Lenz's Law Diagram: Understanding the Principles and Applications

[stmartin]

http://img222.imageshack.us/img222/1203/lenzslawfe6.jpg
Thank you.

 

[Doc Al]

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?

 

[stmartin]

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?

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

 

[Doc Al]

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?

 

[stmartin]

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?

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)?

 

[Doc Al]

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.

 

[stmartin]

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.

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.
http://img247.imageshack.us/img247/3561/74457189pi7.png
http://img247.imageshack.us/img247/8823/48879275gp1.png

 

[Doc Al]

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).

 

[stmartin]

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).

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?
http://img216.imageshack.us/img216/9650/90406307hj7.gif
btw- Why it wants to keep it constant?

 

[jtbell]

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.

 

[stmartin]

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?

 

[stmartin]

Anybody know?

 

[Doc Al]

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?

 

[stmartin]

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?

I just want to 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)?

 

[Doc Al]

I just want to know why the magnetic field of the coil (when it is weaker, like in the top right example), why it wants to get stronger?

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

 

[stmartin]

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

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?

 

[Doc Al]

I just want to know why it wants to have stronger field?

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.

Isn't weaker field better?

No.

 

[stmartin]

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.

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?

 

[stmartin]

Anybody knows?

 

[Doc Al]

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?

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.

 

[stmartin]

When the permanent magnet gets closer to the coil it lines the domains (the atoms) of the coil and it makes field with opposite poles of the permanent magnet. So when I'll close the permanent magnet there will be increasing of the field and when I will pull back the permanent magnet then there will be not lining up of the two fields.
http://img216.imageshack.us/img216/9650/90406307hj7.gif
So I can't see any weaknesses of the field of the coil.
http://img247.imageshack.us/img247/3561/74457189pi7.png
http://img247.imageshack.us/img247/8823/48879275gp1.png

 

[Doc Al]

I think I finally understand your question...

When the permanent magnet gets closer to the coil it lines the domains (the atoms) of the coil

No, this is not the correct way to view things. You are treating the coil as a piece of magnetic material (like iron) that is attracted to the poles of the permanent magnet. But that's not the effect of interest here. (What if the coil were made of non-magnetic, copper wire?)

The right way to view this is to realize that the permanent magnet is surrounded by an non-uniform magnetic field. When that magnet is moved, the field through the coil changes, which induces an EMF that creates a current in the coil.

and it makes field with opposite poles of the permanent magnet.

No, the magnetic field created by the current in the coil is not always oriented opposite to the poles of the permanent magnet. It is oriented in such a way as to oppose the change in the field through the coil due to the movement of the magnet.

So when I'll close the permanent magnet there will be increasing of the field and when I will pull back the permanent magnet then there will be not lining up of the two fields.
http://img216.imageshack.us/img216/9650/90406307hj7.gif
So I can't see any weaknesses of the field of the coil.

This diagram is not true in general--sometimes the poles are opposite; sometimes they are the same.

http://img247.imageshack.us/img247/3561/74457189pi7.png

In this diagram, since you are pushing the magnet towards the coil, the poles of the magnetic field of the coil will point in the opposite direction: You'll have two north poles facing each other, thus repelling and resisting the push.

http://img247.imageshack.us/img247/8823/48879275gp1.png

In this diagram the orientation of the poles of the coil are show correctly, but this creates an attractive force that resists the movement of the permanent magnet.

 

[stmartin]

And how is possible that one field can be weaker? What should I have to make some magnetic field weaker?

 

[jtbell]

The magnetic field of a magnet decreases with increasing distance from the magnet, like the electric field of a charge decreases with increasing distance from the charge. So if you move the magnet further away from some point, you decrease the magnetic field at that point.

 

[stmartin]

The magnetic field of a magnet decreases with increasing distance from the magnet, like the electric field of a charge decreases with increasing distance from the charge. So if you move the magnet further away from some point, you decrease the magnetic field at that point.

So if I pull back the permanent magnet from the coil (like in the top right example) I will decrease the field right? That means that before the magnetic field was increased with the pulling in the permanent magnet?

 

[jtbell]

So if I pull back the permanent magnet from the coil (like in the top right example) I will decrease the field right?

You decrease the field produced by the magnet, at the location of the coil, correct.

That means that before the magnetic field was increased with the pulling in the permanent magnet?

Correct, when you move the magnet near the coil originally, the magnetic field that it produces at the location of the coil, increases. This of course also produces an induced current in the coil, but in the opposite direction to when you pull the magnet away. The induced current goes in one direction when you move the magnet towards the coil, and it goes in the other direction when you move the magnet away from the coil.

 

[stmartin]

You decrease the field produced by the magnet, at the location of the coil, correct.



Correct, when you move the magnet near the coil originally, the magnetic field that it produces at the location of the coil, increases. This of course also produces an induced current in the coil, but in the opposite direction to when you pull the magnet away. The induced current goes in one direction when you move the magnet towards the coil, and it goes in the other direction when you move the magnet away from the coil.

You know, the electrons are reinforcing them selfs to create the best energy state level, that means that also they create the best force level. So when I pull in the permanent magnet there will be increasing of the field and electrons are getting opposion to the increase, so there will be not any increasing right?

 

[stmartin]

What do you think?

 

[jtbell]

You know, the electrons are reinforcing them selfs to create the best energy state level, that means that also they create the best force level.

I'm sorry, I don't understand what you're trying to say here.

 

[stmartin]

I'm sorry, I don't understand what you're trying to say here.

I want to say that always the electrons tend to have lower energy level and also I couldn't understand how do the field is increased when their magnetic field is opposite of the permanent magnet's field.