🚀 go-pugleaf

In article <kevinl-1812951936500001@tsb-oc-26.earthlink.net>, kevinl@earthlink.net (Kevin Luttrell) writes:
|>
|>Hi,
|>
|>I teach 9th grade science and I would like a definitive word on how a
|>radiometer works. They are the "toys" that consist of a clear glass bulb
|>with freely rotating vanes inside. The vanes are painted white on one
|>side, black on the other. The vanes will spin in the presence of bright
|>light.
|>
|>I believe that momentum conservation requires the photons to be imparting
|>their momentum to the vanes. Since photons have no rest mass, the mass
|>they contribute to momentum is relativistic, m = E/c^2. Since they are
|>traveling at speed c, the momentum becomes E/c. Is this view correct?
|>

In theory, this is what you would like the radiometer to do. The photons
are absorbed in the black sides and bounce off the shiny sides, and since
elastic (bouncing) collisions should have a higher momentum transfer than
inelastic ones, the light pressure should turn the radiometer in the direction
which the black vanes are facing.

Unfortunately, this is not what happens. The light pressure effect, although
real, is quite small. 

|>Another teacher I work with thinks it has something to do with convection
|>currents set up inside the bulb. Yet I always assumed the buld was
|>evacuated  (a vacuum).

Well, your colleague is also mistaken, although the reason a radiometer
works is the small amount of air left in the bulb (it is not a PERFECT
vacuum :-)

When the light hits the vanes, the black sides heat up more than the shiny
sides. So, when gas molecules hit the black sides, they absorb energy and
bounce off with greater momentum than the ones which hit the cooler shiny
sides. So, the net effect is to cause the vanes to spin in the direction
which the shiny sides are facing.

The reason the radiometer is in a bulb is that at room pressure, the air
resistance would be too high to let the vanes spin. A radiometer only works
in a partial, but not perfect, vacuum.


|>
|>Thanks for helping out!!
|>

Any time!

|>                                    "Is this a train robbery"?    
|>Thanks! Sincerely,                  "No...it's a science experiment".
|>Kevin Luttrell                               - Back 2 the Future, III
|>

Sam Silverstein