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mikehanson

what happens at a distance when an electromagnet is turned off?

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If there is a permanent magnet and an electromagnet 1 light year apart and separated by the vacuum of space, and say the electromagnet has been running for a number of years and it and the permanent magnet are attracted to one another, and then the electromagnet is turned off, will the permanent magnet "feel" this change in less than 1 year? Will the permanent magnet still remain attracted to the electromagnet for 1 year longer even though the electromagnet has been turned off?

 

 

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The decreasing magnetic field is an EM wave that travels at the speed of light. The affect on the permanent magnet would take a year to propagate from the electromagnetic to the permanent magnet.

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It will take a year for the magnet to get the field from the electromagnet. If the electromagnet is then turned off, the magnet will get the field for 1 year.

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It will take a year for the magnet to get the field from the electromagnet. If the electromagnet is then turned off, the magnet will get the field for 1 year.

English is so imprecise. The permanent magnet will feel the field for the same length of time as the electromagnet is turned on, with a one year delay from time on till felt, and similar delay from time off till felt.

 

I think that is a correct statement, but it seems a bit obtuse.

Edited by EdEarl

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What about the conservation of momentum of both magnets together?

 

When switched on, the electromagnet gets immediately a force from the permanent magnet, while the permanent magnet must wait for 1 year. Meanchile, only the electromagnet has acelerated, adn their common center of mass has gained asymmetric speed too. Then both magnets act, the common center of mass keeps its asymmetric speed. The electromagnet is switched off, the permanent magnet continues to accelerate, until the common center of mass as regained its orgiinal seed BUT not its original position, which I find seriously disturbing.

 

Beware such questions are a can of worms.

 

Up to now I agree with the delayed effect by the electromagnet but could change my mind. Such questions are badly complicated. One solution is known for a charged particle that changes its speed, and it's a mess - essentially, a constant speed would let the electric field point towards the present position of the charge, but speed changes need one propagation time to be felt, and this is over-simplified. I haven't heard of a similar solution for electromagnets. At antennas we clearly say "delayed potentials" and it works.

 

Maybe we must attribute a momentum to the magnetic field, and the interaction of both magnets carries some momentum away.

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When you turn on an electromagnet, you will be creating an EM field (B is changing, which induces an E field), which indeed has momentum. Normally this is symmetric. But I suspect the interaction with the existing field will make it asymmetric.

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Just thought. Ed and I are not considering that the permanent magnet already has a field, so if we assume that field reaches the electromagnet, the interaction will be instantaneous when it's turned on.

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Newton’s Third Law?

 

“For every action there is an equal and opposite action.”

 

Is Newton’s Third Law then temporarily violated as the permanent magnet is pulled towards the electromagnet while the electromagnet is not pulled towards the permanent magnet, or not?

 

?

 

 

 

 

 

 

 

 

 

Although the opposite asymmetry occurs earlier when the electromagnet is first switched on, and so there is “equality” but only over time and only over opposite actions (turning the electromagnet on and then back off).

 

But I always thought Newton’s Third Law was set in the immediate (or, at least, the near immediate) and not set over an (arbitrarily) large amount of time (and over a “hypothetical” dynamic, in that the electromagnet may never be (hypothetically) turned off).

 

?

 

?

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Actually it should be possible to measure/check in laboratory experiment.

 

Imagine beam of electrons emitted by electron-gun inside of vacuum tube.

Couple such setups f.e. 3 with slightly offset between them.

On one side there is electromagnet.

 

Electron guns are emitting electrons:

1) when electromagnet is turned off

2) when electromagnet is turned on

 

We know distances between electron beams and each electromagnet. And can deduce whether change in direction of electron beam is delayed or not.

 

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.

 

 

 

Think about this.

 

What if instead of an electromagnet and a permanent magnet, what if we had two electromagnets?

 

And say they are both switched on at the same time.

 

(I know that “simultaneous events at a distance” can be problematic to verify, but in a thought experiment we can stipulate that they are.)

 

So, in the phase where they become attracted to one another there is symmetry.

 

However, if one is then switched off before the other, we get asymmetry.

 

And then later when the second one is then switched off that asymmetry persists given that the first electromagnet has already been switched off and so whether the second electromagnet remains on or is switched off is irrelevant and has no effect on it (to the already switched off one).

