"Burgerman" wrote in
:

Both Einstein's and Reynolds's forces appear to cause a Crookes
radiometer to rotate, although it still isn't clear which one is
stronger.


It's still magic to me - just like that stirling engine thing. Wish I could
get one. "OI !!! SANTA !!!" :-)

--
Tunku

"end user" v. A command regrettably not implemented in most systems.

On Thu, 21 Dec 2006 14:02:33 GMT, Burgerman wrote:

"Burgerman" wrote in message
...

"Steve Firth" wrote in message
...
On Thu, 21 Dec 2006 12:49:11 GMT, Burgerman wrote:

"Mike G" wrote in message
...

"Burgerman" wrote in message
...
http://www.ltd-stirling.de/index.htm

you can get these cheaper on ebay.

I have one running for 10 days solid sat on top of my ntl set top box!

And I am buggered if I can fathom how it works...

Dunno what it's called, but the gadget I've never been able to figure
out
is the one that has a rotating vane in a sealed glass container shaped
like an electric light bulb. Powered by light.
Mike.


Can you still get them?
Its the "pressure" of light. Absorbed in the black and reflected from
the
white.


Umm no, that's not how they work (sadly) although light pressure is a
real
phenomenon. The vanes on a Crookes Radiometer are too small to get much
of
a push from light pressure and the pressure would be greater on the white
patch than on the black. So if light pressure were the cause the rotation
would have the white patch on the trailing edge. As it is, they are the
other way around, the "push" comes from the black side.

The vanes are in a vacuum, but the vacuum isn't perfect, just good at
about
10^-2 torr. Hence there are some gas molecules in there, just not many.
The
vaccuum is good enough to reduce air friction on the vanes significantly.
When exposed to light, the dark side of the vane absorbs photons and
warms
up slightly. Any air molecule hitting that side of the vane departs with
slightly more energy than it had. Any air molecule hitting the white side
of the card leaves with about the same (or slightly less) energy than it
had. This isn't enough to cause an imbalance in force big enough to make
the vanes turn, but at the edges of the vane there are effects where the
molecules of air move from the cold side to the hot side past the edge of
the vane. The sum of these forces causes an imbalance in force on the two
sides of the vane and it rotates.

Confirmation of this effect can be shown by evacuating more air from the
bulb. By the time you get to 10^-6 torr the vanes stop moving.


But photons have no mass...

But they do have a lot of energy.

Well whatever magic it uses I just ordered one.


[edit] Explanations for the force on the vanes
Over the years, there have been many attempts to explain how a Crookes
radiometer works:

