assume), even if it stays the same, a constant force would provide an acceleration and hence work needs to be done.

No, work is energy that is put into the system from the outside.
But here, the situation is different: you don't have to `push' the magnet to go towards the other magnet, you simply have to let go. No work is required, it's all internal energy that is converted from one form into the other.

Originally Posted by Andhee 

Where does the energy come from to do the work?

From the field energy. In vacuo it's proportional to the integral over B^2 + E^2 (I'm missing constants here, but I'm lazy).

Originally Posted by Andhee 

I'm guessing that magnets come under the 'electromagnetic force', but I am unsure.

Yes.

[Andhee]

Oh, I forgot to edit my post before about changing the plane from being smooth to rough, hence the comment about the block being moved through any kind of distance work is done

[MarkLT1]

Originally Posted by Andhee 

Right, this is what I debated about with my teacher. He says magnetic fields are no way similar to gravitational fields (even though they appear to have the same characteristics). Nobody know hows gravity works, many theories suggest there being particles called gravitons which keep is in together, I don't know the full story, but its not like a north attracts to a south in a magnet.

Also, I found it strange how + and - works differently to a magnets poles, even though a magnet can repel a wire with a current going through it...

I didn't mean to imply that gravity and magnetism are alike, other than the fact that gravity attracts masses, and magnetism attracts magnetic materials, and that this attraction can be represented for both as a field, and that when you keep two masses, or magnets apart, you can "feel" the potential energy. But in terms of the details, gravitational force and magnetic force behave quite differently (such as the poles, etc.. you are mentioning). I was doing an over-simplification for clarity.

As for the force category, yes magnetism goes under the EM force.

[Andhee]

What about + and - charges?

What do they come under? If it were under electrostatic, then why would a magnet in a fully ionised solution not attract the ions and get a deposit on the magnets surface?

[olePigeon]

Now let's discuss spooky action at a distance. You take paired particles in a cat state and separate them the distance of our known universe. Observing either particle will have an instant effect on the other particle 16 billion light years away.

Oooooooo.

[MarkLT1]

Hopefully OC can help you more with charge stuff.. as I am just an armchair physicist (aka- an engineer) and get that kind of stuff mixed up way too easily.

[OreoCookie]

Originally Posted by Andhee 

What about + and - charges?

What do they come under? If it were under electrostatic, then why would a magnet in a fully ionised solution not attract the ions and get a deposit on the magnets surface?

Magnetic forces don't act on charges at rest, it acts on currents (charges in motion) and magnetized particles. There are no magnetic charges*. You're mixing terminology.

Also, don't add friction to have physical laws conform to your expectations, friction can be taken into account in physical theories, but don't add it, just so that you can `you've done work carrying suitcases.'

* There is some theory on magnetic charges, but so far, none have been discovered.

[turtle777]

Alright, guys, too much for me. I have checked out. All my knowledge has been discharged already.

-t

[MarkLT1]

Originally Posted by OreoCookie 

...but don't add it, just so that you can `you've done work carrying suitcases.'

Awww.. thats no fun!!!

[OreoCookie]

You'll be happy to know that I'll be on vacation, doing `no work' on my mountain bike for four days

[Andhee]

Oh yeah, OreoCookie, I remember my teacher saying something about a current or whatever, but I was just too convinced energy came from nowhere anyway.

A question about harnessing energy:
Say a big-arse heavy magnetic material was put on a wheel of some sort (imagine a water wheel in a river), then once it reached the bottom, a magnet pulled it back to the top, but without it touching the magnet, and the magnet is quickly removed. The block would fall back down again. Imagine the wheel collecting the energy from the falling block acting on the wheel. Its only an idea, its not like a full blown proposal or anything, but I just thought it could work.

[OreoCookie]

There are no perpetuum mobile, you can't make energy from nothing
Where did the energy go? Where did it come from? What do you think?

I'll post the answer in a bit … 

[olePigeon]

Wouldn't it suck if your dad was a physicist AND a gym teacher?

"Tommy, quit screwin' around! There're 50 more boxes you have to put up."
"I've already put up 20 boxes, can't I rest? This is too much work."
"They're moving perpendicular! That ain't work. Now get movin'!"

[turtle777]

-t

[turtle777]

Originally Posted by OreoCookie 

There are no perpetuum mobile, you can't make energy from nothing

But it might be energy that comes form the "outside", so it's "free" (i.e. from nothing) for you.

Like solar energy or gravitational energy from changes of the earth-moon constellation.

-t

[OreoCookie]

But then you don't create energy, you're simply missing terms in your `energy balance.'

