# Explaining Magnetism - A Small Bar Magnet and a Massive Solenoid, what would happen?

1. Jun 5, 2009

### PhysicsJunkie

1. The problem statement, all given/known data

A bar magnet is positioned at the mouth of a solenoid. The current indicated in the solenoid is then turned on. (Assume the solenoid is a lot bigger than the bar magnet.) Gravity and friction can be ignored.

Describe what would happen, and why.

For a visual representation:

http://img141.imageshack.us/img141/3614/assignmentq.png [Broken]

2. The attempt at a solution

To try and solve it I drew the above diagram. Since the north pole of the bar magnet is facing the south pole of the solenoid I suppose they would attract?

But I feel like there's something else that would happen (ex. something more drastic since the solenoid is much more massive).

Any help/explanations will be greatly appreciated!

Last edited by a moderator: May 4, 2017
2. Jun 5, 2009

### LowlyPion

Re: Explaining Magnetism - A Small Bar Magnet and a Massive Solenoid, what would happ

Welcome to PF.

I think the reason they say the solenoid is much greater is that the magnetic field lines in the region of the opening will be uniform and parallel to the axis of the solenoid. Also that it is longer so no need to consider what happens if the end of the magnet should poke out the other end.

As you note the force is attractive. The field lines run through the coils, so the bar magnet will experience a force, there is no friction ... so ...

3. Jun 5, 2009

### PhysicsJunkie

Re: Explaining Magnetism - A Small Bar Magnet and a Massive Solenoid, what would happ

Thanks for the welcome!

So the bar would be simply be attracted? How close would it move to the solenoid before stopping? Would anything else occur?

4. Jun 5, 2009

### LowlyPion

Re: Explaining Magnetism - A Small Bar Magnet and a Massive Solenoid, what would happ

As far as that goes yes. If the solenoid is open ended will it be drawn inside? If so, consider what happens after it is inside. Would it be pulled all the way through? Would it stop?

5. Jun 5, 2009

### PhysicsJunkie

Re: Explaining Magnetism - A Small Bar Magnet and a Massive Solenoid, what would happ

I suppose it would stop when the 2 south poles come in contact? Would the bar magnet then be repelled back out?

6. Jun 5, 2009

### LowlyPion

Re: Explaining Magnetism - A Small Bar Magnet and a Massive Solenoid, what would happ

What would stop it? There's no friction or gravity.

Maybe it would oscillate inside?

7. Jun 6, 2009

### PhysicsJunkie

Re: Explaining Magnetism - A Small Bar Magnet and a Massive Solenoid, what would happ

Oh I think I understand the problem now. The magnet would enter the solenoid, then when the south poles meet, it will be repelled back out, then back in, then back out etc.?

So it will oscillate near the opening of the solenoid?

8. Feb 3, 2012

### mishi003

Re: Explaining Magnetism - A Small Bar Magnet and a Massive Solenoid, what would happ

the magnet would be attracted to one side and then it would be situted at the centre of the soenoid . this is proven by a formula which shows tht the magnetic attraction is a function of the distn e btween the magent and the solenoid centre and this becomes a minimum whn the two centres coincide