# Electromagnetic Induction

1. Feb 8, 2017

### Xubi

1. The problem statement, all variables and given/known data

A ring of mass m, radius R having charge q uniformly distributed over it and free to rotate about its own axis is placed in a region having a magnetic field B parallel to its axis. If the magnetic field is suddenly switched off, the angular velocity acquired by the ring
2. Relevant equations

3. The attempt at a solution
All I understand in this problem is that the change in magnetic flux will induce current in the ring. I don't understand why it will rotate, who will provide torque for rotation.

Last edited: Feb 8, 2017
2. Feb 8, 2017

### TSny

Welcome to PF!

The ring can be considered an insulator. So, there will not be any induced current in the ring material.

The key idea in this problem is that there will be an electric field associated with the changing magnetic field.

3. Feb 8, 2017

### Xubi

Thank you for such quick response.
However, it doesn't mention in the question that the ring is an insulator.
Yes, there will be an electric field associated with the changing magnetic flux. But it is still very unclear to me how this electric field will rotate the ring.

4. Feb 8, 2017

### TSny

If the ring is not an insulator, then there will be an induced current in the ring. But this current is not relevant to the problem.

In order to set the ring into rotation, some sort of force must act on the ring. The fact that the ring is charged suggests that the force acts on the charge q spread around the ring. What type of field might be producing the force on the charge?

5. Feb 8, 2017

### Xubi

It does suggest that electric field must be applying the torque necessary for rotation. But still it seems very vague to me.

6. Feb 8, 2017

### Xubi

Is it possible to show in a diagram how charge separation will occur in the ring so that i t becomes clear to me how the electric field will produce torque?

7. Feb 9, 2017

### TSny

There is no need for any charge separation to occur. The ring already has a net charge q uniformly spread over it.

8. Feb 9, 2017

### Xubi

How will electric field be produced? The arrangement of these charges on the ring must change, right?

9. Feb 9, 2017

### TSny

The changing B field "produces" the E field.

The arrangement of q on the ring does not need to change.

Can you picture the E field lines produced by the changing B field?

10. Feb 9, 2017

### Xubi

No

11. Feb 9, 2017

### TSny

Google "induced electric field".

12. Feb 9, 2017

### Xubi

In the typical example of a rod moving on a U shaped rail placed in a magnetic field , it is very clear to me how charge separation will occur and how electric field will be produced. But in this case I am unable to imagine what is happening. You say " the arrangements of charge on the ring doesn't need to change for electric field to be produced". I can't figure out how electric field will be produced without change in arrangemnt of charges on the rod

13. Feb 9, 2017

### TSny

In order to work this problem, you need to be familiar with the idea that a changing B field induces an E field. The induced electric field lines "circle around" the B field lines. The magnitude of the induced E field can be determined from Faraday's law of induction.

For example, see these videos: