Purpose:Shows the damping of pendula due to eddy currents
Pendula with bobs of different materials and geometries are swung through the poles of a strong horseshoe magnet. The amount of damping is greater for those bobs in which strong eddy currents can flow. Bobs include, solid copper, copper loop, broken copper loop, laminated copper, copper with central hole, aluminum, and wood.
After showing the swing of the nonconductive (wood) pendulum, encourage students to make a prediction about what the copper disc will do.
Ideas to ask them about as discussion prompts:
• Will it swing just the same,
• stop immediately in the magnetic field,
• slowly slow down after a couple of swings,
• or gain energy and swing higher/faster?
As a conductive pendulum swings into the magnetic field, the changing magnetic flux induces electrical eddy currents in the metal. Some shapes (e.g. solid disc) offer more opportunity for these currents to form and grow. Outside of the magnetic field, these currents disappear at the magnetic flux does, but each pass through the magnet creates the currents again. This causes a gradual loss of kinetic energy in the pendulum.