The noise is caused by the thermal motion of electrons within a conductor. According to the Nyquist theorem, the power spectral density of this noise is directly proportional to the resistance of the conductor and the temperature, given by the formula:
S_v(f) = 4kTR
where S_v(f) is the power spectral density, k is the Boltzmann constant, T is the absolute temperature, and R is the resistance.
