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Author Topic: How does a solar radiometer work?  (Read 18592 times)

Offline chris

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How does a solar radiometer work?
« on: 22/06/2008 23:25:41 »
These are the pretty devices resembling a lightbulb, inside of which are a series of diamond-shaped "sails" arranged in a carousel around a central hub which is balanced delicately on the tip of a needle. When the sun shines on the device the carousel assembly turns...how?

Chris


 

Offline RD

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How does a solar radiometer work?
« Reply #1 on: 23/06/2008 14:23:38 »
.
Quote
The Crookes radiometer, also known as the light mill, consists of an airtight glass bulb, containing a partial vacuum. Inside are a set of vanes which are mounted on a spindle. The vanes rotate when exposed to light, with faster rotation for more intense light, providing a quantitative measurement of electromagnetic radiation intensity. The reason for the rotation has been the cause of much scientific debate.
http://en.wikipedia.org/wiki/Crookes_radiometer

There is also the YORP effect where sunlight causes irregular asteroids to rotate, but the mechanism is different from the Crookes radiometer. The asteroid is in a vacuum, the radiometer is a partial vacuum. The motion of the air molecules in the radiometer from the hotter (black) to the colder (silver) surfaces of the vanes is essential for the rotation. If all the air is extracted from the bulb (hard vacuum) the rotation stops, there is still a rotational force but not sufficient to overcome friction. In space there is no friction to prevent an irregular asteroid from turning, (see YORP effect).
http://en.wikipedia.org/wiki/YORP_effect
« Last Edit: 23/06/2008 17:37:54 by RD »
 

Offline JP

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How does a solar radiometer work?
« Reply #2 on: 23/06/2008 18:29:25 »
To summarize the wikipedia article, the vanes have one black side and one white/polished side.  The black side gets hotter as its exposed to light, since it absorbs more light.  Then two factors come into play:

1) Molecules hitting the black side get heated up slightly and bounce off with higher energy.  This causes the vanes to recoil in the opposite direction (it turns out that the edges of the vanes contribute to this).
2) There's an airflow from cold-to-hot around the edges of the vanes.  This causes the pressure to drop on the cold side, which makes the vanes turn.

The wrong description that's often given is that the radiation pressure of the light causes the vanes to turn.  While light does exert a pressure, it's too tiny to cause the effects seen in the Crookes radiometer.
 

Offline rosy

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How does a solar radiometer work?
« Reply #3 on: 23/06/2008 22:24:26 »
And wouldn't radiation pressure push harder on the shiny (reflecting) sides where the momentum change is twice the momentum of each photon, than on the black (light absorbing) side on which the momentum transfer is only from converting the photon into excitation energy of some sort in the vane?
 

Online syhprum

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How does a solar radiometer work?
« Reply #4 on: 23/06/2008 22:33:48 »
This device is of course a heat engine of sorts, has any measurement been made of the thermal efficiency, pretty low I guess
 

Offline JP

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How does a solar radiometer work?
« Reply #5 on: 23/06/2008 22:38:27 »
And wouldn't radiation pressure push harder on the shiny (reflecting) sides where the momentum change is twice the momentum of each photon, than on the black (light absorbing) side on which the momentum transfer is only from converting the photon into excitation energy of some sort in the vane?

Yes.  So in reality it also turns the wrong way for radiation pressure to be the culprit.
 

lyner

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How does a solar radiometer work?
« Reply #6 on: 24/06/2008 19:41:15 »
I worked out the force (very approx) due to radiation pressure on a 1cm sq vane in direct sunlight.
It seems it's about 10e-7Newtons (0.1 microNewtons). Not likely to overcome the friction in even the best needle bearing.
But that force can be used, on a huge 'solar sail' to produce quite a few Newtons and it's there 24/7. (at least whilst near the Earth's orbit). It could accelerate a light probe to some pretty high speeds, eventually.
 

Offline chris

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How does a solar radiometer work?
« Reply #7 on: 24/06/2008 22:47:23 »
I worked out the force (very approx) due to radiation pressure on a 1cm sq vane in direct sunlight.
It seems it's about 10e-7Newtons (0.1 microNewtons). Not likely to overcome the friction in even the best needle bearing.
But that force can be used, on a huge 'solar sail' to produce quite a few Newtons and it's there 24/7. (at least whilst near the Earth's orbit). It could accelerate a light probe to some pretty high speeds, eventually.

