Conference hydra::amiga_v1

> those whatchamacallits that look like a light bulb with a weathervane
> inside.    

Radiometers. The vanes rotate because the black side gets hotter than the
white side, so the gas molecules bounce off the black side faster than
off the white side, providing a simple "jet propulsion". The black side
gets hotter because it absorbs more light, which degrades to infrared on
the surface.

If I could only use this scientific trivia to figure out what's wrong
with my disk drive...

John O.

>    inside.  The vanes are black on one side and white on the other.
>    In the presence of light, or due to some other effect that I used
>    to know about 15 years ago in Physics 1010, the vanes will rotate.


I don't know the name of the effect, but the reason is because light
can be treated as a particle.

A black vane will absorb light while a white one will reflect it. Now you
would think the black one would move away from the light source since it
is in effect gaining something in the "impact". But if you calculate the
total momentum imparted in the collision, the reflection (white) surface
actually ends up with more inertial change.

Think of it as the white is hard to the light source and it bounces off
(reflection), while the black is soft to the light source (absorbes) and
can move into it.

John

    The interesting thing about these gizmos is that there has to be
    a partial vacuum within the bulb.  Otherwise the air resistance would
    be so high that the vanes couldn't move.  There has to be some gas
    within the bulb, although, for the hotter (black) side of the vanes
    to heat the gas molecules for the 'push' in one direction.  It is
    strange to see the things turn around.... sort of like perpetual
    motion....  almost like magic.

re: .2, .3

Sorry, John, but light particles (photons) have no mass, the vanes are
really driven by differential heating in a partial vacuum. The radiometer
steals a tiny amount of energy from the environment. Where's the energy
missing? It's the SHADOW of the vanes.

John O.

    re: .4
    
    I'm sure you're right about the differential heating, etc.  But
    I believe photons do have some mass.  I vaguely remember calculating
    the momentum of light back in college.  I always thought that
    proposed SF light "sails" were extrapolated from a real principle.
    
    Sorry, this probably shouldn't be in the Amiga conf...
    
    - steve

    Wait a minute... notes .2 and .3 seem to disagree on the principle
    involved here as well as the direction of spin.  If the light heats
    the black side and makes gas molecles bounce off of it, (thus pushing),
    then the vanes should move 'toward' the shiny side.  If the other
    reply is true and the motion is caused by the photons bouncing off
    of the shiny side, then the vanes should move toward the black side.
    
    Which is it?  (My curiosity's been piqued now.   Either somebody's
    gotta tell me or I gotta dig out my old Physics textbooks.  :-)
    
    
    					John

Re. .6

I was about to make the same point. I'd like to know which it is also. Which way
does the animation spin? 

While we're at it, how about somebody generating an animation of the "bobbing
bird" drinking a glass of water. I once heard a story that, to his dying day,
Einstein commented that he never could figure out how those things worked. 

Dave

re: .6

>    Which is it?    

*** pompous and bombastic on ***

As Indiana Jones says in "Raiders of the Lost Ark", trust me.
The vanes in the radiometer spin with the dark side behind. A friend of
mine has one in the office- we took it to the window, and that's what it
did. (Like me, it tends to grind to a halt under fluorescent light.)

I don't believe light has pressure- photons have a "rest mass" of 0.
Nothing which has any mass can go the speed of light, and they do. Solar
sails would work because the solar wind contains other particles which
do have mass and don't go quite as fast. 

*** pompous and bombastic off ***

John O.

    
    Re: .4
    
    >Sorry, John, but light particles (photons) have no mass, ...
    
    My physics book (Introduction to Physics for Scientists and Engineers,
    claims that photons DO have mass, not at rest, but when in motion.
    The mass is dependant upon the velocity, which is always c.
    
    Ed.

re: .9
> photons DO have mass, not at rest, but when in motion.

Yep, stupid of me... light bends in gravity (hence black holes...), and
gravity only works on things with mass. Solar sails work with light pressure.
Well, at least I got a chance to go off on a boring diatribe, and I REALLY
think the earth is flat...

John O.

>> photons DO have mass, not at rest, but when in motion.


Precisely. And that mass is produced from it's motion at the speed of light.

