Magnetic force on a charge carrying wire

[thatgirlyouknow]

[SOLVED] Magnetic force on a charge carrying wire

Homework Statement

A current carrying wire is in a magnetic field as shown below. The magnet is on a scale. Given this setup, does the scale read more or less weight? Explain using the Right Hand Rule and Newton's Third Law. Note: The apparatus depicted is a magnet sitting on the mass pan of a balance.

Homework Equations

The RH rule is a bit long to explain here.. basically, the current is flowing W, and the magnetic force is N to S.

Newton's 3rd: For every action, there is an equal and opposite reaction.

The Attempt at a Solution

The wire is carrying current, but the current is not given. Current must be positive, so therefore it has a charge of >0. Positive current exerts a force upward based on the drawing, so the magnet will weigh less. I don't know how to explain this in terms of the RH rule, though. My best guess:

With my thumb pointing in the direction of the current and fingers wrapped around the wire, the magnetic force flows N to S. My open palm would face left? right? up? down?, which corresponds to the force the magnet will feel. Because of Newton's Third Law, the weight will _____.

 

[Doc Al]

Keep it simple: Find the magnetic force on the current-carrying wire. See: http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magint.html#c1"

 

[thatgirlyouknow]

So if the magnetic force on the wire is upward, then the magnet itself will push downward, resulting in a weight gain.

Right?

Thanks for the links, also. Much more informative than my prof's lecture notes.

 

[Doc Al]

Right. If the magnet pushes the wire up, the wire pushes the magnet down--increasing its apparent weight.

 

[thatgirlyouknow]

Awesome. Thanks for the help!

 

[Hdeasy]

Right. If the magnet pushes the wire up, the wire pushes the magnet down--increasing its apparent weight.

Hmm... What if a wire is attached to the N surface of a very broad magnet? Now if the current is switched on, and the whole setup is free to rotate on a wheel, what will happen? Will the BIL force continue to rotate the wire with the magnet for as long as the current flows?