For electrical circuits, let n be the concentration of free charged particles (in m) and let q * be the charge of each particle (in coulombs). (For electrons, q * = e = 1.6 × 10 C.) Then nQ is the number of particles in the volume Q, and nQq * is their total charge. This is the charge that flows through the cross section per unit time, i.e. the current I. Therefore, I = nQq *. Consequently, Q = ⁠ I / nq * ⁠, and