An equation which gives the @[email protected] per unit area of an @[email protected] (electrode): \[(\frac{\partial \gamma }{\partial E_{\text{A}}})_{T,p,\mu _{i}\neq \mu }=- Q_{\text{A}}\] where \(\gamma \) is the @[email protected], \(E_{\text{A}}\) is the potential of a cell in which the @[email protected] has an interfacial equilibrium with one of the ionic components of A, \(Q_{\text{A}}\) is the charge on unit area of the @[email protected], \(\mu _{i}\) is the @[email protected] of the combination of species \(i\) whose net charge is zero, \(T\) is the @[email protected] and \(p\) is the external pressure. Since more than one type of @[email protected] may be chosen, more than one quantity \(Q\) may be obtained. Consequently \(Q\) cannot be considered as equivalent to the physical charge on a particular region of the @[email protected] It is in fact an alternative way of expressing a @[email protected] or combination of @[email protected] of charged species.