Magnetic force.

[Gareth56]

I note from reading my physics book that "experiments show that the direction of the force is always perpendicular to the direction of the current and also perpendicular to the direction of the magnetic field". What the book doesn't go on to say is the reason for the preferred perpendicular direction. Does anyone know why the preferred direction is perpendicular?

 

Thanks

[ajb]

The mathematical explanation is due to the Lorentz force which ("turning off" the electric field) is given by

 

[math]\mathbf{F} = q (\mathbf{v} \times \mathbf{B})[/math],

 

where [math]q[/math] is the particles electric charge, [math]\mathbf{v}[/math] is the velocity and [math]\mathbf{B}[/math] is the magnetic field.

 

The cross product is a vector that is perpendicular to the "input" vectors. Thus the force is perpendicular to the direction of the current and the magnetic field.

 

This is the famous right-hand-rule.

Edited November 4, 2010 by ajb

[Gareth56]

The mathematical explanation is due to the Lorentz force which ("turning off" the electric field) is given by

 

[math]\mathbf{F} = q (\mathbf{v} \times \mathbf{B})[/math],

 

where [math]q[/math] is the particles electric charge, [math]\mathbf{v}[/math] is the velocity and [math]\mathbf{B}[/math] is the magnetic field.

 

The cross product is a vector that is perpendicular to the "input" vectors. Thus the force is perpendicular to the direction of the current and the magnetic field.

 

This is is the famous right-hand-rule.

 

 

Thanks.

 

Now I know why there's no explanation in my particular physics book!

[TonyMcC]

You should note that simple electric motors and simple electric generators are really the same thing as far as construction is concerned. In other words you need to decide whether you want to determine direction of movement of a current carrying conductor in a magnetic field or whether you want to determine direction of current in a conductor that is moving in a magnetic field. These are different and there are two similar rules that cover the two situations. These are Fleming's left hand rule for motors and fleming's right hand rule for generators. You can easily find details by googling - Fleming's right left hand rules-. As a memory aid to decide whether you should use the Right or Left hand rule I used to say to myself gener-right-ers.

[swansont]

Thanks.

 

Now I know why there's no explanation in my particular physics book!

 

It gives the physics professor a chance to smugly remind you that you were supposed to earn it in the math prerequisite. It's a conspiracy against students who proclaim they'll never use the math they learn, and as a cautionary tale for the ones who tend to treat math and science subjects as compartmentalized, stand-alone classes.