Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Homopolar Generator
Name: Mike S.
Status: other
Age: 30s
Location: N/A
Country: N/A
Date: 2000-2001


Question:
I think I understand the basics of a homopolar generator. A disc is spun in front of a magnet and an electrical current flows in the disc. my question is, Can the disc be attached to the magnet (insulated and glued) and have both spun together and have an electrical current formed on the disc? The reason I ask is that there seems to be conflicting information about it and I cannot seem to find any "reliable" information on it. Thank you.


Replies:
No, that does not work. You have a question on one of the most subtle parts of physics. A charged particle moving through a stationary magnetic field feels a magnetic force. However, if that field is moving with the particle, though the field may be of the same strength as in the previous case, the particle feels no force!

It can, perhaps, best be explained by realizing that the force is really between the (moving) charged particle producing the field and the (moving) charged particle acted on by the field. The field is a construct which (sometimes) simplifies the thinking about magnetic forces (and sometimes complicates the thinking).

The physics is even more subtle. Two positive charged particles at rest repel each other. If they move at the same speed parallel to each other, there is an attractive magnetic force. Newtonian relativity says that if you run with the moving charges, the physics must be the same. However, that eliminates the magnetic force for the running observer!

In fact, this problem was a main impetus for Lorenz and Einstein to develop the theory of special relativity. If you watch the particles flying by, Einstein says their clocks slow down and they are repelled more slowly, which is another way of saying the magnetic attraction slows their motion apart induced by the electrostatic repulsion.

I hope this is clear. It is NOT simple. Let me know if you'd like me to try again. Best, Dick...

Richard J. Plano



Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory