Wednesday, November 08, 2006

Efston Science needs to learn some science

Efston Science, a large retailer of microscopes, telescopes, and other scientific gadgets in Toronto, had this ad attached in the latest issue of SkyNews which contains numerous errors.

The ad is for a device called a Radiometer. I've had one since I was a kid. It's pretty simple. Put it in sunlight and it spins. The glass case protects it from wind, and the sunlight shines on the vanes, powering it. The question is, how?

Efston's ad gets it wrong, two different ways. One, light is not made of atoms, it's made of photons. Two, light does not have a pressure that forces the vanes. (Sunlight (or any light) does have a pressure, but it is not strong enough to have any effect once it has passed through Earth's atmosphere.

So, how does it work? Wikipedia has a good explanation involving some of history's great physicists. Theory 2 is the one I've known since I was a kid. Sunlight strikes the sides of the vanes. The black side absorbs more heat than the white side, so it heats faster. Air currents flowing around the vane's edges push it along.

Ultimately, it comes down to two forces:

1. A partial explanation is that gas molecules hitting the warmer side of the vane will pick up some of the heat i.e. will bounce off the vane with increased speed. Giving the molecule this extra boost effectively means that a minute pressure is exerted on the vane. The imbalance of this effect between the warmer black side and the cooler silver side means the net pressure on the vane is equivalent to a push on the black side, and as a result the vanes spin round with the black side trailing. The problem with this idea is that the faster moving molecules produce more force, they also do a better job of stopping other molecules from reaching the vane, so the force on the vane should be exactly the same — the greater temperature causes a decrease in local density which results in the same force on both sides. Years after this explanation was dismissed, Albert Einstein showed that the two pressures do not cancel out exactly at the edges of the vanes because of the temperature difference there. The force predicted by Einstein would be enough to move the vanes, but not fast enough.

2. The final piece of the puzzle, thermal transpiration, was theorized by Osborne Reynolds, but first published by James Clerk Maxwell in the last paper before his death in 1879. Reynolds found that if a porous plate is kept hotter on one side than the other, the interactions between gas molecules and the plates are such that gas will flow through from the cooler to the hotter side. The vanes of a typical Crookes radiometer are not porous, but the space past their edges behave like the pores in Reynolds's plate. On average, the gas molecules move from the cold side toward the hot side whenever the pressure ratio is less than the square root of the (absolute) temperature ratio. The pressure difference causes the vane to move cold (white) side forward.

Strangely, scientists still aren't sure which is providing more force.