Son's science experiment

I used to have one - they were called something-ometers.

ahah - found it:

http://en.wikipedia.org/wiki/Crookes_radiometer

Black and white colored triangles tilted and set up on a pinwheel type of afair i believe.

Yes: I do remember seeing such a device. It is a vertical spindle with a sharp pivot at the lower end to reduce friction torque carrying a number of radial vanes with their leading surfaces silvered and back surfaces blackened; all enclosed in a transparent evacuated glass bulb. The spindle rotates when exposed to sun light. Then you have the problem of explaining why the spindle should rotate!!

Check Wikipedia for radiometers.

I'm sorry, here's the link you're looking for:

http://en.wikipedia.org/wiki/Crookes_radiometer

yep all teh below are right.

They thing turns because the light hits one side (white side) and bounces off, the other side it is absorbed (black side) teh implalance causes teh turning effect, Oh no need for a vacuum

I disagree. If the glass is full of gas, the air molecules will bounce off each side and prevent it from turning. The effect of sunlight will then be negligible compared to this effect.

You are close but not quite there yet. You definately do not want a vacuum. The turning is caused by the gas. As the light hits the vanes they are heated un-equally. The dark side absorbs more radiation and gets slightly hotter. This heats the air adjacent to the dark side increasing the speed of the molecules which impact the dark side. This imballance in the speed of the impacting molecules is what caused the torque. No gas = No impacting molecules = No torque. I am sure that google will turn up a couple thousand sites which explain it far better than I have, maybe with pictures.

This corresponds to what I remember. The vanes spin with the black side moving away from the light and the reflective side toward the light. If it was due to the momentum of photons hitting the vanes it would spin in just the opposite direction as more momentum is imparted when an object bounces off than when it sticks.

I recall a video of an experiment where they did demonstrate that light photons have momentum. In this experiment they hung a very reflective metal foil from a very thin ribbon. This was encased in a glass container under full vacuum. Imagine the foil bisected by the vertical axis of the ribbon. They aimed a powerful beam of light at the foil on one side of the axis of the ribbon. By alternating the beam on the one side and then off, they were able to get the foil to rotate back and forth slightly about the axis of the supporting ribbon.

"The vanes spin with the black side moving away from the light and the reflective side toward the light. If it was due to the momentum of photons hitting the vanes it would spin in just the opposite direction"

Correct. And the explanation relying solely on photon momentum is also refuted by the observation that a working Crooke's radiometer fails to turn at all when the pressure inside is reduced further. This proves that gas molecules inside the radiometer play a crucial role.

I understood the explanation to be that the gas pressure is lower at black surfaces of the vanes due to their higher temperature. Since the pressure is higher at the white sides of the vanes, a net pressure difference creates thrust against the white sides, causing the vanes to spin with the black side leading.

http://www.howstuffworks.com/framed.htm?parent=question239.htm&url=http://math.ucr.edu/home/baez/physics/General/LightMill/light-mill.html

Darn, I just looked at my radiometer and realized that the last paragraph of my previous post is wrong. I gave the correct explanation in so far as ruling out photon momentum as the sole cause of spinning (a typical Crooke's radiometer does stop spinning if too much gas is sucked out of it).

But I erred in explaining the direction in which it turns. The vanes spin with the *white* sides leading, not the black sides. In other words, the vanes spin as if a net thrust pushed against the black sides. So my previous explanation based on a "net pressure against the white sides" is obviously wrong. This is a tricky device to explain correctly -- should be especially challenging for a kid to get it right. I trust the detailed explanation at the link (in my previous message), but it gets fairly technical.

They are available from Edmund's Scientific.

http://scientificsonline.com/search.asp?t=ss&ss=Radiometers&x=12&y=9

A radiometer. I had one when I was a kid. I thought I could make it go faster by focusing sunlight with a magnifying glass. Big mistake. There was a little puff of smoke and it never worked so well after that.

It has a fair (not perfect) vacuum inside to make it work. The few gas molecules rebound harder off the black (warmer) side.

