Crookes's radiometer: Difference between revisions
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(Created page with "<!-- Crooke's Radiometer is a near-vacuum with a low-friction rotary thing in it, with sides to it it that are black on one side, metaly on the other. When light hits these,...") |
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Anti-intellectualism says "science just doesn't know how this works", | |||
which is an example of if you repeat it enough, it ''still ain't bloody true''. | |||
This is not so much a trick question, | |||
but it ''is'' more complex than it looks at first, | |||
in that the correct answer | |||
relies only partly on figuring out what kinds of physcal effects are involved, | |||
but also on figuring which one has the most effect. | |||
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you may figure that said bouncing would impart more force and so the white moves away from the light. | you may figure that said bouncing would impart more force and so the white moves away from the light. | ||
Yes | Yes. But also that's negligible. | ||
It turns out that the black surface being warmer | |||
Is it a black body radiator? | |||
Well, both are, really; the black is irrelevant to what is ''emitted''. | |||
But also the radiation is negligible. | |||
It turns out that the black surface being warmer imparts more energy to air coming in physical contact - that air will bounce and move away with slightly higher speed. | |||
And because of equal-and-opposite-reaction (conservation of momentum), that means slightly force on the black surface. | |||
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It won't work in a ''full'' vacuum either, because there is no more air to move away. | It won't work in a ''full'' vacuum either, because there is no more air to move away. | ||
There is in fact a relatively narrow window where this even works noticeably at all. | |||
In that sense, this ''is'' a bit of a trick question. | |||