 

In other words, when the two electromagnets are turned on at the same time we get the same attraction by each one towards the other (Third Law cool). However, when one is switched off before the other then the switched off one loses its attraction towards the other while the other remains attracted to the switched off one (Third Law not cool). And we never get this asymmetry in the opposite direction back because the already switched off one is not affected by the other one whether it remains on or is also eventually switched off too.

 

Yes? No?

 

How do we get cool with Newton’s Third Law in this situation?

 

?

 

?

 

 

 

.

 

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.

 

 

Does this mean Newton’s Third Law is false and not true?

 

 

Does this mean Newton’s Third Law is a “Principle” (generally true) rather than a “Law” (always true)?

 

 

 

?

 

 

.

 

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Newton's third law is only seemingly violated in such situations. You need to remember that the electromagnetic field itself carries momentum.

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"Newton's third law is only seemingly violated in such situations. You need to remember that the electromagnetic field itself carries momentum." - ajb

 

 

 

 

 

Okay. So make your case.

 

Assertions are like opinions ... everybody's got one.

 

 

 

 

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Okay. So make your case.

 

Assertions are like opinions ... everybody's got one.

It is well known and well understood physics that the electromagnetic field (or photons) carry energy and momentum. You can derive expressions for the momentum carried by an electromagnetic wave. The momentum density is essentially the Poynting vector. You can look up details yourself, I don't exactly want to teach electromagnetic theory from scratch here.

 

And for the opening question, it may make sense to think not about switching on and off the electromagnetic field, but rather consider small fluctuations and examine how these propagate. Doing so you will rediscover electromagnetic radiation which we all know in vacuum travels at the speed of light. Thus ,small changes in the electromagnetic field travel at a finite speed c.

Edited by ajb

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There is nothing in your last post that I disagree with.

 

However, there is nothing in you last post that addresses the issue.

 

No one is asking you to give us a MOOC on electromagnetic theory. The issue raised is whether or not the third law of motion is violated in this case. And no one is even asking you in particular to address this either. However, if you seemly purport to have addressed it and seemly purport to have put the matter to rest, but you actually don't, then you are going to get called out on it.

 

You seem to conclude with the speed of changes in a magnetic field in a vacuum transmit at the speed of light ( c ). Okay? Again, I don't disagree with you, but what does this have to do with addressing the issue? As far as I can tell, this is not a conclusion as to why there is not a violation, but rather this is a restatement of an initial premise.

Edited by Zet

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However, if you seemly purport to have addressed it and seemly purport to have put the matter to rest, but you actually don't, then you are going to get called out on it.

 

 

ajb pointed out that you have to analyze the Poynting vector, which carries energy (and since it's photons propagating, has momentum) for the field created by the electromagnet. I hinted at this earlier.

 

Newton's third law is fairly well established. The burden of proof at this point would be in showing that there is a violation, if that's what one wishes to do. Short of that, drawing attention to the fact that the momentum of the EM field had been ignored is not a claim for which one should be "called out" on. It is a true statement: the field has momentum, and that was not accounted for in the initial statement of the problem.

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There is nothing in your last post that I disagree with.

 

However, there is nothing in you last post that addresses the issue.

 

No one is asking you to give us a MOOC on electromagnetic theory. The issue raised is whether or not the third law of motion is violated in this case. And no one is even asking you in particular to address this either. However, if you seemly purport to have addressed it and seemly purport to have put the matter to rest, but you actually don't, then you are going to get called out on it.

 

You seem to conclude with the speed of changes in a magnetic field in a vacuum transmit at the speed of light ( c ). Okay? Again, I don't disagree with you, but what does this have to do with addressing the issue? As far as I can tell, this is not a conclusion as to why there is not a violation, but rather this is a restatement of an initial premise.

 

!

Moderator Note

Zet, by creating this account to hold discussions with your mikehanson account, you're in violation of our rules on sockpuppetry. You can only have one account here.

 

Please PM me and explain why this was necessary. Don't discuss it off-topic here.

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.

 

 

Yes, the supplicant has the burden of proof and not the pedagogue, if he is trying to prove something.

 

However, if the supplicant asks a question and the pedagogue does not directly answer the question but rather points to a field of study, then, presumably, the teacher is saying that the answer lays in that area of study, and simply leaves it to the student to put the pieces together.

 

So, in the thought experiment above where there are two electromagnets and they are turned on and off in such a way that the one is set into greater motion towards the other, if the response to “how is momentum is conserved?” is “electromagnetic fields have momentum” then presumably the answer lays in this aspect of physics (and it is just up to the student to put the pieces together).

