However, instead, suppose that the voltage between the plates V is maintained constant as the plate moves by connection to a battery, which is a reservoir for electric charges at a constant potential difference. Then the amount of charges Q is a variable instead of the voltage; Q and V are the Legendre conjugate to each other. To find the force, first compute the non-standard Legendre transform U ∗ ∗ with respect to Q (also with using C = Q V ),