Concepts

Faraday's law, magnetic fields from currents, electrical conductivity and temperature

Overview

An AC electromagnet levitates aluminum rings under sustained current and causes them to "jump" when a large current is suddenly introduced. The physical properties of the ring, largely the electrical conductivity and mass, determine if and how high it will jump. The slit ring, iron ring, and acrylic ring will not jump at room temperature. A ring cooled in liquid nitrogen will jump higher than an identical ring that is at room temperature.

Contrary to popular belief, this is not a demonstration of Lenz' law. The current induced in the ring has an inductive phase shift relative to the induced EMF, and this phase-shifted current feels a Lorentz force from a radial magnetic field from the electromagnet's iron core. The phase shift is essential to having an upward force. Details are in the linked reference below.

Details

Equipment

• [1] Electromagnet
• [1] Iron core lifter
  
• [1] Variac
• [1] Set of rings (aluminum rings, slitted aluminum ring, copper ring, iron ring, plastic ring)
• [1] Aluminum tube
• [1] Aluminum foil ring
  
• [1] Coil of wire with light bulb
  
• [1] Forceps
• [1] Small (~2 L) dewar of liquid nitrogen
• [1] Power bar (if necessary) 

Safety Equipment

• [1] Safety glasses
• [2] Safety glove

Classroom Assembly

1. Start with the variac at zero volts.
2. Plug the electromagnet into the variac, and the variac into the wall outlet.
   
3. Put the white iron core lifter in position under the iron core.
4. Ensure a ring is being precooled in the dewar of LN₂.
5. Confirm the demo works!

Important Notes

• Ensure demo is not under low-hanging lights or a low ceiling!
• Do not exceed 100 V on the variac.
• Liquid nitrogen can cause severe burns and blindness. Use with caution. Wear gloves when catching the liquid nitrogen cooled ring.
• Do not lean over the apparatus when operating it. The ring could hit your head at high speed.
• Do not float rings for very long. They can get very hot.

Script

Jumping rings:

1. Put on your safety glasses.
2. Turn the variac to about 100 V.
3. Place a ring on the electromagnet.
4. Turn on the variac. The ring will jump off the electromagnet.
   
5. Catch the ring.
6. Turn off the variac.
7. Repeat for a variety of rings, including the identical aluminum ring cooled with liquid nitrogen. Use tongs to remove the cooled ring. Wear gloves when catching the liquid-nitrogen-cooled ring!

Floating rings:

1. Put on your safety glasses.
2. Turn the variac to 0 V.
3. Place a ring on the electromagnet.
4. Turn on the variac and slowly turn up the voltage. The ring should float.
   
5. Turn the variac to 0 V and turn it off.

Parallel currents:

1. Put on your safety glasses.
2. Turn the variac to 0 V.
3. Place the aluminum foil ring on the electromagnet, and then put the aluminum tube on top of the ring.
4. Turn on the variac and slowly turn up the voltage. The aluminum pieces should float.
5. Separate the pieces and slowly bring the tube closer to the foil ring. They should attract.
6. Turn the variac to 0 V and turn it off.
   

Additional Resources