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| D.3.40 Crookes radiometer (camera) | |
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Concept: Four vanes are suspended inside a glass bulb containing vacuum. Each vane has a black and a shiny side. When the radiometer is lit or heated the vanes rotate with the black side away from the source. When the radiometer is cooled it reverses the direction of its rotation. Keywords: Crookes radiometer, heat, light, photon, radiation, radiation pressure, temperature, absorption, emission, Kirchhoff's law, thermal transpiration, thermal creep, mean free path, kinetic theory |
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Time to setup: 5 min
Time to demo: 10 min
Warning: This demonstration uses liquid nitrogen or water ice as an option. Please, place your request in advance, and indicate your choice of cooling.
Description, explanation
This experiment has a long and controversial history. Sir William Crookes built the radiometer in 1875 and reported his observations. James Clerk Maxwell saw the effect as a demonstration of radiation pressure predicted by his theory of electromagnetism. But the vanes were rotating in the wrong direction! Several other incorrect explanations were given. Maxwell showed in 1879 that it is not the force from the gas pressure on the warm side. Furthermore it cannot be the recoil from the gas molecules either, because the mean free path is about a millimeter or a fraction of it at this pressure (10E-3 to 10E-4 atm). The correct explanation was given by Osborne Reynolds in 1879. It is thermal transpiration on the edge of the vanes giving a net force spinning the vanes. It is possible to measure radiation pressure with a more sensitive apparatus, a torsion balance in extra high vacuum. That experiment was performed in 1901 by Pyotr Lebedev, and E.F. Nichols and G.F. Hull. (Ref: The Physics Teacher, Oct. 1968, pp358.)
Equipment list, location(s)
Two different models of Crookes radiometer, semi cylindrical metal container, desk lamp, laser pointer.
Notes and suggestions to instructor
Use the desk lamp first. Then shine laser light on the vanes. Does the laser light make the vanes rotate? You can fill the metal container with liquid nitrogen, crushed ice or simply chilled water from the drinking fountain. The radiometer is sensitive, even the cold water will work to spin it in the opposite direction. After cooling, put your hand next to the radiometer. The warmth from your palm will be enough to spin the vanes. After long enough periods of warming or cooling the glass bulb, the gas and vanes termalize and the spinning stops. If you run the radiometer with sunlight it does not stop, because it never comes to equilibrum with the temperature of the incident light, that is the temperature of the Sun's surface.
