Cavity Radiation EX-9912 Page 4 of 4
Cavity Radiation
EQUIPMENT
INCLUDED:1 / Thermal Cavity / TD-8580
1 / Patch Cords / SE-9750
1 / 60 cm Optics Bench / OS-8541
1 / Linear Translator / OS-8535
1 / Aperture Bracket / OS-8534
1 / Light Sensor / CI-6504A
1 / Infrared Sensor / CI-6628
1 / Thermistor Temperature Sensor / CI-6527
1 / Power Amplifier II / CI-6552A
NOT INCLUDED, BUT REQUIRED:
1 / ScienceWorkshop 750 Interface / CI-7650
1 / DataStudio Software / CI-6870
INTRODUCTION
An aluminum cube has sides that are black, white, polished aluminum, and matte aluminum with a hole in it. The cube is heated to approximately 90oC and an infrared light sensor and a visible light sensor are moved across the face with the hole in it to compare the amount of radiation emitted by the cavity to the amount of visible light reflected off the surface. Also, the emitted intensities from the different colored surfaces are compared.
THEORY
The light intensity (I) emitted by an object is given by
(1)
where s is the Stefan-Boltzmann constant (5.6703x10-8), T is the absolute temperature of the body, and e is the emissivity of the surface. In general, e ≤ 1. For a perfect emitter, e = 1.
The wavelength with the greatest intensity is given by
(2)
where T is the absolute temperature of the body.
Figure 1: Thermal Cavity Setup
SET UP
1. Mount the Rotary Motion Sensor on the rack of the Linear Translator and mount the Linear Translator to the end of the optics track (see Figure 1). Put the Infrared Light Sensor with the Aperture Bracket (set on slit #6) in the Rotary Motion Sensor rod clamp. Adjust the height of the Light Sensor to match the height of the center of the cavity in the cube.
2. Begin with the Infrared Light Sensor gain switch set on 10x and if the intensity goes off scale, turn it down to 1x.
3. Plug the Rotary Motion Sensor into Channels 1 and 2 on the ScienceWorkshop 750 interface and the Infrared Light Sensor into Channel A.
4. Mount the Thermal Cavity apparatus to the base.
5. Plug the DIN connector for the Thermistor Sensor into Channel B on the ScienceWorkshop 750 interface. Plug the red and black cables extending from the Thermistor Sensor into the upper white-colored jacks on the Thermal Cavity apparatus (see Figure 2).
6. With the power turned off, plug the DIN connector from the Power Amplifier into Channel C on the ScienceWorkshop interface. Insert the banana jacks into the output channels of the Power Amplifier.
Figure 2: Thermal Cavity Wiring
7. Connect the same set of leads from the Power Amplifier to the lower red plugs on the Thermal Cavity apparatus.
8. Open the DataStudio file called "Cavity Radiation".
9. Turn on the Power Amplifier. (The on/off switch is located in the back.) Then turn the Power Amplifier on to 10 Volts in DataStudio by double-clicking on the Power Amplifier in the Setup window and clicking on ON. Let the cube heat up for about 15 minutes and then reduce the voltage to 8 V. Monitor the temperature and don't let it go over 100oC.
CAUTION: The cube is very hot during this experiment. Never touch it. When rotating the cube, use the plastic knob below the cube.
Figure 3: Scanning the Cavity
PROCEDURE
1. Once the cube has come to equilibrium, rotate the cube so the face that has the cavity in it is facing the light sensor. Position the cube face about 2 cm from the front of the light sensor.
2. This part of the experiment can be done with the room lights on. However, be sure there is not a window or other bright source in position to enter the light sensor. Position the Infrared Light Sensor about 2 cm to the right or left of one side of the cube to start the scan. Set the clamp on the scanner rack to mark the starting position of the Rotary Motion Sensor.
3. Press the zero button on the Infrared Light Sensor and click on START in DataStudio. Rotate the Rotary Motion Sensor pulley until the Infrared Light Sensor has passed completely by the cube (see Figure 3). Don't rotate too slowly because the Infrared Light Sensor will heat up. Click on STOP.
4. Record the temperature of the cube. Calculate the wavelength of the maximum intensity using Equation (2).
5. Rotate the cube to the black side and repeat the scan, starting from the same position and pressing the zero button on the Infrared Light Sensor at the start of the scan.
6. Rotate the cube to the polished aluminum side and repeat the scan. Then rotate the cube to the white side and repeat the scan.
7. Replace the Infrared Light Sensor with the Light Sensor. Remove the Aperture Bracket so the cube will not be shadowed by it. It helps to have a light behind the Light Sensor so it reflects off the surface of the cube. Set the Light Sensor on 10x gain. Adjust the height of the Light Sensor to match the height of the center of the cavity in the cube. Position the Light Sensor at the same starting position as before.
8. This part of the experiment is done with the room lights on. Click on START. Rotate the Rotary Motion Sensor pulley until the Light Sensor has passed completely by the cube. Click on STOP.
QUESTIONS
1. Compare the graphs of the reflected visible light and the emitted infrared radiation. What do the graphs look like in the region of the hole?
2. Is the calculated wavelength that has the greatest intensity in the visible or infrared part of the spectrum?
3. Compare the intensity from each of the sides (matte aluminum, black, polished aluminum, and white). Which surface emits the greatest amount of radiation? Which surface emits the least amount of radiation?
4. How do the black and white surfaces compare?
5. Which emits more radiation: the black side or the cavity?
6. Why are automobile radiators painted black?
Written by Ann Hanks