 

So, we have the one mass (the one electromagnet) set into more motion in one direction, and if momentum is conserved because “electromagnetic fields have momentum” then this must mean that the electromagnetic field moving towards this now moving magnet speeds up to offset the magnet’s increased velocity. But this would mean that the electromagnetic field is now moving faster than the speed of light. And it cannot.

 

So, yes, this tangent was ignored in the statement of the thought experiment. But noting this absence, and noting it in such a way as to suggest that in it lays the answer to how this law is not violated is misleading at best. It appears to answer, or it appears to point to an answer, to the question but it does not. And this should then be called out.

 

Yes? No?

 

When the one electromagnet is set into more motion than the equal mass of the other electromagnet, how is Newton’s Third Law not violated?

 

 

?

 

 

.

 

 

Edited by Zet

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So, we have the one mass (the one electromagnet) set into more motion in one direction, and if momentum is conserved because “electromagnetic fields have momentum” then this must mean that the electromagnetic field moving towards this now moving magnet speeds up to offset the magnet’s increased velocity.

No, that is not implied.

 

 

But this would mean that the electromagnetic field is now moving faster than the speed of light.

Not implied, either. A photon's momentum is related to its energy, not its speed.

 

So, yes, this tangent was ignored in the statement of the thought experiment. But noting this absence, and noting it in such a way as to suggest that in it lays the answer to how this law is not violated is misleading at best. It appears to answer, or it appears to point to an answer, to the question but it does not. And this should then be called out.

 

Yes? No?

No.

 

"X seems to violate Newton's third law"

 

"You have not accounted for Y"

 

is perfectly reasonable. But you have to account for Y properly, which you have not done.

 

 

When the one electromagnet is set into more motion than the equal mass of the other electromagnet, how is Newton’s Third Law not violated?

You have not established that it is, as you have not accounted for the momentum of the EM field.

 

But I'll give it a shot:

 

You have a permanent magnet with static field. You turn on the electromagnet and it feels a force, immediately, because the static field is already in place. Repulsive for one direction of the field, and attractive for the other. Let's pick attractive — opposite poles, meaning the field will be in the same direction.

 

The electromagnet creates a field, and with our choice, the net field increases. The energy density, proportional to B2, increases. But now this energy density is not symmetric.

 

Griffiths et al. explain that if the center of the energy density is moving, then there is net momentum in the fields (pdf)

http://www.ate.uni-duisburg-essen.de/data/postgraduate_lecture/AJP_2009_Griffiths.pdf

 

Since the center of the energy density began at the permanent magnet, and it should end up being between the two, that says the energy density (and thus momentum) is moving toward the electromagnet. The electromagnet must acquire a momentum in the opposite direction, to compensate. The amount will depend on the strengths of field, which were not given.

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I get it.

 

I was wrong.

 

(Well, I don't totally get it. I need to spend some time studying this area of physics. But I get, I think, the basic overall logic of it.)

 

 

So, when we have two electromagnets attracted to one another (and they have been for some time) the "center of energy density" is halfway between them (assuming the two electromagnets are the same).

 

They are accelerating towards one another.

 

One is then turned off. It stops accelerating. But the other one (the still turned on one) continues to accelerate for a while. This increased amount of momentum of the one electromagnet in one direction is offset by the "center of energy density" moving in the opposite direction towards this still accelerating magnet (the still turned on magnet).

 

And so increased motion in one direction is matched by increased motion in the other direction. The increased mass times velocity of the accelerating electromagnet is equally matched by the mass times velocity of the moving "center of energy density" (and the decreasing amount of energy density) in the opposite direction.

 

If I've stated this right, then I get it. ("Get it" is the broadest of meanings.)

 

Cool. Thank you.

 

I realize my snootiness is not forgivable, but I do apologize.

 

Take care!

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However, there is nothing in you last post that addresses the issue.

The electromagnetic field carries momentum. When you analyse interactions of material bodies with the electromagnetic field you need to take this into account. Quite generally it may seem that momentum is not conserved, but what is really happening is that the electromagnetic field carries this momentum away.

 

I am quite confident that the momentum carried by the electromagnetic field explains your seeming lack of conservation of momentum. I have not analysed the situation you describe very carefully, I don't have the time nor will to sit down very carefully and look at this. The ball is now in your court so to speak. People here have provide you with the clues to properly understand what is going on.

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