Thermal noise known as “Johnson noise” or “Nyquist noise” is well known and characterized. This noise is a result of thermal agitation of charge carriers inside a conductor. Its power spectral density is given by:(adsbygoogle = window.adsbygoogle || []).push({});

v^2=4kTR

where kB is Boltzmann's constant in joules per kelvin, T is the resistor's absolute temperature in kelvins, and R is the resistor value in ohms.

Imagine a resistor in an adiabatic enclosure (one incapable of heat transfer) connected to a lossless diode. The wires leave the enclosure through sealed ports to power a circuit. Shouldn’t the rectification of this random fluctuation in the voltage across the resistor create a net positive voltage and hence allow us to violate the second law of thermodynamics (creating free energy)?

I once asked a physics professor this question and he excitedly gave me a rushed explanation of why this wouldn’t work. It involved Brownian motion and he quickly digressed into a tangent about proving the continuity between classical (Rayleigh Jeans) and quantum blackbody theory. It involved some esoteric thought experiment where he defiled my adiabatic enclosure with a big parabolic dish. I wasn’t impressed.

edit: I just realised the parallels between this hypothetical and Maxwell's demon, which seems to be still up for debate.

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Can i exploit johnson noise to violate the second law?

**Physics Forums | Science Articles, Homework Help, Discussion**