Electromagnetic Induction

This animation shows a copper rod being moved up and down through a horse-shoe magnet. Notice that as the rod moves down, the electrons (the black spheres) are forced toward the front of the rod by the changing magnetic field. As the rod is moved up, the electrons are forced to the back of the rod by the changing magnetic field. You should also notice that the positive charges (the red spheres) are not free to move because they are the protons and make up the mass of the copper rod, their position is fixed. When the electrons move to one end of the rod that end is now negatively charged and the other end is positive. As a result of this difference in charge you could look at that copper rod as though it were a battery or some other source of electrical potential difference (voltage).

This demonstrates that a changing magnetic field causes a potential difference (voltage) in a conductor. This concept is called Electromagnetic Induction. Electromagnetic Induction is the principle used in electric generators (also called alternators), microphones, electric guitars, transformers, and Tesla coils. (see the induction devices) One more thing you should notice is that the current created in this conductor is alternating current because it flows back and forth. This is a result of the conductor being raised then lowered in the magnetic field. This should begin to give you an idea of why our household current is alternating.

1998 Science Joy Wagon