Interestingly enough, I recall that I held the thing under cold water and it actually rotated backwards. I'm not sure why that is. I may have to buy another one and try that again.

Black not only absorbs heat better, it also radiates heat better as well. By holding your radiometer under cold water, the black sides of the vanes are discharging heat energy better than the silver sides are. Thus, the same uneven heating (or in this case, cooling) effect is taking place, resulting in air molecules on the warmer side (in this case, the silver sides) exerting greater force on the vanes than on the cooler side.

Regarding Crooke's Radiometer, please also see the links listed on this page:

http://www.borderschess.org/energy.htm#radiometer

Some free advice; worth what you paid: To take this into a lesson of problem solving and how to research / prove a hypothesis . . . . . when you buy the radiometer, and watch it work, he must research why it works (and not stop there with a report copied from a web site), but then try to prove his theory with things he makes with his two hands. Failing with proving the theory with a home made version isn't failing as long as he can predict his success, and explain his failure (which may prove his theory anyway). Failing is the best teacher, but what you are teaching is 'the process', not to be successful. If he explain how the store bought one works to his mom or neighbor, and explain why his home made version didn't work (in case it didn't), without referring to his notes or 'reading' from a script, then you have reached the goal. And he doesn't need to 'spin' something to prove it . . . . just move something 1 mm by the same process. Something suspended by a human hair ?

And take a video of you two working on this, about 15 minutes long each in several steps. Just the simple stuff, not only the finale. You will treasure these simple videos of you two when he is older.

My son recently passed away, but I have many videos of us just coloring together, making paper airplanes, making a 'deen-o-soar' with clay, making science projects (like yours) . . . just simple videos from 3 YO to about 10 YO. So you see, his memory is alive though his body is gone.

Remember, this is worth what you paid and I hope I didn't offend you with obvious stuff.

Enjoy and thanks for being a good dad !

"My son recently passed away, but I have many videos of us just coloring together, making paper airplanes, making a 'deen-o-soar' with clay, making science projects (like yours) . . . just simple videos from 3 YO to about 10 YO. So you see, his memory is alive though his body is gone."

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I'm really sorry to hear that, PetroPower. I nearly lost my four-year-old to a massive brain hemorrhage after he fell from the bleachers at an ice-skating rink in Houston. The surgeon didn't expect him to live. While he was in a coma, I finally let him go and gave him to the Lord. Within the hour he woke up and asked for his shoes. The surgeon was thunderstruck and confided later that, until that moment, he'd never believed in miracles. Interestingly, like Abraham's son, my son's name is also Isaac.

Thanks Euro. Your deal was more rough than mine I imagine, but with a blessed outcome. I'll chat privately with you so we don't take away from the intent of this original post (science project).

PetroPower writes: "Failing with proving the theory with a home made version isn't failing as long as he can predict his success, and explain his failure (which may prove his theory anyway). Failing is the best teacher, but what you are teaching is 'the process', not to be successful. If he explain how the store bought one works to his mom or neighbor, and explain why his home made version didn't work (in case it didn't), without referring to his notes or 'reading' from a script, then you have reached the goal. And he doesn't need to 'spin' something to prove it..."

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It took me a awhile as child to understand that my experiments didn't "succeed" or "fail," but that I was, in fact, defining success and failure by the wrong measure. The real objective was not to see it work, but to learn something I hadn't known before. When I saw my first radiometer (in a store window in Lewiston, Idaho) I was stunned to see that the vanes rotated with the white sides leading. I expected the opposite, as I had read that light carries momentum. I really had to do some digging (the Internet was still very far in the future) to discover what it was that actually imparted momentum to the vanes. As the thing was in the shape of a light bulb - and you could see the pinched-off stem where they had evacuated the bulb - I had assumed the bulb was completely evacuated, increasing my puzzlement. (Btw, light bulbs are not completely evacuated, but contain a small amount of inert gas to increase the life of the bulb by reducing the rate at which the filament boils away.) So it took a bit of research on my part to figure out what the heck was going on. Success was in finding out what really was going on, not that the vanes turned. There was no failure, because I had learned something new. For me it was one of those epiphanies.