Magnets near a current carrying wire

[vasya]

Homework Statement

if we place a bar magnet near current carrying wire, will it move in circles?

Relevant Equations

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Hi! I'm trying to understand electromagnetism. So I had a question: if we place a bar magnets on the low friction surface near vertical wire, then switch on the current, what will happen? As I been told they will align with the magnetic lines, and then nothing will happen. I also suggest that they will be bumped a little bit at the moment of turning on the current. But how this situation is different from a solenoid? Solenoid can act as a permanent magnet. But they draw the same magnetic lines in a solenoid and a current carrying wire. It's due to acceleration, right? Can you explain this please?

 

[vasya]

At least tell me if my suggestion is right and magnets will be bumped in direction tangential to a wire in a moment of switching.

 

[berkeman]

Hint -- In addition to aligning with the B-field that circulates around a current carrying wire, in which direction does that B-field get stronger? Can you think of how that gradient might affect the motion of the bar magnet?

 

[vasya]

in case of a current carrying wire gradient will be stronger near the wire, in solenoid it will will be stronger near the poles. So can you please explain me how gradients affect permanent magnets? Or provide a link to good explanation?

Hint -- In addition to aligning with the B-field that circulates around a current carrying wire, in which direction does that B-field get stronger? Can you think of how that gradient might affect the motion of the bar magnet?

 

[berkeman]

Good. I was just thinking that in addition to the bar magnet aligning with the circulating B-field from the wire, the gradient in the strength of the B-field would cause a different kind of motion of the bar magnet. Maybe do a search to see how the gradient of a B-field affects a magnetic dipole...

 

[berkeman]

Spoiler

https://en.wikipedia.org/wiki/Force_between_magnets#Magnetic_dipole-dipole_interaction

 

[vasya]

now I understand even less than before

 

[berkeman]

Glad to help!

Er wait...

Did you read the link in my Spoiler post? That gave away the answer sort of...

 

[berkeman]

Another hint in case it helps while you re-read the Spoiler link -- If I fix one bar magnet on a horizontal frictionless surface and set another bar magnet down on that surface nearby, what happens? (watch your fingers!)

 

[vasya]

If we are assuming magnet-charge model, the gradient thing will become clear. If we are talking about ampere model things are not clear

 

[berkeman]

What are the differences in the B-fields for those two models? Are there gradients in each?

 

[vasya]

What are the differences in the B-fields for those two models? Are there gradients in each?

it will take some time. its 2am in local time, so excuse me for going offline for a while

 

[vasya]

Ok. I got it. There is an imaginary loop of charged particles in each atom of the magnetic material. If we will apply F=qvBsin to every "moving charged particle" inside the magnet everything will become clear. The force acting on a loop of charged particles is proportional to the divergence of magnetic field. It's not about any gradients. Problem solved, at least for now, so thank you, and have a nice day!