Okay, so I’m bored outta’ my skull waiting for my computer to render an image, and in a fit of listlessness I shoot the radiometer next to my desk…
http://www.creationscience.com/onlinebook/images/radiometer.gif
… with supercooled liquid (a can of compressed gas dust remover turned upside-down). To my surprise it starts spinning wildly! WTF!? Any amateur physicists want to try to explain that?
Your 1,1,1,2-Tetrafluoroethane cooled one side of the radiometer, which created an imbalance in the radiation hitting the vanes. Essentially, a radiometer at ambient temperature is bombarded by IR from the glass on all sides, but you cooled off one side so that it no longer emitted IR.
Either that, or you have a can full of neutrinos and your radiometer vanes (as well as the can) are made out of something that is neutrino-opaque.
I’m leaning toward the first explanation.
Basically what Wikkit said, but it’s more complicated.
It’s important to note that the so-called radiometer does not spin because of the photonic pressure on it, as it’s normally explained.[sup]*[/sup]
If it did, it would spin counter-clockwise (if seen from above),as the reflected photons on the shiny side deliver more momentum. All the ones that I have seen spin clockwise, which is most easily explained by the gas closest to the dark vanes heat up more than on the other side, this higher temperature means that the gas pushes the black vanes more than the cooler gas on the other (shiny) side. (What direction does your thing move, Inky-?)
So - in theory any temperature difference will make it move.
[sup]*[/sup]Or at least that’s true for all the (two) things like that that I have seen…
In addition to what Wikkit and Popup said, I’ll note that I’ve been told that if you put a radiometer in the freezer for a while, then bring it out and use it it will spin in the “right” direction (the direction Crookes thought it should – white going away from the light source).
Incidentally, photon pressure is still as big issue. Since they successfully obserrved real light pressure back in the thirties or so, there have been a lot of papers on this stuff. I noted this week that Optics Index has an entire section on “Mechanical Effects of Light”.
I am more inclined towards the temperature gradient explanation. Cooling parts of the glass surface causes such a gradient, causing it to spin.
The fault in this is that the radiometers are vacuum filled (?) not filled with vacuum? not filled with gas?.
Anyway - they’re empty.
Actually, they’re not empty. If they were, they wouldn’t spin. It is the residual gas that was left behind during the first build that caused all the confusion. The effect of the photon momentum hitting black vs white is greatly overpowered by the black vanes heating the gas. These days, the residual gas is left in on purpose.
Right, they’re NOT empty. I once cracked the seal on a radiometer, then stuck in under a bell jar. After pumping for an hour with an old vacuum pump, the radiometer just started responding to IR again. I expect that the vacuum was down to a few millimeters of mercury. A proper radiometer must be pumped down even more. (I think the mean free path of the gas molecules has to be many mm long, otherwise the “hot” molecules would bounce back and hit the vanes again.)
Yes, if you put your radiometer in the freezer, it will spin backwards for awhile. And when you take it out after cooling it down, it will “see” the walls as being very hot, and will spin “forwards” until it warms up. Leave it in very bright sunlight so it spins forwards, then put it in cool shadow and it will spin backwards until the vanes cool down again. Show your radiometer a hunk of metal that has been heated to a few hundred degrees, and it will spin forwards. Show it a hunk of metal that was cooled in liquid nitrogen, and it will start spinning, only backwards.
…But if it’s the second, could I borrow those for a little while? I promise to give them back, really! 