3B Scientific Radiation of Heat__Crookes Radiometer - Manual

2
Instruction Sheet
3B SCIENTIFIC® PHYSICS
U14300 Crookes radiometer
5/03 ALF
®
The Crookes radiometer is used for demonstrating the
conversion of radiation energy into kinetic energy.
1. Safety instructions
• Caution! Handle the glass vessel carefully to avoid
breakage and resulting injury.
2. Description, technical data
The apparatus consists of a rotary-vane wheel mounted
on a metal tip and equipped with four vertical plates
connected to a shaft that is free to rotate around a vertical
axis. Each vane is colored black on one side. It is housed
in a more or less evacuated glass bulb.
Height: 210 mm
Ball diameter: 80 mm
3. Operation and operating principle
• Allow light from the sun, from a light bulb or the heat
from an oven to incident on the radiometer.
• The vane wheel starts rotating with the bare surfaces
leading.
3B Scientific GmbH • Rudorffweg 8 • 21031 Hamburg • Germany • www.3bscientific.com • Technical amendments are possible
The rotation is caused by a temperature difference be-
tween the plain surface and the blackened surface of the
vaned wheel (turbine), which produces a slight differ-
ence in the gas pressure on the two surfaces.
Incoming photons are mostly absorbed by the black
vanes and mostly reflected by the bright vanes. The
air next to the black vanes is then hotter, and so the
air molecules there have greater average kinetic
energy, pushing the black vanes and causing the
device to spin.
• Heat the radiometer over a radiator to just above
room temperature.
• Screen it against direct radiation.
• The vane rotates for a short time in opposite direc-
tion.
Heat is transported in the opposite direction and the
blackened surfaces cool off more rapidly than the bright
vanes.
This second experiment proofs that the rotation is not
caused by the radiation pressure of the incident radiation
but by the greater recoil.