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Al Tobias (wat4y) - Office: Gibson S123 & Physics 218, (434) 924-0538

Physics Demo Manual

Demonstrations are cataloged according to PIRA Bibliography


Due to Physics Building renovations, the lead time to set up demo requests has increased due to the need to transport equipment across campus. Please be kind and let me know well ahead of time what you need.

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 Thermal Properties Of Matter
 Heat And The First Law
 Change Of State
 Kinetic Theory
  video  - Brownian Motion
 Crookes Radiometer (Light Mill)
  video  - Kinetic Theory
 Gas Law
 Entropy And The Second Law

Crookes Radiometer (Light Mill)


To demonstrate the "Radiometer Effect". This demo shows the effects of heat on black vs. white (mirrored) vanes illuminated by light in a partially evacuated glass bulb. There is still some air in the bulb.


The manual for this device is here.

The radiometer has vanes inside that are coated black on one side and white or mirrored on the other. Thus, when light is directed on the vanes, the reflected light should transfer more momentum than the absorbed light and the vanes should rotate. They do -- but in the wrong direction. This occurs because of the distribution of molecular velocities inside the radiometer as the vanes heat up. In a perfect vacuum, this would not happen, as there would be no gas molecules. In 1903, Nichols and Hull measured radiation pressures and their success was due in a large part to the care they took to eliminate spurious "radiometer effects".



E. F. Nichols and G. F. Hull, A Preliminary communication on the pressure of heat and light radiation, Phys. Rev. 13, 307-320 (1901).
E.F. Nichols and G.F. Hull, The Pressure due to Radiation, The Astrophysical Journal,Vol.17 No.5, p. 315-351 (1903).


  • 600 W light
  • Radiometer