DEMONSTRATION TOPICS

This is a Whimhurst electrostatic generator. When the crank is turned, two glass disks rotate in opposite directions and aluminum pads on the discs are charged by electrostatic induction. Charge is removed from the pads by needles just above the disc surfaces. The charge is stored in Leyden jars until the electrostatic field between two spheres is high enough to ionize air and a spark jumps between them.

The Crooke's tube is an evacuated tube in which electrons can be accelerated by a large potential difference. The phosphor screen glows when hit by electrons. A magnetic field perpendicular to the electron path exerts a force perpendicular to both. This can be used to demonstrate how magnetic fields can be used to steer electrons beams in devices like a television screen or a cathode ray tube (CRT).

The force exerted on electrons as they cross magnetic field lines can be used to generate current as in this generator. If the wires from the generator are not connected to anything, the generator does no work and its handle turns easily. If the generator is pushing electrons through resistance (like light bulb filaments) the handle is harder to turn. The generator can push electrons into a storage device like a capacitor. When you stop turning the handle, the electrons flow back through the generator and it acts like an electric motor.

A simple electric motor can be made by attaching a coil of wire to a thin wooden shaft and taping the ends of the wire to either end of the shaft. The shaft can be supported on two wires (paper clips) attached to the terminals of a battery. Insullation is scraped from the wires on the shaft so that current flows through the coil for an instant each time it completes a revolution. While the current flows, the coil becomes a magnet and is repelled by the magnet on the base of the motor.

An alternating current through this coil causes an alternating magnetic field along the axis of the coil. This induces a current around the aluminum ring above the coil. The magnetic field resulting from the current in the aluminum ring repels the field in the coil and the ring is levitated (vigorously).

The ceramic, Yttrium Barium Copper Oxide (YBCO), is superconducting at 77 K, the boiling point of liquid nitrogen. In a superconductor, electrical currents can flow without resistance and magnetic fields are expelled (the Meissner effect). A small rare earth magnet placed on a superconducting disc will be levitated as its magnetic field is expelled from the superconductor.

This demonstration of energy lossess due to eddy currents was provided by the Canadian Association of Physicists to mark the International Year of Physics. When a penny and a strong magnet are allowed to slide down an inclined plexiglass sheet, the smooth magnet arrives at the bottom first. The magnet does not stick to aluminum but when they are allowed to slide down an inclined aluminum sheet, the magnet is much slower. Most of the gravitational potential energy of the penny is converted to kinetic energy as it slides but the magnet induces eddy currents as it slides and some of its gravitational potential energy is also converted to kinetic energy of the electrons in the aluminum.