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Fiona Rozario
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Why is it that the induced emf opposes the change in magnetic flux? Is it to follow the law of conservation of energy?
Yes. I believe this article explains it.Fiona Rozario said:Why is it that the induced emf opposes the change in magnetic flux? Is it to follow the law of conservation of energy?
It applies to the net field (external+ autoinduced).xareu said:Does the Lenz law apply to external flux across the coil or the total field (external plus autoinduced)?
Yes. The emf in this situation is the time derivative of the "net flux". Study the working of a current transformer. The concept of "burden" is very important while operating a CT.xareu said:In a general situation, when the resistance is not zero, is the EMF ever changing because the total flux is time dependent not only by "external" field change but also by the EMF itself (that is, the EMF "changes itslef")?
The reason is because iron is a conductor, so that there are free electrons that can move throughout the material. These are responsible for the eddy currents, just as occurs in copper which is non-ferromagnetic, and weakly diamagnetic (The eddy currents are the diamagnetic portion.) Bound electrons, and it is actually their spin components if I'm not mistaken, but you can loosely consider it as bound currents that are circulating in the same direction, e.g. clockwise, cause the ferromagnetic state. When these currents are all going clockwise, the net effect is a clockwise current around the outer surface of the solid. You can think of it like a checkerboard where each square has a square loop of current=currents in adjacent squares precisely cancel, wit the net effect being a current circulating on the outer edge of the checkerboard. This is the model for the bound surface currents. For additional info, see the Insights article about Permanent Magnets explained by Surface Currents that I authored: https://www.physicsforums.com/insights/permanent-magnets-ferromagnetism-magnetic-surface-currents/kiskrof said:Thanks Charles, but I am even more in trouble that before. Do you know of any experiment that would show that repulsive force? I thought that ferromagnetism was more or less the same thing as eddy currents. Why on Earth would there be some eddy electrons behaving in a a certain way and ferromagnetic electron behaving the opposite way?
I don't know of a specific one but eddy currents are only there when the flux is changing so you could measure this as a drag force on, say, an oscillating (moving) magnet. You can use laminations with or without insulation between them to allow eddy currents or not and measure the difference in amplitude or impressed force with and without eddy currents.kiskrof said:Do you know of any experiment that would show that repulsive force?
I believe if you bring a permanent magnet quickly up to a block of copper, you will see a repulsive force acting, due to the B field from the eddy currents. (The magnetic field of the permanent magnet will introduce a changing flux in the copper as it gets closer.) There may also be a torque that tries to flip the permanent magnet around. In any case, if you propel a permanent magnet with a constant velocity towards a block of copper, you may observe a decelerating force. Another experiment that has been mentioned on another posting is to run a permanent magnet down a copper tube. Because of Lenz's law, etc, it will get slowed down appreciably. You could allow gravity to pull it through the tube or you could even pull it with a string, etc...sophiecentaur said:I don't know of a specific one but eddy currents are only there when the flux is changing so you could measure this as a drag force on, say, an oscillating (moving) magnet. You can use laminations with or without insulation between them to allow eddy currents or not and measure the difference in amplitude or impressed force with and without eddy currents.
If you observe a deceleration, it would be the result of a force. In the case of pulling the magnet through a copper tube with a string, you could pull a non-magnetized piece of iron over the same path, etc. It wouldn't be terribly difficult to set up a "control" case.sophiecentaur said:Yes. That's fine but if you want to detect a force, you need to know the difference between a situation with and without its possible cause, using a 'control' case.
I thought the subject of this thread was differentiating the effect of a magnet on iron due to ferromagnetism from the magnetic force from eddy currents. The experiment you describe doesn't address this problem because it involves a copper tube - which only causes eddy currents.Charles Link said:If you observe a deceleration, it would be the result of a force. In the case of pulling the magnet through a copper tube with a string, you could pull a non-magnetized piece of iron over the same path, etc. It wouldn't be terribly difficult to set up a "control" case.
It would be interesting to get more feedback from the other posters=the OP's original question didn't contain much detail, and subsequent postings have asked numerous questions. To separate the eddy current effect from the ferromagnetism is just one of several questions that arose, and we'll need to hear from @kiskrof (who is not the OP) to see if my response was somewhat helpful to answer his question(s), or if he has additional questions.sophiecentaur said:I thought the subject of this thread was differentiating the effect of a magnet on iron due to ferromagnetism from the magnetic force from eddy currents. The experiment you describe doesn't address this problem because it involves a copper tube - which only causes eddy currents.
The "control case' for your phenomenon is to use a copper tube with a slot running down it and compare the force with that which occurs with a complete tube. But where is your ferromagnetism or your Magnet?
Lenz's law is a basic law of electromagnetism that states that the direction of an induced current in a conductor will always be such that it opposes the change that produced it. This means that when there is a change in magnetic flux through a conductor, it will create an induced current that creates a magnetic field in the opposite direction of the change.
Magnetic flux is a measure of the strength of a magnetic field passing through a given area. It is represented by the symbol Φ and is measured in units of webers (Wb). One weber is equal to one tesla-meter squared (T·m²) and is a derived unit of magnetic flux in the International System of Units (SI).
Lenz's law is a consequence of Faraday's law of induction, which states that a changing magnetic field will induce an electric field in a conductor. Lenz's law specifies the direction of the induced current in the conductor, while Faraday's law describes the magnitude of the induced electromotive force (EMF).
Lenz's law is important in understanding how electromagnetic devices work, such as generators and motors. It also plays a crucial role in the concept of energy conservation, as it shows that the induced current in a conductor will always oppose the change in the magnetic field, thereby conserving energy in the system.
Lenz's law is used in many practical applications, such as in the design of transformers, electric generators, and induction motors. It is also used in electromagnetic braking systems, where the opposing force created by the induced current is used to slow down a moving object. Additionally, Lenz's law is used in the field of magnetic levitation, where it helps to keep objects suspended in the air by opposing the pull of gravity.