Thought experiment on magnetic field propagation

[lxrose]

Some 30 years ago I learned in the elementary school, that if I move a permanent magnet in a coil which ends are somehow electrically closed, than I need to apply force on the magnet, because the generated current creates a magnetic field which is against the magnet's field. However if the propagation of magnetic field is the same as the speed of light, than this counter effect is not instant.
What happens if the generated magnetic field arrives back to the magnet when the movement is the opposite? It still has to push when I pull the magnet, but I cannot find the reason why it happens if the change in the magnetic field is not instant. Could someone explain why this PM cannot exist?

 

[mfb]

Your accelerations of the magnet would be so fast, you would basically create an antenna and emit a lot of electromagnetic radiation.
The counter-effect can exist, but it would be weaker than the amount of energy you need to keep the magnet oscillating. The coil can just reduce your emission of radiation a bit (so the amount of energy you have to put in goes down a bit, but stays positive), if the frequency and orientation is right.

 

[lxrose]

Your accelerations of the magnet would be so fast, you would basically create an antenna and emit a lot of electromagnetic radiation.
The counter-effect can exist, but it would be weaker than the amount of energy you need to keep the magnet oscillating. The coil can just reduce your emission of radiation a bit (so the amount of energy you have to put in goes down a bit, but stays positive), if the frequency and orientation is right.

Thanks for your answer. In a thought experiment one can create a coil with a huge diameter like a couple thousand miles or kilometers. In this case the frequency can be low. What's the explanation of the magnet not moving back and forth of its own after pushing it for the first time?

 

[Dale]

Changing the size of the apparatus doesn't change the answer. It just changes the wavelengths of the emitted radiation that would "fit" inside the loop, and lowers the frequency of any resonance with the magnet.

 

[lxrose]

Yes I know that, but I still don't understand the situation. I thought that working with a "human delay" can help to enlighten the problem better, or I will get an answer that I understand. I know mostly what the first answer is trying to say, but something is still missing for me.

 

[mfb]

You need power to have the magnet oscillating. Without coil, all this power is radiated away as electromagnetic waves. With a coil, a part of this power can be "captured" and transferred back to the magnet, so the oscillation needs less external power. You cannot capture 100% of the power and you cannot "send back" 100% of the captured power, so you will always need some energy input to keep the system running. And even if 100% would be possible, you still would not gain anything.

 

[lxrose]

So depending on the size of the coil and the duration of the movement the generated magnetic field either helps or slows the movement of the magnet?
The initial movement can be short then the magnet could oscillate by itself while emit some EM radiation. Magnets can be strong, what would prevent the oscillation to take longer than the initial movement?

 

[mfb]

You'll need very special conditions to "help" the magnet, but yes.

What do you mean by initial movement? Without a continuous external driving power, the oscillation will get smaller and smaller.

 

[lxrose]

You got what I meant, it seems.
I need to think more about this.
Thanks for your answer.

 

[lxrose]

You'll need very special conditions to "help" the magnet, but yes.

What do you mean by initial movement? Without a continuous external driving power, the oscillation will get smaller and smaller.

I don't know where I am wrong but I think it cannot work that way. Here is my reasoning.
Let's compare the following two cases.
Case no 1: I move back and forth a piece of metal.
Case no 2: I move back and forth a piece of magnet with the same weight (and mass of course) inside a coil which has a lightbulb attached.
With the right frequency in the second case it is easier to move the magnet than in the first case while I have an illuminated lightbulb and some radiation. It would break the law of conservation of energy, isn't it?

 

[mfb]

You need energy to move the magnet back and forth (more than you need for the non-magnetic piece). This energy is used to generate radiation, and to drive your light bulb. If you replace the light bulb and use a superconducting coil, you'll need less power (but you still need power) because you eliminated one power drain, and the now stronger back-reaction will help more to move the magnet.

 

[lxrose]

In the case when I continuously push the magnet, the "resistance" of the generated magnetic field needs more energy from me which is converted to electric energy. But in the case above we are talking about a magnetic field which helps to move, so what explains the effect you wrote in your first sentence?

"You need energy to move the magnet back and forth (more than you need for the non-magnetic piece)."

 

[sophiecentaur]

In the case when I continuously push the magnet, the "resistance" of the generated magnetic field needs more energy from me which is converted to electric energy. But in the case above we are talking about a magnetic field which helps to move, so what explains the effect you wrote in your first sentence?

"You need energy to move the magnet back and forth (more than you need for the non-magnetic piece)."

You are correct to put 'resistance' in inverted commas because there are two different aspects. Resistance, in electrical terms, gives energy loss, like friction. The other effect is called Reactance (it is analogous to the reaction against an applied force which accelerates an object) . Changing the state of a magnet (rotating or switching current on and off) will always involve some energy loss - radiating EM energy AND there will be induced Electric Fields too. If there is another magnet / object nearby, some energy will go towards causing currents and electrical polarisation of that object and it will produce its own fields. When they are close together, there will be an appreciable interaction and the first magnet will 'feel' that effect of the other. (General term is Mutual Impedance) If the other object is a magnetic substance, then the interaction will be stronger. In the limit, of course, you have the Transformer Effect, where two coils are strongly coupled by a common iron core.

