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carrz
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disk and magnet spin together -> induced currentI ask can anyone properly explain why there is induced current in the setup?
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vanhees71 said:(a) Magnet and disk are rotating together (i.e., the magnet is fixed on the disk). There is a time-dependent magnetic field, giving rise to a electric field and thus a Lorentz force [itex]\vec{F}=-e(\vec{E}+\vec{v} \times \vec{B}/c)[/itex] is acting on each electron in the disk. Here you can assume [itex]\vec{v}=\vec{\omega}\times \vec{x}[/itex] for the velocity of the electrons, because the velocity according to the drift due to conductivity can be neglected for such usual household setups. So there will be a current flowing through the light bulb. If it's large enougth, the light bulb will be "on".
Personally, I am not certain that there is an induced current in this setup.carrz said:disk and magnet spin together -> induced current
DaleSpam said:The one thing that I do know is that there will be eddy currents induced, since without the lightbulb and associated connections this is the design of an eddy current brake.
DaleSpam said:Personally, I am not certain that there is an induced current in this setup.
The Lorentz force has two terms, one due to E fields and one due to the cross product of the v and B field. The second term is clearly non-zero, but because a time-varying B field induces an E field the first term is also non-zero.
If this were linear motion then it would be completely clear that these two terms cancel out exactly, but since it is rotational motion there may be some slight non-cancellation. I would have to see or do the math to be certain either way.
I am not sure why you consider that unfortunate. I personally enjoy having multiple correct explanations. Often different people will prefer different explanations, so having multiple ones helps.carrz said:unfortunately, explanations do vary
The magnet is the source of the field so as its position changes over time then the field at any fixed point necessarily changes over time.carrz said:Where do you see "time-varying B field" when the magnet is fixed to the disk?
That is true, but not relevant. The criterion for determining if the field is time varying is whether or not the fields stay the same (magnitude and direction) for every fixed point in a given inertial frame. The disk is not fixed in any inertial frame.carrz said:Does magnetic field magnitude not stay the same for any point on the disk itself, just like if they were completely stationary?
No, they both create time varying fields. The difference is that for straight line motion the resulting E field clearly cancels out the force from the B field.carrz said:Are you saying rotating magnet creates "time-varying" B field, but a magnet that moves in a straight line does not create "time-varying" B field?
Drakkith said:Carrz, i doubt that Faraday was using this setup when he discovered the paradox, as it doesn't work the same as the descriptions say.
In this example rotating the magnet will result in a flow of current.
Usually the setup used to explain the paradox considers the magnetic field to be homogenous throughout the disk, as if you used a big cylindrical magnet on each side instead of a small horseshoe one. This can easily be seen in different examples by looking at how the magnetic field is setup. In the picture the wiki article uses, they don't even bother to show a magnet, they just show the direction of the magnetic field (which is probably detrimental to explaining the paradox and contributes to the confusion here).
carrz said:I'm pretty sure in dozens of papers and articles I went through someone would have mention something about it, but instead what I saw is that the same effects are described for either type of homopolar generator.
Magnet rotating alone when the disk is stationary will not produce current, that's one paradox. Magnet rotating together with the disk will produce current, that's the second paradox. And as far as I know both types of homopolar generator produce these same two effects.
I don't see how to settle this. Either you show me some reference that explains how the two types of generator produce different effects, or I show you some reference that explains those same "disk magnets" effects with the horseshoe type of homopolar generator. I'll search for it later on.
DaleSpam said:I am not sure why you consider that unfortunate. I personally enjoy having multiple correct explanations. Often different people will prefer different explanations, so having multiple ones helps.
The magnet is the source of the field so as its position changes over time then the field at any fixed point necessarily changes over time.
That is true, but not relevant. The criterion for determining if the field is time varying is whether or not the fields stay the same (magnitude and direction) for every fixed point in a given inertial frame. The disk is not fixed in any inertial frame.
No, they both create time varying fields. The difference is that for straight line motion the resulting E field clearly cancels out the force from the B field.
Here's a quick course about magnetism of electrons:carrz said:Why, how? To properly explain it is not sufficient to just say Lorentz force did it. There is always Lorentz force between the magnet's magnetic field and electron's magnetic fields in the conducting disk. But to actually strip atoms of their electrons in the disk and move them to the rim or the center, this Lorentz force must therefore be somehow different, or stronger, than when both the magnet and the disk are stationary. Proper explanation must address how and why Lorentz force changes the way it does for each particular setup.
That is not true, but it is also off topic, so I won't argue the point. However, if you want only one explanation then asking on an internet forum is not going to accomplish your objective.carrz said:There can be only one correct explanation.
What is "it". What do I say is because of the time-varying B field? I believe that all I said is that I don't know if there is a current in this scenario and would need to see the math (or an experiment).carrz said:For the 3rd scenario when both the disk and the magnet are spinning together, some say it's because magnetic field stay static, some say it's because current is induced in connecting wires, and as far as I know only you say it is because of "time-varying" B field.
Change over time in the B field at any fixed location relative to the inertial frame where the center of the disk is at rest which is caused by the rotation of the magnet and disk about that center.carrz said:Change over time in what property, what location, relative to what, caused by what?
That is not how Maxwell's equations work.carrz said:In any case we are not talking about any absolute fixed points, we are talking only about relation between magnetic fields in the magnet and magnetic fields in the disk.
Nonsense. They don't have an inertial frame. They are rotating, so they are non-inertial by definition.carrz said:Yes, and both the disk and the magnet are in the same inertial frame when they are spinning together. So why exactly do you think magnitude and direction, of either magnetic fields of the magnet or magnetic fields of the disk, would change for any fixed point in their own inertial frame?
Consider a charge, q, at rest in a magnetic field, B. The Lorentz force is:carrz said:How does that work?
Faraday's Paradox is a phenomenon discovered by British physicist Michael Faraday in the 1830s. It states that when a magnet is moved inside a coil of wire, an electric current is induced in the wire. This means that a changing magnetic field can produce an electric current, which is known as induced current.
Faraday's Paradox is the basis for understanding electromagnetic induction, which is the process of generating an electric current by changing the magnetic field around a conductor. This occurs because the changing magnetic field causes the electrons in the conductor to move, creating an electric current.
Faraday's Paradox and induced current have many practical applications, such as in generators, motors, and transformers. Generators use induced current to convert mechanical energy into electrical energy, while motors use induced current to convert electrical energy into mechanical energy. Transformers use induced current to step up or step down the voltage of an alternating current.
While Faraday's Paradox is a fundamental principle in electromagnetism, there are some limitations and exceptions to it. For example, the magnitude of the induced current is dependent on the speed at which the magnet is moved and the strength of the magnetic field. Additionally, there are certain materials, such as superconductors, that do not follow Faraday's Paradox.
Faraday's Paradox is a crucial concept in the study of electricity and magnetism. It helped to establish the relationship between the two and provided evidence of the electromagnetic nature of light. It also led to the development of important technologies, such as generators and transformers, which are essential in our modern world.