| Author | Message | ||
     Jlnance |
Have you ever seen a radiometer? It's an evacuated glass bulb containing a needle on which sits a finned ring. The finns are painted black on one side and white on the other. If you shine light on the fins, the ring spins on the needle. So here is what I am confused about. Angular momentum is conserved. So if the ring spins in one direction, something else must spin in the opposite direction. Whats spinning the other way and where are the forces making it spin comming from? | ||
| Fran_dog |
Have fun. http://science.howstuffworks.com/question239.htm | ||
| Prior |
From How Stuff Works: A Crookes' radiometer has four vanes suspended inside a glass bulb as you've described. Inside the bulb, there is a good vacuum. When you shine a light on the vanes in the radiometer, they spin -- in bright sunlight, they can spin at several thousand rotations per minute! The vacuum is important to the radiometer's success. If there is no vacuum (that is, if the bulb is full of air), the vanes do not spin because there is too much drag. If there is a near-perfect vacuum, the vanes do not spin unless they are held in a frictionless way. If the vanes have a frictionless support and the vacuum is complete, then photons bouncing off the silver side of the vanes push the vanes, causing them to rotate. However, this force is exceedingly small. If there is a good but incomplete vacuum, then a different effect called thermal transpiration occurs along the edges of the vanes, as described on this page. The effect looks as though the light is pushing against the black faces. The black side of the vane moves away from the light. (Message edited by awprior on March 23, 2006) | ||
| Ftd |
Interesting question as I have one on my window sill at work and I am watching it spin as I type. Left over prop from when I used to teach Physical Science. Frank Prior nailed the answer. | ||
| Jlnance |
So whats spinning the other way? | ||
| Fran_dog |
It's not that something spins the other direction. The photons bounce off of the vanes in the opposite direction that the vane being affected by the photons is pushed. The vane spins because it is on an axis. The linear action is turned in to rotation by the axis that the vanes are to mounted in the glass bulb. | ||
| Jlnance |
Fran - Yes, I think you're right. It's the axis that makes it spin, and that also answers my qustion. To spin something in one direction requires something else to spin in the opposite direction. I couldn't see how the force to do that got generated. But it comes down the axis. | ||
| Panic |
What "spins the other way" when the wheels on your bike rotate? | ||
| Jlnance |
Panic - The earth. Really. There is a principle in physics called "conservation of angular momentum," which says there is a contant amount of spin in the universe. It can't change. So if you make something spin one way, something else has to spin the other way. Since the earth is heavy it doesn't have to spin very fast to compensate for your bike wheels turning, so you don't notice it. | ||
| M1combat |
Alright... June 15th... Everyone with more than 75 HP get out there and at 7:00AM GMT point your bike/car/truck as East as you can get it and launch... Sorry... Back to your regularly scheduled programming  . Wait... make that west... I'm trying to lose a few pounds for better acceleration.  , I know... (Message edited by M1Combat on March 24, 2006) |