 

[mfb]

Compare it with a swing: gravity will help you swing back and forth every time, but you still have to do something to keep it swinging at the same amplitude as you have some friction.

 

[lxrose]

You are correct to put 'resistance' in inverted commas because there are two different aspects. Resistance, in electrical terms, gives energy loss, like friction. The other effect is called Reactance (it is analogous to the reaction against an applied force which accelerates an object) . Changing the state of a magnet (rotating or switching current on and off) will always involve some energy loss - radiating EM energy AND there will be induced Electric Fields too. If there is another magnet / object nearby, some energy will go towards causing currents and electrical polarisation of that object and it will produce its own fields. When they are close together, there will be an appreciable interaction and the first magnet will 'feel' that effect of the other. (General term is Mutual Impedance) If the other object is a magnetic substance, then the interaction will be stronger. In the limit, of course, you have the Transformer Effect, where two coils are strongly coupled by a common iron core.

Thanks for joining to the conversation. I meant resistance as we use in everyday language, but it may not be the correct English word for this. Most of the time I am trying to find the right words as used in physics, because I learned physics in Hungarian. Thanks for your detailed clarification.

 

[sophiecentaur]

Thanks for joining to the conversation. I meant resistance as we use in everyday language, but it may not be the correct English word for this. Most of the time I am trying to find the right words as used in physics, because I learned physics in Hungarian. Thanks for your detailed clarification.

Just look up the difference between Resistance (Resistors) and Reactance (Capacitors and Inductors). (Friction vs Inertia?) Both factors are at work here - even when the nearby object is a perfect conductor or insulator, because there is always a finite amount of energy radiated out into space.

 

[lxrose]

Compare it with a swing: gravity will help you swing back and forth every time, but you still have to do something to keep it swinging at the same amplitude as you have some friction.

Yes, in the case of the swing there is friction due to air and the swing itself. But in our case what is the way a force acting upon my hand when I push and pull the magnet if it is not the force of the fields interacting?

 

[lxrose]

Yes, in the case of the swing there is friction due to air and the swing itself. But in our case what is the way a force acting upon my hand when I push and pull the magnet if it is not the force of the fields interacting?

I mean the fields not acting against my hand...

 

[mfb]

Yes, in the case of the swing there is friction due to air and the swing itself. But in our case what is the way a force acting upon my hand when I push and pull the magnet if it is not the force of the fields interacting?

It is a force of the field directly from the magnet, acting on the magnet, resisting acceleration.

 

[lxrose]

So the magnets own magnetic field acting against the magnet. It might explain what happens during acceleration, but does not explain the constant speed motion that may come after it finished acceleration, but before the counter effect occurs. Or during constant speed of the magnet there is no induced voltage?
(And for the above effect you wrote we don't even need a coil.)

 

[mfb]

I don't see where you would get constant speed in the easiest way of oscillation or why that would be relevant.
Don't add more and more things to the setup that make it more complicated before you understand the easier setups!

 

[lxrose]

I don't think it makes things much more complicated if there is some time in the setup when the magnet moves at constant speed.
In the beginning I described two ways to do the experiment, and since it is only in thought we can play with some details.
My goal is not to find a way to create a PM (I know it cannot exist), but to get some explanation on this problem which stands all circumstances, otherwise something is wrong or missing.
Is constant speed of the magnet difficult to explain or understand if we don't care how the magnet gets into this state?
I've got some other thing in mind, but I would like to get an answer for the above first.
Thanks

 

[mfb]

I don't think it makes things much more complicated if there is some time in the setup when the magnet moves at constant speed.

It does.

Is constant speed of the magnet difficult to explain or understand if we don't care how the magnet gets into this state?

It makes the analysis more complicated.

 

[lxrose]

If we focus only on the time range when I move the magnet in the coil at constant speed, it should be the simplest case.
What happens with the lightbulb?
What about the force I need to act against?

 

[mfb]

Sorry, I don't see how it would help if you continue to make the setup more and more complicated before you understand the easiest cases. I'm out.

 

[lxrose]

I think I simplified it by taking only a small part, so we can agree on the most basic part of magnetic induction.
Perhaps you cannot explain the whole thing together, that's why you don't want to answer this piece otherwise I don't see why you leave right now if you answered all before.

 

[lxrose]

If anybody else still reading this thread, could he or she explain the "extra" energy that is required by me if we compare the following two cases.
Moving the magnet inside the coil with normal speed and distances back a forth. (This is harder to push.)
Moving the magnet inside the same coil with the same speed but shorter distances so the magnetic field that is of the induced current arrives back when I already moving the magnet backwards.
If I put in more energy it should come out somewhere. Is it the connected lightbulb? Would the light be more intensive?
Or is it radiated out?