# Magnetic field rotation

1. Jun 27, 2008

### chukie

I know that rotating the loop in a constant magnetic field induces an emf in the coils, but supposing the magnetic field is constant, will an emf be induced in the coil when the motion of the coil through it is without rotation?

2. Jun 27, 2008

### Dick

The emf is induced by a change in flux through the coil. If the magnetic field is constant and the coil doesn't rotate, can the flux change?

3. Jun 27, 2008

### chukie

Oh so the coil must rotate in order for the flux to change. If there is no rotation there would be no induced emf?

4. Jun 27, 2008

### Dick

If there is no change in flux, there is no induced emf. You tell me what the answer is.

5. Jun 28, 2008

### chukie

Thanks, I get it now. I just didn't know that you had to rotate in order for a change in flux. I got a bit confused by the textbook I'm reading.

6. Jun 28, 2008

### Nick89

Sounds like you don't fully understand what (a change in) flux actually is...

(Magnetic) Flux could be described as the quantity of magnetism. In simple conditions (constant magnetic field, constant area) the flux is just the magnetic field strength (B) multiplied by the area (A). If the conditions are less simple (for example suppose the magnetic field has different values in different places in the area) then you have to integrate.

A change in magnetic flux is therefore achieved by either a change in the magnetic field strength, or a change in the area (or both).

In the case of a loop in a constant magnetic field, is the area the same for each orientation of the loop? No! If you rotate the loop slightly the area changes. Suppose you are looking in the direction of the magnetic field, and you are looking down on a loop. If you rotate it, the area you see will get smaller, until the loop is upright (parallel to the field). Then the area is even 0. If you rotate it further it increases again until it is again perpendicular to the field, where the area is at a maximum.

So rotating the loop will change the area in question, which changes the flux, which induces an emf.

7. Aug 28, 2008

### Robin07

I hope I'm not taking this out of context...
I'm not disagreeing with the above. But am I to understand that.. If you took a permanet magnet(PM) and rotated it within a cylindercal coil an EMF will not be realized since the flux field is moving parrallel to the wires but now if I move that same PM perpendicular to the wire it will set up an EMF. Will there be an EMF if you move the magnet accross the open end of the coil, which is bisecting the wire at a perpendicular angle? I would imagine that the EMF in the later question would be very much less in strength...Yes?

Thanks
Robin07

8. Aug 28, 2008