# Simple EM situation

1. Jul 30, 2010

### ehilge

You have a coil of wire inside a solid cylinder of iron or some other magnetic material. There is a DC current going through the coil of wire so you have an electromagnet. If you were to spin the outer shell, would the coil also spin in the same direction.

My first thought when approaching this was no because the magnetic force between the coil and cylinder is the same in the either outward or inward. Essentially, you can spin it all you want but it won't change to force on the inner coil. However I don't think this is correct. The way I came across this situation is someone explaining the properties of a magnetic coupling to me. Essentially, you could have a motor driving the cylinder, and this would put torque on the inner coil which would spin whatever you want to drive. By changing the current through the coil, you can control how fast whatever you're driving rotates. So, I'm pretty sure I'm misunderstanding either the design or the principle. Your thoughts?
Thanks

2. Aug 2, 2010

### marcusl

Your intuition is correct. Because the iron shell is axially symmetric, and so are the fields from the solenoid, there should be no torque when they spin relative to one another.

3. Aug 2, 2010

### ehilge

curious, I guess I'll have to look into my magnetic coupling in a bit more detail.

thanks

4. Aug 2, 2010

### marcusl

I'm assuming that your coil of wire is a solenoid that is aligned with and located on the axis of a hollow iron cylinder.

5. Aug 3, 2010

### ehilge

Correct. Any thoughts on how you could possibly transfer power in something like the arrangement I described. What if the outside cylinder was a magnet or electromagnet? I still don't think that would change anything because of the symmetry. Any other ideas?