# Lenz's law

73

Thank you.

### Staff: Mentor

In the diagram: The magnet provides a magnetic field through the coil indicated by the blue arrow marked "B". As the magnet moves, the magnet field changes; the change is marked $\Delta B$. The induced current creates a magnetic field that opposes that change; the current direction is shown by the thick red arrows; the induced field from that current is shown by an arrow opposed to $\Delta B$ marked $B_{induced}$.

Does that help?

3. ### stmartin

73
Ok, I understand, and how is possible decreasing of the field? I can't understand that.

### Staff: Mentor

In the top right and bottom left diagrams, the change in the field is opposite to the field and thus the magnitude of the field is decreasing. Taking the top right as an example: The field from the magnet points to the right (since its a south pole). Since it's also moving to the right, the field is getting weaker--the change in field points to the left. Make sense?

5. ### stmartin

73
So you say that the magnet (from the example top right) is "pulling" the magnetic field from the wire (since the wire its it self a magnet)?

### Staff: Mentor

In that diagram (top right) the field from the magnet is being pulled to the right, thus reducing the field inside the coil due to the magnet. This action induces a current in the coils which creates a field that opposes this change.

7. ### stmartin

73
I still can't see the difference between the decreasing and increasing examples. Just the magnet is moving in different direction. I think that in the both examples the field is increasing.

### Staff: Mentor

Realize that the magnetic field of a bar magnet is strongest near the poles. So if the magnet is moving away from the coil, the field through the coil due to the magnet is decreasing (getting weaker); if it's moving towards the coil, the field is increasing (getting stronger).

9. ### stmartin

73
Is it like two permanent magnets? Ex. Let's say that there is one permanent magnet. I put close to it other same permanent magnet so they are attracting each other and there are 2 magnetic fields together. So when I'll pull out the second permanent magnet the magnetic field will be weaker, right?

btw- Why it wants to keep it constant?

### Staff: Mentor

No, that's not what Doc Al is talking about. Imagine that you start with a coil made out of plastic, not metal, so that it can't carry a current. Start with the magnet close to the coil. The flux through the coil, of the magnetic field produced by the magnet, is relatively large. Now pull the magnet away from the coil. The flux through the coil, of the magnetic field produced by the magnet, decreases, because the magnetic field is weaker far from the magnet than close to it. Of course, this doesn't have any other effect, because the coil is non-conductive, so there is no induced current.

Now, replace the plastic coil with a metal one, wired into an electric circuit, and perform the same motion with the magnet. The flux through the coil, of the magnetic field produced by the magnet, changes in exactly the same way as before. But now, because the coil is conductive, and it's part of an electric circuit, this changing flux induces a current in the coil. This current produces an induced magnetic field, which is indeed rather like the field produced by a bar magnet (a dipole field). This induced field is in addition to the original field produced by the magnet.

In this case, the induced field is in the same direction as the field produced by the magnet, so as to try to "reinforce" it and maintain a constant total magnetic flux through the coil.

Last edited: Aug 4, 2007
11. ### stmartin

73
Ok, thank you. But look. Always the electrons and in general the atoms tend to have lower magnetic or electric force. So when you get close the permanent magnet to the conductor, the field increases, but when the field is decreasing (getting weaker) why it wants to get increased again?

12. ### stmartin

73
Anybody know?

### Staff: Mentor

I think you are asking why there's a negative sign in Faraday's law--why does the induced field oppose the change due to the moving magnet. Think of it as a consequence of the conservation of energy. If, as you went to push the north pole of a magnet towards the coil, the induced current created a field in the other direction then the bar magnet would be sucked into the coil. It would speed up (increasing its kinetic energy) and the current in the coil would increase (increasing its energy as it heats up or drives some other device)--you'd end up getting free energy. The way things actually work--as described by Lenz's law--is that it takes work to push the magnet into the coil (or pull it out): No free energy here. To create that current in the coil you have to exert a force--do work--on the magnet.

Does that help?

14. ### stmartin

73
I just wanna know why the magnetic field of the coil (when it is weaker, like in the top right example), why it wants to get stronger? Isn't the atoms tend to have weaker force (weaker magnetic field)?

### Staff: Mentor

All four examples in the diagram operate according to the same principle. What's special about the top right?

16. ### stmartin

73
I said the example when the magnetic field is getting weaker no matter the top right or below left. I just want to know why it wants to have stronger field? Isn't weaker field better?

### Staff: Mentor

It "wants" to keep the field constant, not stronger or weaker. If the field through the coil (from the moving magnet) is getter stronger, the current acts to make it weaker; if the field is getting weaker, the current acts to make it stronger.
No.

18. ### stmartin

73
And when the magnetic field of the coil gets weaker, is it like when it was in first time, when there is no permanent magnet around the coil?

Last edited: Aug 5, 2007
19. ### stmartin

73
Anybody knows?

### Staff: Mentor

Again, it's not clear what you are asking. You must distinguish between (1) the induced magnetic field due to the current in the coil, and (2) the magnetic field in the coil due to the permanent magnet.

Note that the induced EMF, which creates the current in the coil, is proportional to the rate of change of the magnetic field in the coil due to the moving permanent magnet (what I labeled as (2) above). If you stop the magnet from moving, the induced EMF goes to zero. If you pull the magnet out far enough, eventually the field goes to zero and the rate of change of the field is essentially zero and the induced EMF goes to zero.