Superconductors are materials that conduct electric current with no resistance. Electrical resistance is the property of a material to oppose current passing through and is found in nearly everything from light bulb filaments to your own body. Superconductivity occurs at extremely low temperatures and induces, or creates, a strong magnetic field. The current that flows at the surface of the superconducting material creates these magnetic fields. Inside the superconductor, the magnetic fields are cancelled out, but outside they are strong and can levitate a magnet.
Resistance in a material generally decreases with decreasing temperature. Superconductivity occurs as a phase change in materials, and when at a low enough temperature, the current in a superconductor can flow forever without a power source. The temperature at which a material becomes a superconductor is called its critical temperature.
Observe the Meissner Effect in a superconductor.
- Thick Styrofoam cup
- Cubic neodymium magnet
- YBa2Cu3O7 ceramic disk
- Liquid nitrogen
Safety: Liquid nitrogen is extremely cold, and it boils into a gas at 77K, or -321°F. This can burn you and cause damage to your skin. Do not handle objects covered in liquid nitrogen with your bare hands. Be sure to perform this experiment in a well ventilated area.
- Use the scissors to cut off the “walls” of the Styrofoam cup. A small dish about half an inch high will do.
- Very carefully pour about a quarter inch of liquid nitrogen into the Styrofoam cup and wait for it to stop boiling. Yes, it’s boiling! Nitrogen boils at 77K, which is -321°F!
- Using the tongs, gently place the ceramic disc in the liquid nitrogen.
- Using the tongs again, bring the magnet directly above and very close to the ceramic disc.
- Let go of the magnet. Record your observations.
After cooling with liquid nitrogen, the magnet will float above the surface of the ceramic disc.
Below the critical temperature, superconductors are diamagnetic, which means they strongly repel magnetic fields and do not allow magnetic fields to enter. The repulsive magnetic field around the superconductor repels the magnet, causing it to levitate above the surface of the ceramic disc. When the liquid nitrogen boils off and the ceramic disc begins to warm, the magnet will no longer float.