"DAVID WILLIAMS" bravely wrote to "MIKE ROSS" (26 May
05  10:08:04)
 --- on the heady topic of "Pnu 732"

 ->  PR> Well there must be SOME explanation for radiometers.

 -> Well, radiometers are explained by gas molecules recoiling off the
 -> black vanes (gas expansion).
 DW>
 DW> Right. Radiometers turn with the shiny sides of the vanes leading. If
 DW> the cause of the motion were radiation pressure, the shiny sides would
 DW> trail, since more pressure would be exerted by photons reflecting off
 DW> them than by photons being absorbed by the black sides. Also, in real
 DW> radiometers, the vanes have to be made of some material that conducts
 DW> heat poorly, so the black sides of the vanes can be warmer than the
 DW> shiny sides. If the vanes are made of metal, the radiometer works
 DW> badly if at all. Radiation pressure would work irrespective of the
 DW> temperatures of the sides.

The vanes are typically made of aluminum or tin. They both conduct
heat but that doesn't matter because the heating is very localized and
very brief. Remember that the incident photon energy is transformed
into a mechanical torque via the gas molecules so that the vanes don't
actually get hotter and hotter. As such there is a net transfer back
out of the partially evacuated bulb. I think there is an electron
cloud boiling off the surface of the vanes (or thermal emission just
like in old radio tubes). These free electrons readily give up their
energy to any gas molecule that comes into their vicinity and then
plunge back into the metal lattice. That is a property of conductive
metals in that they have free electrons which are responsible for
electrical conduction. These can readily escape the surface if they
are excited with the least amount of photon quanta required.


 DW> Having no charge, neutrons aren't affected by electromagnetic
 DW> radiation.

A neutron doesn't have a net charge but its constituent parts do.
Recall a neutron is about equal to a proton and an electron. A neutron
weighs a bit more than the sum of a proton and electron though. There
is also some amount of energy making up the difference in mass and a
neutrino IIRC.


 DW> It isn't really an electron that is affected, but a combination of
 DW> electrons and protons. Their separation is increased, or whatever. But
 DW> since protons are so much more massive than electrons, the electrons
 DW> are moved far more than the protons.

There is some kind of resonnance effect taking place between the
electron and the nucleus of the atom because there are distinct energy
steps that the electron can exist at only. There is a constant
exchange of energy between the protons and electrons but the net
exchange is 100% efficient and always adds up to the original amount.
Kind of like the "ideal" tank circuit in an electronic oscillator.
A true form of perpetual motion, second only to the quarks orbiting
inside the proton.


 -> Say if I had a vacuum tube free floating in space would it exhibit a
 -> net acceleration when an electron current flowed through it?

 DW> Why should it?

Why it shouldn't is really the question, because the energy recoil as
the electron leaves the surface is equal to the impact energy as it
arrives at destination, on the plate electrode, so the net effect is
always zero. However if there is no impact then one has a rocket
engine. In practice it is done with ions instead, since these are much
more massive, in what is called a plasma ion drive rocket engine.
Basically an ionized gas is accelerated by a high electric potential.

 M*i*k*e

... It's always darkest the moment before you step on the cat.
--- Blue Wave/DOS v2.30