Class Experiment #5
Is a Superconductor a ‘perfect metal’?
In this experiment we will discuss the following topic:
Metals exhibit the Lenz law: currents, induced by an electromotive force(voltage), are created inside a metal when exposed to a changing magnetic flux:
Experiment:
1. Take a coin and drop it horizontally above the rectangular magnets of the handheld device.
2. The coin will land smoothly on the magnets.
3. Repeat the experiment with the coin after it has been cooled in liquid nitrogen.
Observation:
1. The coin slows down as it gets closer to the magnets.
2. The cooled coin slows down even more than the room-temperature coin.
3. The downfall of the coin never stops but only slows.
Discussion:
1. The coin is metallic and the abrupt change in magnetic flux as it nears the magnets induces internal currents.
2. The currents create an opposite magnetic field that causes the coin to be repelled by the external magnets, slowing down the fall.
3. As the fall becomes slower so does the currents, since the induced voltage is proportional to the change in the magnetic field:
4. Cooling the coin reduces its electrical resistance and increases the currents (Ohm’s law: ) and with them the magnetic forces.
5. Will a superconductor, which posses no electrical resistance, behave like a super-cooled coin?