The momentum of the system before and after light particle impact
is:
		mv = mv1 - mv2

where v1 is the particle velocity before, and v2 is velocity after.

Look at the momentum of the system in the black side:

	photon impacts the black side and absorbs into it. Total momentum
	is that of the particle:

		mv = mv1 - mv2 (where v2=0) so mv = mv1

On the reflecting side,

	photon impacts and reflects:

		mv =  mv1 - mv2 (where v2 ~= -v1) so
		mv =  mv1 - (-mv1) = 2mv1

Since 2mv1 (light side) > mv1 (dark side), the light side moves away.

John

    
    The current issue of Discover magazine has an interesting article
    about Tachyons, mysterious particles that DO travel faster than
    light.  They also have mass, so how can they coexist within Einstein's
    world?
    
    Simple.  (Right).  Einstein said that no particle with mass MAY
    REACH THE SPEED OF LIGHT.  Tachyon physicists have no problem with
    that.  They just say that Tachyons are born travelling faster than
    light.
    
    Don't you just love physicists?
    
    Ed.
    

	re: Tachyons

	The notion of "always faster than light" particles has been around
	for about 15 or 20 years.  Like Superstrings, tachyons have one
	major problem: there's never even been a HINT of any kind of
	experimental verification...

	Geo Bernard Shaw:  If 30 million people say a silly thing, it's still
	a silly thing.

Well, I just consulted a physics book. Basically what I said is correct theory
and John Osborne had the practical results of reality explained.

To recap (And end this digression!):

	Momentum imparted from light partical impact would cause
	the vane to rotate, black leading white trailing under Ideal
	conditions (most notably, no air in the glass). The effect
	is known as "radiation pressure".

	Air in the glass and the uneven heating between the white
	and black surfaces causes directed turbulence to force the
	vane to rotate in the other direction (i.e. white leading,
	black trailing) since the hot black surface produces high energy
	air molocules which impart thier own momentum on the surface.

John

And if we're not carefull, Ed will never post another neat demo like this
again!

    
    By the way, has anyone bothered to look at this thing?
    
    Ed.

Evaporation is a cooling process.

Start with an upright bird that has a wet beak.  As the water on the
beak evaporates, it cools, and also cools the gas chamber inside, which
(Boyles law) lowers it's pressure, which provids the impetus to slurp
up some mass (liquid) from the base of the bird up towards the head,
which has the effect of eventually moving the center of mass to the
beak side of the bird's fulcrum, which causes the bird to tilt until
it's beak hits the water.  Capilary action in the fabric surrounding
the beak pulls water up from the glass, warming the gas chamber, which
causes the gas to expand until it pushes some mass back into the base,
sending the center of gravity back across the fulcrum, and up goes the
head for a new round of evaporation.

I've seen this bird ray-traced at ACM SigGraph, but I don't recall who
did it.

	Oh boy! I just ordered one of those birds from Edmund Scientific,
	now I'll know what's *really* going on. (My goal is one line of
	C code every time the bird drinks...  :-)

< Note 2292.17 by PRNSYS::LOMICKAJ "Jeff Lomicka" >
             -< How does that bird work anyway?  My speculation: >-

<	the beak pulls water up from the glass, warming the gas chamber, which
<	causes the gas to expand until it pushes some mass back into the base,
<	sending the center of gravity back across the fulcrum, and up goes the
<	head for a new round of evaporation.

I don't think it's the warming effect of the fresh water on the beak that sends
the fluid back down the bird's neck. When the neck is horizontal, gas from the
"stomach" can flow up the neck, allowing the fluid to flow back down to replace
it. That gas had been warmed by the ambient temperature of the fluid, etc. in
the environment away from the head (which is cooling by evaporation). Once the
head is upright again, the cooling/contraction cycle starts again. The bottom
line is that the device is a thermal engine which depends on a difference in
temperature between the head and the "tail". 

Dave

    The cooling beak makes a partial vacuum, thus pulling fluid from
    the belly, the rising fluid raises the center of gravity of the
    bird so that it tips and so doing does an inside burp returning
    the fluid to the belly, returning the center of gravity to what
    it was while it was upright so the bird returns to its former 
    position.....
    
    Jean
    

    Re:.16
    
    Yes Ed, I looked at it... Great demo.
    
    
    ...Ed