Charge Fluctuations in Finite Coulomb Systems

Abstract

When described in a grand canonical ensemble, a finite Coulomb system exhibits charge fluctuations. These fluctuations are studied in the case of a classical (i.e. non-quantum) system with no macroscopic average charge. Assuming the validity of macroscopic electrostatics gives, on a three-dimensional finite large conductor of volume $V$, a mean square charge $<Q^2>$ which goes as $V^{1/3}$. More generally, in a short-circuited capacitor of capacitance $C$, made of two conductors, the mean square charge on one conductor is $<Q^2>=TC$, where $T$ is the temperature and $C$ the capacitance of the capacitor. The case of only one conductor in the grand canonical ensemble is obtained by removing the other conductor to infinity. The general formula is checked in the weak-coupling (Debye-Hückel) limit for a spherical capacitor. For two-dimensional Coulomb systems (with logarithmic interactions), there are exactly solvable models which reveal that, in some cases, macroscopic electrostatics is not applicable even for large conductors. This is when the charge fluctuations involve only a small number of particles. The mean square charge on one two-dimensional system alone, in the grand canonical ensemble, is expected to be, at most, one squared elementary charge.