Figure 1: Beam of cold parabolic mirror setup

Equipment:

• 2 Parabolic mirrors [Cabinet E5]
  • Light bulb [Cabinet E5 (with mirrors)]
  • Copper ball [Cabinet E5 (with mirrors)]
• Electronic Thermometer [Cabinet D3]
• Fisher Scientific power supply [Cabinet K4]
• Liquid nitrogen

Demo:

Two parabolic mirrors are placed a couple of meters apart. The instructor puts a thermometer in the focus of one of the mirrors, and a heat source (a light bulb) in the focus of the other, and observe that the thermometer reads an increase in temperature. This result is trivial and expected by students. Now replace the heat source with a cold source (copper ball filled with liquid nitrogen) and notice the thermometer shows a decrease in temperature.

Explanation:

The second law of thermodynamics states that heat can flow spontaneously from a hot object to a cold object; heat will not flow spontaneously from a cold object to a hot object. In this demo the heat source (light bulb) moves towards the thermometer because it is colder. However when we put the liquid nitrogen in set in the mirror, the cold does not shoot a beam to the side where the thermometer is. The heat from the thermometer moves to the cold side causing the temperature on the thermometer to decrease.

Note:

• The lightbulb has maximum ratings of 8 Volts and 4 Amps
  • Exceeding this will burn out the bulb
• Mirrors should be precisely aligned
  • For alignment look through the stainless steel tube and make sure you see the focus of the thermometer in the focus of the distant mirror.
• For reading the temperature, use the big screen LCD thermometer

Edited by Noah Peake