1.. Crookes incorrectly suggested that the force was due to the pressure
of light. This theory was originally supported by James Clerk Maxwell who
had predicted this force. This explanation is still often seen in leaflets
packaged with the device. The first experiment to disprove this theory was
done by Arthur Schuster in 1876, who observed that there was a force on the
glass bulb of the Crookes radiometer that was in the opposite direction to
the rotation of the vanes. This showed that the force turning the vanes was
generated inside the radiometer. If light pressure was the cause of the
rotation, then the better the vacuum in the bulb, the less air resistance to
movement, and the faster the vanes should spin. In 1901, with a better
vacuum pump, Pyotr Lebedev showed that in fact, the radiometer only works
when there is low pressure gas in the bulb, and the vanes stay motionless in
a hard vacuum. Finally, if light pressure were the motive force, the
radiometer would spin in the opposite direction as the photons on the shiny
side being reflected would deposit more momentum than on the black side
where the photons are absorbed. The actual pressure exerted by light is far
too small to move these vanes but can be measured with devices such as the
Nichols radiometer.
2.. Another incorrect theory was that the heat on the dark side was
causing the material to outgas, which pushed the radiometer around. This was
effectively disproved by both Schuster's and Lebedev's experiments.
3.. A partial explanation is that gas molecules hitting the warmer side of
the vane will pick up some of the heat i.e. will bounce off the vane with
increased speed. Giving the molecule this extra boost effectively means that
a minute pressure is exerted on the vane. The imbalance of this effect
between the warmer black side and the cooler silver side means the net
pressure on the vane is equivalent to a push on the black side, and as a
result the vanes spin round with the black side trailing. The problem with
this idea is that the faster moving molecules produce more force, they also
do a better job of stopping other molecules from reaching the vane, so the
force on the vane should be exactly the same - the greater temperature
causes a decrease in local density which results in the same force on both
sides. Years after this explanation was dismissed, Albert Einstein showed
that the two pressures do not cancel out exactly at the edges of the vanes
because of the temperature difference there. The force predicted by Einstein
would be enough to move the vanes, but not fast enough.
4.. The final piece of the puzzle, thermal transpiration, was theorized by
Osborne Reynolds, but first published by James Clerk Maxwell in the last
paper before his death in 1879. Reynolds found that if a porous plate is
kept hotter on one side than the other, the interactions between gas
molecules and the plates are such that gas will flow through from the cooler
to the hotter side. The vanes of a typical Crookes radiometer are not
porous, but the space past their edges behave like the pores in Reynolds's
plate. On average, the gas molecules move from the cold side toward the hot
side whenever the pressure ratio is less than the square root of the
(absolute) temperature ratio. The pressure difference causes the vane to
move cold (white) side forward.
Both Einstein's and Reynolds's forces appear to cause a Crookes radiometer
to rotate, although it still isn't clear which one is stronger.

See also: photophoresis.



And yes I know its html but I dont care...

Yes, that's what I meant by:

"at the edges of the vane there are effects where the molecules of air move
from the cold side to the hot side past the edge of the vane. The sum of
these forces causes an imbalance in force on the two sides of the vane and
it rotates."

Nice Wiki cut and paste BTW.

"Steve Firth" wrote in message
...
On Thu, 21 Dec 2006 14:02:33 GMT, Burgerman wrote:

"Burgerman" wrote in message
...

"Steve Firth" wrote in message
...
On Thu, 21 Dec 2006 12:49:11 GMT, Burgerman wrote:

"Mike G" wrote in message
...

"Burgerman" wrote in message
...
http://www.ltd-stirling.de/index.htm

you can get these cheaper on ebay.

I have one running for 10 days solid sat on top of my ntl set top
box!

And I am buggered if I can fathom how it works...

Dunno what it's called, but the gadget I've never been able to figure
out
is the one that has a rotating vane in a sealed glass container
shaped
like an electric light bulb. Powered by light.
Mike.


Can you still get them?
Its the "pressure" of light. Absorbed in the black and reflected from
the
white.


Umm no, that's not how they work (sadly) although light pressure is a
real
phenomenon. The vanes on a Crookes Radiometer are too small to get much
of
a push from light pressure and the pressure would be greater on the
white
patch than on the black. So if light pressure were the cause the
rotation
would have the white patch on the trailing edge. As it is, they are the
other way around, the "push" comes from the black side.

The vanes are in a vacuum, but the vacuum isn't perfect, just good at
about
10^-2 torr. Hence there are some gas molecules in there, just not many.
The
vaccuum is good enough to reduce air friction on the vanes
significantly.
When exposed to light, the dark side of the vane absorbs photons and
warms
up slightly. Any air molecule hitting that side of the vane departs
with
slightly more energy than it had. Any air molecule hitting the white
side
of the card leaves with about the same (or slightly less) energy than
it
had. This isn't enough to cause an imbalance in force big enough to
make
the vanes turn, but at the edges of the vane there are effects where
the
molecules of air move from the cold side to the hot side past the edge
of
the vane. The sum of these forces causes an imbalance in force on the
two
sides of the vane and it rotates.

Confirmation of this effect can be shown by evacuating more air from
the
bulb. By the time you get to 10^-6 torr the vanes stop moving.


But photons have no mass...

But they do have a lot of energy.

Well whatever magic it uses I just ordered one.


