Radiometer

The black side of a panel heats up more quickly than the white side. The heated air on the black side of the panel causes the panel to move.

Category:

Howto & Style

Tags:

• Radiometer
• Thermodynamics

[JosephJonathanGould]

If it were photons it would spin faster in a vacuum; it doesn't spin at all in a vacuum.

If it were air pressure imbalance on the black side of the vanes alone; it would not reverse rotation when moved to a cold area.

The sphere causes a micro whether system to develop similar to the spinning weather systems we see in earth's atmosphere.

Hope this is helpful.

[JosephJonathanGould]

Circulating air eddy currents cause the rotation.

1. Air density ratio inside to outside the bulb is critical.

2. Moving the radiometer to a cold dark area causes reverse rotation until vanes reach the same temperature.

3. Air near the black side of vanes is heated/excited more than the air near the silvered side causing a slight pressure embalance.

4. The silvered side of the vanes must be insulated from the black side of the vains so that the two sides do not reach the same temperature.

[huskykhtam]

photons carry hardly any momentum per unit energy (if it's photons the device would spin the other way round since reflection changes the momentum twice as much as absorption). it's the air molecules' momentum. Black gets heated and the black material's molecules vibrate. When an air molecule hits the black side it recoils more than if it hits the white (cooler) side.

[advthinker]

air in a vaccum?

[J0Boa]

It spins because the black side absorbs the momentum and energy of the photons and the white side absorbs less photons (reflects more photons). that's why it turns in the direction that you see

[NoizHed]

Look which way the mill is turning!

[NoizHed]

That sounds like the obvious explanation at first. But if you watch the video you'll see the light mill doesn't turn the direction you would expect if that explanation were correct.

[coldplasma]

It doesn't spin because the black side heats up faster than the white side. It spins because the photons transfer less momentum to the black side to the white side. The black side absorbs all of the momentum from the photons, whereas the white side reflects the photons and thus receives twice the momentum required to stop the photon.

[coolbluelights]

no way, I used to have one it spins wicked fast in the sun!

[dewiltheone1]

imposibble!!!