Why does the compass spin when a current passes?

1. Jul 24, 2006

pivoxa15

Could someone give a basic and concise explanation for it? Its surprising that textbooks talk about this example as how electricity and magnetism were first considered together but not explain why it was the case.

Does the needle spin indefinitely? Or does it eventually point towards a specific direction?

I'll have a stab at it. A current in a wire sets up a magnetic field around the wire. It is this magnetic field which interferes with the magnetic field set up by the compass thereby disrupting its field causing the compass to spin. This is because the north pole 'want' field lines to be coming out and the south pole 'wants' field lines inwards. The compass will have to move from its original position in order to achieve this configuration again. But once it has achieved this with the new field (assuming a constant current) the compass should stop moving?

Last edited: Jul 24, 2006
2. Jul 24, 2006

leright

Well, do you understand why the magnetic dipole moment of a ring of current that spins freely on an axis of rotation (could be a ring or a square) aligns itself with a uniformly applied B-field? Do the analysis yourself (analyze the torques that are applied to the ring of current and determine what the equilibrium position of the ring ends up being). Take a square ring of current lying in the x-y plane which has an axis of rotation along the x-axis and then apply a uniform B-field in the y direction. You will find that the magnetic moment of the ring of current will align itself with the externally applied B-field.

Now, realize that a simple bar magnet's (for instance, the arrow of a compass is a bar magnet) magnetic moment is also caused by currents, which is the same way a ring of current's moment is produced. However, the current in a bar magnet is formed by Eddy currents flowing in the magnet. However, much like the moment of the ring of current aligns with the external B-field, the moment in the bar magnet aligns with the B-field set up by the current. This can be clearly understood when you understand how a ring of current aligns with a B-field.

This is probably confusing and I apologize. I just got off work and I'm tired.

3. Jul 24, 2006

quasar987

But leright, why does the arrow of the compass spins full circles like crazy instead of just oscillating about the position of dipole moment alignment with the B field?

4. Jul 24, 2006

leright

I've seen this experiment performed and the needle does not spin. It aligned with the field and stabilized. LEt me try and find a video of the experiment being performed. It was performed in the same way Orsted originally performed it, and it was the experiment that made it apparent that electricity and magnetism were intimately connected.

Load lecture #11 in the link below and fast forward to time 6:55. Prof. Lewin performs the experiment.

http://ocw.mit.edu/OcwWeb/Physics/8-02Electricity-and-MagnetismSpring2002/VideoLectures/index.htm [Broken]

Is this the setup you were referring to?

Last edited by a moderator: May 2, 2017
5. Jul 24, 2006

Gokul43201

Staff Emeritus
That's more or less correct. The reason the compass does not stop immediately is similar to the reason a pendulum bob that is released from a certain angle does not come to a screeching halt when it reaches its lowest point. Instead it executes oscillatory motion about this equilibrium position. If there's some friction in the system, the oscillations will be damped and the needle becomes essentially stationary in the direction of the field after sufficient time.

6. Jul 24, 2006

quasar987

leright,

I had in mind that same experiment but the arrow spining wildly as a result. I'm probably confusing that experiment with one where the compass is moved around in random directions near the wire. In this case, wild behaviors of the arrow could occur.

7. Jul 24, 2006

quasar987

As is the case with actual compasses used to indicate the north I suppose.

8. Jul 24, 2006

leright

this is definitely true. OBviously as the compass needle aligns with the B-field it will build up momentum and will overshoot and seem to oscillate, and this may be what you are thinking of when you say it spinned, but these oscillations are damped due to friction and it will eventually stabilize in an equilbrium position, which is in the direction of the B-field set up by the current.

9. Jul 24, 2006

leright

Exactly. If you push the needle out of equilibrium for a moment and then let it go it will realign with the Earth's field, but not before oscillating back and forth about the equilibrium position.