# B Lenz' law and magnetic flux

1. Nov 24, 2016

### Fiona Rozario

Why is it that the induced emf opposes the change in magnetic flux? Is it to follow the law of conservation of energy?

2. Nov 25, 2016

### cnh1995

https://en.m.wikipedia.org/wiki/Lenz's_law
The induced emf tends to drive a current which causes either transfer of electrical energy (in transformers) or conversion of energy (in electrical machines). "Opposing the cause" is a feedback of the energy conversion or energy transfer process.

Last edited: Nov 25, 2016
3. Jan 12, 2017

### Fiona Rozario

Thanks much...

4. Jan 14, 2017

### xareu

One more question related to this topic: imagine you have a circuit consisting of a superconductive closed loop crossed by a time changing magnetic field. In this situation, the circuit is closed so an induced EMF should appear to counteract this magnetic field change. This leads to induced current and then an induced magnetic field. Does the Lenz law apply to external flux across the coil or the total field (external plus autoinduced)?As there is no resistance (then the total EMF is zero) does it mean that the total field in the coil is then zero? 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")?

5. Jan 14, 2017

### sophiecentaur

Lenz's Law is the electrical equivalent to Newton's Third Law in mechanics. If it weren't for N3, you would push something and it would accelerate away from the cause of the push, leaving the original 'push' behind. So it's Energy Conservation, of course.

6. Jan 14, 2017

### cnh1995

It applies to the net field (external+ autoinduced).
I am not an expert in superconductors, but it appears that the net magnetic field should be zero in order to have zero emf and it is possible bacause there is no resistance involved.
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.
https://www.physicsforums.com/posts/5654661/

7. Feb 24, 2017

### kiskrof

When a magnet comes close to a piece of iron, we all know what happens. Isn't that an exception to Lenz's law? Shouldn't the eddy currents in the iron be such as to push the magnet back? Let's say you have a plate of metal on a table. Your magnet goes down, north first. You increase the magnetic field in the downward direction. So the induced currents in the plate want to produce a magnetic field upwards, i.e., a north pole pointing upwards, which is supposed to repel the magnet.
What is my mistake?

8. Feb 24, 2017

(To answer the question in post #7), The process of induced ferromagnetism, where the magnetic field is enhanced in the material from the applied magnetic field, is both an exception to LeChatlier's principle and Lenz's law. The induced eddy currents will indeed create the opposite effect and obey Lenz's law and LeChatlier's principle. The ferromagnetism, whose overall effect can be explained as bound surface currents, serves to enhance the applied magnetic field and is indeed an exception to what you would expect. The effect from the ferromgnetism in ferromagnetic materials is much larger than any eddy current effect.

9. Feb 25, 2017

### kiskrof

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?

10. Feb 25, 2017

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/

Last edited: Feb 25, 2017
11. Feb 25, 2017

### sophiecentaur

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.

12. Feb 25, 2017

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...

13. Feb 25, 2017

### sophiecentaur

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.

14. Feb 25, 2017

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.

15. Feb 25, 2017

### sophiecentaur

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?

16. Feb 25, 2017