Concepts

Temperature-dependent resistivity of copper

Overview

A copper coil is connected in series with a flashlight. When the coil is cooled in liquid nitrogen, the resistance drops and the flashlight bulb glows brighter. The resistivity of copper is temperature-dependent, with its resistance about 2 times greater at room temperature (293 K) than at liquid nitrogen temperature (77 K). 

Details

Equipment

• [1] Copper coil wrapped around acrylic rod and connected to a flashlight
• [1] Small dewar
• [1] Small dewar holder
• Liquid nitrogen

Safety Equipment

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

Classroom Assembly

1. Place the dewar in its holder.

Important Notes

• Wear safety gloves and glasses to avoid direct contact with liquid nitrogen. Liquid nitrogen can cause severe burns and blindness. Use with caution.
• The bulb may glow dimly at room temperature if the batteries in the flashlight are new.
• The coil warms quickly when removed from the liquid nitrogen. The bulb brightness decreases rapidly as the coil warms.

Script

1. Put on safety gloves and glasses.
2. Dim the room lights.
   
3. Turn on the flashlight.
4. Notice the flashlight is dim.
5. Submerge the coil in liquid nitrogen.
6. Point out that the flashlight lights up and grows brighter as the temperature of the coil decreases.
7. Turn off the flashlight.

Additional Resources