[edit] Explanations for the force on the vanes
Over the years, there have been many attempts to explain how a Crookes
radiometer works:

1.. Crookes incorrectly suggested that the force was due to the
pressure
of light. This theory was originally supported by James Clerk Maxwell who
had predicted this force. This explanation is still often seen in
leaflets
packaged with the device. The first experiment to disprove this theory
was
done by Arthur Schuster in 1876, who observed that there was a force on
the
glass bulb of the Crookes radiometer that was in the opposite direction
to
the rotation of the vanes. This showed that the force turning the vanes
was
generated inside the radiometer. If light pressure was the cause of the
rotation, then the better the vacuum in the bulb, the less air resistance
to
movement, and the faster the vanes should spin. In 1901, with a better
vacuum pump, Pyotr Lebedev showed that in fact, the radiometer only works
when there is low pressure gas in the bulb, and the vanes stay motionless
in
a hard vacuum. Finally, if light pressure were the motive force, the
radiometer would spin in the opposite direction as the photons on the
shiny
side being reflected would deposit more momentum than on the black side
where the photons are absorbed. The actual pressure exerted by light is
far
too small to move these vanes but can be measured with devices such as
the
Nichols radiometer.
2.. Another incorrect theory was that the heat on the dark side was
causing the material to outgas, which pushed the radiometer around. This
was
effectively disproved by both Schuster's and Lebedev's experiments.
3.. A partial explanation is that gas molecules hitting the warmer side
of
the vane will pick up some of the heat i.e. will bounce off the vane with
increased speed. Giving the molecule this extra boost effectively means
that
a minute pressure is exerted on the vane. The imbalance of this effect
between the warmer black side and the cooler silver side means the net
pressure on the vane is equivalent to a push on the black side, and as a
result the vanes spin round with the black side trailing. The problem
with
this idea is that the faster moving molecules produce more force, they
also
do a better job of stopping other molecules from reaching the vane, so
the
force on the vane should be exactly the same - the greater temperature
causes a decrease in local density which results in the same force on
both
sides. Years after this explanation was dismissed, Albert Einstein showed
that the two pressures do not cancel out exactly at the edges of the
vanes
because of the temperature difference there. The force predicted by
Einstein
would be enough to move the vanes, but not fast enough.
4.. The final piece of the puzzle, thermal transpiration, was theorized
by
Osborne Reynolds, but first published by James Clerk Maxwell in the last
paper before his death in 1879. Reynolds found that if a porous plate is
kept hotter on one side than the other, the interactions between gas
molecules and the plates are such that gas will flow through from the
cooler
to the hotter side. The vanes of a typical Crookes radiometer are not
porous, but the space past their edges behave like the pores in
Reynolds's
plate. On average, the gas molecules move from the cold side toward the
hot
side whenever the pressure ratio is less than the square root of the
(absolute) temperature ratio. The pressure difference causes the vane to
move cold (white) side forward.
Both Einstein's and Reynolds's forces appear to cause a Crookes
radiometer
to rotate, although it still isn't clear which one is stronger.

See also: photophoresis.



And yes I know its html but I dont care...

Yes, that's what I meant by:

"at the edges of the vane there are effects where the molecules of air
move
from the cold side to the hot side past the edge of the vane. The sum of
these forces causes an imbalance in force on the two sides of the vane and
it rotates."

Nice Wiki cut and paste BTW.

No it was on the link on the site that I bought it from.

"Burgerman" wrote in message
...

No it was on the link on the site that I bought it from.

If you don't mind me asking. How much and where?
All the UK suppliers seem very expensive, considering they can be baught for
around $10 in the states.
Mike.

"Tunku" wrote in message
. 109.145...
"Burgerman" wrote in
:

Both Einstein's and Reynolds's forces appear to cause a Crookes
radiometer to rotate, although it still isn't clear which one is
stronger.


It's still magic to me - just like that stirling engine thing. Wish I
could
get one. "OI !!! SANTA !!!" :-)

Just won one on eBay. £10.06 inc airmail delivery from Australia.
Mike..