Thermodynamics and Quantum Mechanics

Goal: To Measure emissivity

Be sure that you play with the two student exercises so that you understand Planck's distribution formula.

Materials

• A Crooke's radiometer (can be purchased for under $10 ) OR
• IR thermometer (kitchen equipment)
• Rotating table (a lazy-susan arrangement will work).
• A Leslie's cube (commercial ones can be purchased for $10-25, depending on suppliers and quality. ).

  You can also make a substitute Leslie's cube using a square metal container, and painting the outside of one side matte black, one side shiny black, one side white, and leaving one side as shiny metal. The box should be large enough to hold a small 40W lightbulb and socket. Make a "floor" and lid of aluminum foil to retain the heat, with a hole in the floor for the socket cord. Make a hole in the lid so that you can suspend a thermometer above the light bulb.

  

Procedure

1. Predict emissivity (amount of energy escaping through a side) for each side of the box in general terms: which side will emit the most energy? Which the least?
2. Record the dimensions of the sides of your box.
3. Create a data table for each set of measurements (you will need multiple tables, one for each temperature reading):

   	T0 = val	T0 +20°C = val	T0 +40°C = val	T0 +60°C = val	T0 +80°C = val
   Calculated Flux
   White Side rev/min
   Metal Side rev/min
   Black Matte Side rev/min
   Black Glossy Side rev/min

4. Set up the box as shown, with the lightbulb inside. Leave the lightbulb off.
5. Place the box on your rotating table.
6. Place the radiometer in a fixed distance from one side of the box, on a line normal to the center of the side.
7. Record the ambient temperature T₀ and calculate the Stefan-Boltzmann flux for that temperature; record this on the table under the temperature.
8. Using the rotating table, turn each face to the radiometer and record the revolutions per minute (for at least 30 seconds) for each side OR your IR thermometer reading.
9. Turn on the light bulb and let the temperature increase at least 20 °C (T₀ + 20°C). Observe each side for at least 30 seconds and record the revolutions per minute.
10. Repeat your measurements for T₀ + 40°C, T₀ + 60°C, T₀ + 80°C, T₀ + 100°C if possible.

Data Handling

1. Plot the flux as a function of temperature.
2. For each side, plot the revolutions/minute as a function of temperature.
3. Discuss your results.

Report

1. Describe your experiment in sufficient detail that another similarly equipped student could perform your tests.
2. Include your assessment that the light bulb acts/doesn't act as a blackbody over various temperature ranges.

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