Can you publish the calculation please SC? I thought the first rule of answering any question was "always show your working"! Tch Tch. What would your students say...!
 

lyner

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How does a solar radiometer work?
« Reply #8 on: 24/06/2008 23:46:42 »
If you insist.
For the photons absorbed by the black surface.
Momentum of a 600nm light photon is h/λ  (1.1e-27 Ns) My ' average value'.
Power received on 1cm sq is 10e-4kW = 10e-1 W (Spotted a mistake already  grrrr.)
Energy of a photon = hc/λ = 3.3e-20J
Number of photons falling in one second = power/(energy per photon) = 3e18
The force is the change in momentum per second =
number of photons per second times photon momentum = 3.3e-9N
or 3.3 nN. (1/100th of my original so even less than I thought)
For the reflective surface the value is twice this so the difference is equal to the above.
Someone check please- but I think it's right this time.

To get 1N of force, you'd need an area of 300m sq - a 17m sided square.
But 1N on a 100kg mass, would increase the speed by 1m/s/s every 100 seconds. A 100kg spacecraft powered by a sail that big would increase its speed by 860m/s every day - that's 300km/s after a year. You could easily make a much bigger solar sail and get things moving even quicker. Of course, as you get further away from the Sun, the force would drop off (inverse square law).
 

Offline Bored chemist

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How does a solar radiometer work?
« Reply #9 on: 25/06/2008 21:48:19 »
OK sunlight couldn't drive one of these by radiation pressure against the friction of a good bearing. What power laser do I need to steal to demonstrate radiation pressure with an evacuated version? How about seeing if the microbalance at work could detect the force? It will resolve down to 100ng.
 

lyner

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How does a solar radiometer work?
« Reply #10 on: 25/06/2008 22:27:05 »
Piece of cake with a laser - you could get a lot more than 0.1W on a cm sq. >200W is the sort of power I've seen quoted.
So, as your 100ng (mass) would weigh just 1nN, you could blast your microbalance out of sight!
 

Offline Bored chemist

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How does a solar radiometer work?
« Reply #11 on: 26/06/2008 21:12:23 »
Does that mean the balance should be able to "weigh" a sunbeam?
 

Offline chris

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How does a solar radiometer work?
« Reply #12 on: 17/12/2008 20:53:29 »
Have you tried this BC? What power is a laser-pointer?

C
 

lyner

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How does a solar radiometer work?
« Reply #13 on: 17/12/2008 22:49:21 »
Piddle power, I'm afraid.
Just as well - for your eyes' sakes.
 

Offline RD

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How does a solar radiometer work?
« Reply #14 on: 17/12/2008 22:59:33 »
I've seen a video on youtube of a large precariously balanced ballbearing apparently being moved by a laser beam,
 or rather being moved by the current of air created by the heat from laser.

Unfortunately I cannot find the video clip : there are thousands of "laser" videos on youtube.
It was a green laser, I suspect the powerful ~100mW type.
« Last Edit: 17/12/2008 23:07:48 by RD »
 

lyner

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How does a solar radiometer work?
« Reply #15 on: 20/12/2008 17:11:46 »
Do you mean MW???
 

Offline RD

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How does a solar radiometer work?
« Reply #16 on: 20/12/2008 19:41:54 »
~ 100 milliWatts like this one ...  http://uk.youtube.com/watch?v=CeMpdiDZ1iI    (typical laser pointers are < 5mW)

There have been incidents of individuals attempting to blind pilots with these powerful laser pointers ...
http://www.abc.net.au/news/stories/2008/04/08/2211257.htm


In the Youtube video where a ballbearing was apparently moved by the laser, the ballbearing was balanced on the edge of a bowl:
 the the slightest breath of wind would have caused it to fall into the bowl. [I still can't find the video]. 
« Last Edit: 20/12/2008 20:15:06 by RD »
 

lyner

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How does a solar radiometer work?
« Reply #17 on: 20/12/2008 20:58:50 »
Not radiation pressure then. Heating can do a lot to move light objects, tho'.
 

Offline Syphered

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How does a solar radiometer work?
« Reply #18 on: 26/01/2009 02:50:36 »
Has anyone considered using solar cells for the dark or light blades?
 

Offline Madidus_Scientia

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How does a solar radiometer work?
« Reply #19 on: 26/01/2009 03:25:29 »
Why?
 

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How does a solar radiometer work?
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