[nonhuman]

True but solar energy is being pumped out into space whether we use it or not. For us to utilize solar energy has pretty much zero impact on anything; nothing changes outside of our essentially closed system, so it really can be treated like free energy.

[turtle777]

Originally Posted by OreoCookie 

But then you don't create energy, you're simply missing terms in your `energy balance.'

I know, it's not creating it, but it a strict sense, you can't make energy at all.

You can only transform one energy into another.

-t

[olePigeon]

Originally Posted by turtle777 

I know, it's not creating it, but it a strict sense, you can't make energy at all.

-t

There is an extremely rare instance in which the laws of conservation of energy & matter are violated (well, sort of bent.) When paired particles are traveling along and skim the event horizon of a black hole, one of the particle gets stripped and stuck in the event horizon while the paired particle continues. For a moment, there is more mass in the universe than there's supposed to be.

Or is there?

Some recent studies (grudgingly supported by Stephen Hawking) suggests that, while there is a singularity at the center of a black hole, the "information" is still retained. If this turns out to be true, then the laws of conservation wouldn't 've been violated in the first place. It's science!

[Andhee]

olePidgeon, did you watch that show about Stephen Hawking or do you just generally know this stuff? I watched a proper intriguing program about him and the theories of everything, really interesting.

Also I've thought of a method for perpetual motion, but its probably not doable anyway, but I'm going to inquire about it.

[turtle777]

Originally Posted by Andhee 

Also I've thought of a method for perpetual motion, but its probably not doable anyway, but I'm going to inquire about it.

Probably very, VERY, VERY slim chance

-t

[Andhee]

Lol i'll let you know about its success (or mass failure)!

[olePigeon]

Originally Posted by Andhee 

olePidgeon, did you watch that show about Stephen Hawking or do you just generally know this stuff? I watched a proper intriguing program about him and the theories of everything, really interesting.

Also I've thought of a method for perpetual motion, but its probably not doable anyway, but I'm going to inquire about it.

I've seen a few shows on Stephen Hawking, but nothing recently. That's just stuff I've read in science/nature journals.

Perpetual motion is not possible.

[nonhuman]

Originally Posted by olePigeon 

I've seen a few shows on Stephen Hawking, but nothing recently. That's just stuff I've read in science/nature journals.

Perpetual motion is not possible.

Although there was a fairly interesting article I read recently about a guy that's apparently discovered something that approaches it. Something to do with magets on the shaft of an electric motor... I don't really remember.

[Mrjinglesusa]

Originally Posted by Andhee 

Also I've thought of a method for perpetual motion, but its probably not doable anyway, but I'm going to inquire about it.

Probably not doable?

[Mrjinglesusa]

Originally Posted by nonhuman 

Although there was a fairly interesting article I read recently about a guy that's apparently discovered something that approaches it. Something to do with magets on the shaft of an electric motor... I don't really remember.

There are a lot of things that might "approach" it. But approaching it is not perpetual motion.

By definition, and by the laws of physics, a perpetual motion machine is not possible.

If someone thinks they have an idea how one could work - they don't.

If someone thinks they have made one - they haven't.

[nonhuman]

Originally Posted by Mrjinglesusa 

There are a lot of things that might "approach" it. But approaching it is not perpetual motion.

By definition, and by the laws of physics, a perpetual motion machine is not possible.

If someone thinks they have an idea how one could work - they don't.

If someone thinks they have made one - they haven't.

I'm aware of that. What this guy did (and I'll see if I can find the article) was basically just find a way that significantly increased the efficiency of an electric motor in a way which was completely surprising both to him and to a number of scientists he had look at it. At the time that I read it, no one was exactly sure what was going on or how it actually worked.

[Edit: it appears that a Google search for anything involving the words 'perpetual motion' is unlikely to yield anything productive...]

[CharlesS]

Originally Posted by nonhuman 

I'm aware of that. What this guy did (and I'll see if I can find the article) was basically just find a way that significantly increased the efficiency of an electric motor in a way which was completely surprising both to him and to a number of scientists he had look at it. At the time that I read it, no one was exactly sure what was going on or how it actually worked.

The problem with that is that anything you power with that motor here on Earth is going to be encountering friction and air resistance, which is going to muck up the efficiency.

[nonhuman]

Originally Posted by CharlesS 

The problem with that is that anything you power with that motor here on Earth is going to be encountering friction and air resistance, which is going to muck up the efficiency.

Yes, but the more efficient the motor is the less power you need to accomplish the same result no matter what it is the motor is driving.

[CharlesS]

True, but not enough that you could call it approaching perpetual motion.

[peeb]

Efficiency is one thing - the def of a pm machine is getting more energy out than you put in - a different kind of a fish.