# Attraction between coaxial solenoids

1. Dec 27, 2013

### LordBerkley

Can anyone help me understand why two coaxial solenoids separated by a distance x, with current flowing in the same direction in each solenoid, would be attracted to each other using the right hand rule?

Im sure they would be attracted to one another (with the current flowing in the same direction, the poles are arranged N-S / N-S), but when i try to use the right hand rule I end up saying that there would be a repulsive force.

Anybody know where I'm going wrong?

Any help appreciated

LB

2. Dec 28, 2013

### jartsa

Let's consider a small solenoid in the magnetic field of a large solenoid. Using the right hand rule we see that the small solenoid is not attracted or repelled, it's streched. (radially)

Let's say the small solenoid is elastic. When it enlarges, the direction of the magnetic field that it's in gradually changes.

So: First we imagine a small solenoid, we are using the right hand rule, our thumb is pointing in the direction of the streching force pulling some part of the solenoid, then we start turning our middle finger away from the axis of the solenoid, our thumb is turning to the appropriate direction.

3. Dec 28, 2013

### LordBerkley

Thank you for the reply, but is there a way you can show that graphically?

I've attached a diagram of why I think using the right hand rule the solenoids would repel if that helps.

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4. Dec 28, 2013

### jartsa

There is a repelling force then, let's believe whatewer the right had rule says. But there's an attractive force at the other ends of the coils.

The repelling forces in the picture are actually attractive forces towards the other end of the same coil.

(I would advice leaving one coils magnetic field out of the picture)

Last edited: Dec 28, 2013