Understanding the Lorentz Force

[ehrenfest]

[SOLVED] lorentz force

Homework Statement

Say you a magnetic field perpendicular to the direction of a current. If you consider the current as electrons moving forward then the Lorentz force on the electrons points up (say). But if you consider the current is a lot of positive charge carriers moving backward, then the Lorentz force on the positive charges points up also because the sign of the cross-product and the sign of the charge changes. That makes ABSOLUTELY NO SENSE TO ME because how can the positive charge carriers and the electrons both move up?

Homework Equations

The Attempt at a Solution

 

[olgranpappy]

because they are moving in different directions.

 

[ehrenfest]

because they are moving in different directions.

but the current is moving in the same direction in both cases

 

[Dick]

but the current is moving in the same direction in both cases

Then that's a good thing. It's telling you the magnetic force exerted on the wire just depends on the current, not on the sign of the charge carriers. I don't see why that would bother you. Don't we already know that?

 

[ehrenfest]

Then that's a good thing. It's telling you the magnetic force exerted on the wire just depends on the current, not on the sign of the charge carriers. I don't see why that would bother you. Don't we already know that?

What bothers me is that for a given current, the sign of the charge carriers determine the sign of the hall voltage

http://en.wikipedia.org/wiki/Imageall-effect.png

 

[Dick]

Sure. As you've observed if the charge carriers are negative, they accumulate on the 'up' side of the conductor. If they are positive they also accumulate on the 'up' side. That's two different signs of potential across the conductor.

 

[ehrenfest]

Sure. As you've observed if the charge carriers are negative, they accumulate on the 'up' side of the conductor. If they are positive they also accumulate on the 'up' side. That's two different signs of potential across the conductor.

So, I guess it was just naive to think that the sign of the transverse electric field would depend only on the current not on what is actually carrying the charge. Maybe that is how they discovered that electrons were the ones in motion...

 

[olgranpappy]

indeed.