Brownian motor

[Hearts]

Actually I came across this quite some time back but never really understood why won't it work.

Wiki it if you need a memory jog~

anyway quote from wiki

 

Although at first sight the Brownian ratchet seems to extract useful work from Brownian motion, Feynman demonstrated that if the entire device is at the same temperature, the ratchet will not rotate continuously in one direction but will move randomly back and forth, and therefore will not produce any useful 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 paddle, 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.^[2] A simple but rigorous proof that no net motion occurs no matter what shape the teeth are was given by Magnasco. ^[3]

 

 

and from another site,http://www.elmer.unibas.ch/bm/index.html.

The energy does not come from the heat bath but from the ratchet potential when it is switched on. At that moment the potential energy of the particle will be suddenly increased. In the simulation this can be seen by a sudden increase of the energy bar. But most of the energy pushed into the system will be just dissipated into the heat bath due to the relaxation of the particle into a potential minima. Only a tiny portion will be used for doing work. Thus a Brownian motor does not violate any law of thermodynamics it only turns one type of work into another one.

 

I'm not entirely convinced yet though~

I thought thermo's second law don't work on extremely small scales..

can anyone explain the problem wih this motor?