Science Buddies: "Ask an Expert"

My son has built a homopolar motor and we can not figure out the science reason for the following:
When the magnet is flipped over the copper wire spins in a different direction, why????

Hello and welcome to the forums.

The best and most complete description about how this motor works I found is here: http://www.evilmadscientist.com/2006/mh ... -a-minute/. Here's a quote from the site:

The horizontal current arrow is the important one. The electric charges moving through the magnet feel a Lorentz force whose strength is determined by the angle between the magnetic field orientation and the direction of current flow; the force is strongest if they are 90 degrees apart. The direction of the resulting force is perpendicular to those two directions.

The key is the current arrow and magnetic field arrows. If you change either one, you will reverse the spin direction. Try all 4 options and see! (Note, the Wikipedia article on monopolar motors says that it is NOT the Lorentz force. I don't know which one is right.)

Best, Keith

Explaining exactly *why* this is true is a very complicated subject, but basically the answer is that electromagnetism follows the "right hand rule" (http://en.wikipedia.org/wiki/Right-hand_rule).

Basically, hold your right hand out in a fist with the thumb sticking out. When electric current flows in the direction your thumb is pointing, the magnetic field curls in the direction that your fingers curve.

When you flip the magnet, the magnetic field generated by the electric current stays the same, but now it reacts with the magnet backwards.