![\"Radiometer](\"\/physics\/wp-content\/uploads\/sites\/2\/2015\/06\/radiometer.jpg\")
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–Explanations for\u00a0the force on the vanes–<\/span><\/p>\n <\/p>\n Over the years, there have been many attempts <\/p>\n 1. Crookes incorrectly suggested that the force was due to the pressure of light. <\/p>\n 2. Another incorrect theory was that the heat on the dark side was causing the <\/p>\n 3. A partial explanation is that gas molecules hitting the warmer side of the <\/p>\n 4. The final piece of the puzzle, thermal transpiration, was theorized by Osborne Both Einstein’s and Reynolds’s forces appear to cause a Crookes radiometer to <\/p>\n <\/p>\n <\/p>\n <\/p>\n","protected":false},"excerpt":{"rendered":" Temperature difference at edge of metal vanes causes rotor to spin. Located in L02, section C3 Below explanation taken from Wikipedia (so it must be true). –Explanations for\u00a0the force on the vanes– Over the years, there have been many attempts to explain how a Crookes radiometer works: 1. Crookes incorrectly suggested … Continue reading Radiometer<\/span><\/a><\/p>\n","protected":false},"author":4,"featured_media":4794,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[12],"tags":[412],"_links":{"self":[{"href":"https:\/\/demos.swarthmore.edu\/physics\/wp-json\/wp\/v2\/posts\/3696"}],"collection":[{"href":"https:\/\/demos.swarthmore.edu\/physics\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/demos.swarthmore.edu\/physics\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/demos.swarthmore.edu\/physics\/wp-json\/wp\/v2\/users\/4"}],"replies":[{"embeddable":true,"href":"https:\/\/demos.swarthmore.edu\/physics\/wp-json\/wp\/v2\/comments?post=3696"}],"version-history":[{"count":1,"href":"https:\/\/demos.swarthmore.edu\/physics\/wp-json\/wp\/v2\/posts\/3696\/revisions"}],"predecessor-version":[{"id":5455,"href":"https:\/\/demos.swarthmore.edu\/physics\/wp-json\/wp\/v2\/posts\/3696\/revisions\/5455"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/demos.swarthmore.edu\/physics\/wp-json\/wp\/v2\/media\/4794"}],"wp:attachment":[{"href":"https:\/\/demos.swarthmore.edu\/physics\/wp-json\/wp\/v2\/media?parent=3696"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/demos.swarthmore.edu\/physics\/wp-json\/wp\/v2\/categories?post=3696"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/demos.swarthmore.edu\/physics\/wp-json\/wp\/v2\/tags?post=3696"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}
\nto explain how a Crookes radiometer works:<\/span><\/p>\n
\nThis theory was originally supported by James Clerk Maxwell who had predicted
\nthis force. This explanation is still often seen in leaflets packaged with the
\ndevice. The first experiment to disprove this theory was done by Arthur Schuster
\nin 1876, who observed that there was a force on the glass bulb of the Crookes
\nradiometer that was in the opposite direction to the rotation of the vanes.
\nThis showed that the force turning the vanes was generated inside the radiometer.
\nIf light pressure was the cause of the rotation, then the better the vacuum
\nin the bulb, the less air resistance to movement, and the faster the vanes should
\nspin. In 1901, with a better vacuum pump, Pyotr Lebedev showed that in fact,
\nthe radiometer only works when there is low pressure gas in the bulb, and the
\nvanes stay motionless in a hard vacuum. Finally, if light pressure were the
\nmotive force, the radiometer would spin in the opposite direction as the photons
\non the shiny side being reflected would deposit more momentum than on the black
\nside where the photons are absorbed. The actual pressure exerted by light is
\nfar too small to move these vanes but can be measured with devices such as the
\nNichols radiometer.<\/p>\n
\nmaterial to outgas, which pushed the radiometer around. This was effectively
\ndisproved by both Schuster’s and Lebedev’s experiments.<\/p>\n
\nvane will pick up some of the heat, bouncing off the vane with increased speed.
\nGiving the molecule this extra boost effectively means that a minute pressure
\nis exerted on the vane. The imbalance of this effect between the warmer black
\nside and the cooler silver side means the net pressure on the vane is equivalent
\nto a push on the black side, and as a result the vanes spin round with the black
\nside trailing. The problem with this idea is that while the faster moving molecules
\nproduce more force, they also do a better job of stopping other molecules from
\nreaching the vane, so the net force on the vane should be exactly the same \u2014
\nthe greater temperature causes a decrease in local density which results in
\nthe same force on both sides. Years after this explanation was dismissed, Albert
\nEinstein showed that the two pressures do not cancel out exactly at the edges
\nof the vanes because of the temperature difference there. The force predicted
\nby Einstein would be enough to move the vanes, but not fast enough.<\/p>\n
\nReynolds, but first published by James Clerk Maxwell in the last paper before
\nhis death in 1879. Reynolds found that if a porous plate is kept hotter on one
\nside than the other, the interactions between gas molecules and the plates are
\nsuch that gas will flow through from the cooler to the hotter side. The vanes
\nof a typical Crookes radiometer are not porous, but the space past their edges
\nbehave like the pores in Reynolds’s plate. On average, the gas molecules move
\nfrom the cold side toward the hot side whenever the pressure ratio is less than
\nthe square root of the (absolute) temperature ratio. The pressure difference
\ncauses the vane to move cold (white) side forward.<\/p>\n
\nrotate, although it still isn’t clear which one is stronger.<\/p>\n