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Schematic figure of a Brownian Ratchet
Schematic figure of a Brownian Ratchet

The Brownian ratchet is a thought experiment about an apparent perpetual motion machine postulated by Richard Feynman in a physics lecture at the California Institute of Technology on May 11, 1962 as an illustration of the laws of thermodynamics. A thought experiment (from the German Gedankenexperiment) is a proposal for an Experiment that would test a Hypothesis or Theory The term perpetual motion, taken literally refers to movement that goes on forever Richard Phillips Feynman (ˈfaɪnmən May 11 1918 – February 15 1988 was an American Physicist known for the Path integral formulation of quantum Physics (Greek Physis - φύσις in everyday terms is the Science of Matter and its motion. The California Institute of Technology (commonly referred to as Caltech) is a private, Coeducational research university located in Pasadena Events 330 - Byzantium is renamed ''Nova Roma'' during a dedication ceremony but is more popularly referred to as Constantinople Year 1962 ( MCMLXII) was a Common year starting on Monday (the link is to a full 1962 calendar of the Gregorian calendar. In Physics, thermodynamics (from the Greek θερμη therme meaning " Heat " and δυναμις dynamis meaning " In particular, it is an example of a Maxwell's demon, a machine that is purported to extract useful work from random fluctuations in systems otherwise at equilibrium. Maxwell's demon was an 1867 Thought experiment by the Scottish Physicist James Clerk Maxwell, meant to raise questions about the possibility

The device consists of an asymmetric gear known as a ratchet that rotates freely in one direction but is prevented from rotating in the opposite direction by a pawl. In Mechanical engineering, a ratchet is a device that allows linear or rotary motion in only one direction while preventing motion in the opposite direction The ratchet is connected by a massless and frictionless rod to a paddle wheel that is immersed in a bath of molecules at temperature T1. The molecules constitute a heat bath in that they undergo random Brownian motion with a mean kinetic energy that is determined by the temperature. A heat bath is a system whose Heat capacity is so large that when it is in Thermal contact with some other system of interest its temperature remains constant This article is about the physical phenomenon for the stochastic process see Wiener process. The kinetic energy of an object is the extra Energy which it possesses due to its motion Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature Each time a molecule collides with a paddle, it imparts an impulse that exerts a torque on the ratchet. Because the pawl only allows motion in one direction, the net effect of many such random collisions should be for the ratchet to rotate continuously in that direction. The ratchet's motion then can be used to do work on other systems, for example lifting a weight against gravity. The energy necessary to do this work apparently would come from the heat bath, without any heat gradient. Were such a machine to work as advertised, its operation would contradict one form of the second law of thermodynamics, which states that

It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work. The second law of Thermodynamics is an expression of the universal law of increasing Entropy, stating that the entropy of an Isolated system which

Although at first sight the Brownian ratchet seems to extract useful work from Brownian motion, Feynman demonstrated that its operation would be self-defeating, and would in fact not produce any work. A simple way to visualize how the machine might fail is to remember that a ratchet and pawl small enough to move in response to individual molecular collisions also would be small enough to undergo Brownian motion as well. The pawl therefore will intermittently fail, allowing the ratchet to slip backward. Feynman demonstrated that if the temperature T2 of the ratchet and pawl is the same as the temperature T1 of the bath, then the failure rate must equal the rate at which the ratchet ratchets forward, so that no net motion results over long enough periods or in an ensemble averaged sense.

If, on the other hand, T2 is smaller than T1, the ratchet can indeed ratchet forward. In this case, though, energy is extracted from the temperature gradient in agreement with the second law.

Although the Feynman ratchet cannot extract work from a heat bath in equilibrium, this model and related ideas led to the development of Brownian motors, which do extract useful work from thermal noise, but do not violate the laws of thermodynamics. Brownian motors are nano-scale or molecular devices by which thermally activated processes (chemical reactions are controlled and used to generate directed motion in space and to do mechanical

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