Bfield and work

[chiddler]

Magnetic forces do no work because all the force is perpendicular to movement.

A bar is moving in a Bfield. "How much work is done on an electron moving across the bar?"

Why is there work in this case?

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[milski]

What's moving the bar? And the work is done by which force on which object?

 

[chiddler]

What's moving the bar? And the work is done by which force on which object?

Some external force. The work is done by the Bfield on the electron.

 

[milski]

There is displacement in direction non-perpendicular to the force. But I'm sure you already knew that. The manifestation of this work is that the magnetic field made it harder for the external force to move bar around.


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[chiddler]

hmm i don't understand what you mean.

 

[milski]

The magnetic force will perpendicular to the movement only if it is the only force acting on the electron. If there is another force, as you describe, the net force and thus the movement will not be perpendicular to the magnetic field.


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[chiddler]

Oh so you're saying that the work must be done by the force that moves the bar?

 

[milski]

Oh so you're saying that the work must be done by the force that moves the bar?

Both forces will do work - as soon as you have the electron change path from what it would if there was only magnetic field they are both doing work. The outside force to change its path and the magnetic force to try to keep it following it's circular path.

 

[chiddler]

Ok I understand, thanks.

 

[chiddler]

Thinking more. It's the electrons that are moving. They can drag the bar up or down along with them. So the assumption is that all forces exerted on the bar by the B field is suppressed as the bar is forced to move linearly in one direction and therefore there is work done.

 

[milski]

Thinking more. It's the electrons that are moving. They can drag the bar up or down along with them. So the assumption is that all forces exerted on the bar by the B field is suppressed as the bar is forced to move linearly in one direction and therefore there is work done.

I'm not sure I understand what you're saying. What does it mean for a force to be suppressed? The forces from the magnetic field are still there, but they are combined with the forces moving the bar.

Think about this: You're rotating a ball on an elastic string along a vertical axis. Is the tension force doing any work? No, because it's perpendicular to the direction of motion. Now let's say that you hit the ball from behind as it is making its rotations and it goes almost straight for a bit. Is the tension doing work now? Yes, since it's not perpendicular to the direction of motion anymore. It's somewhat similar with the magnetic field and the electrons.