Electric current I = dQ/dt is the rate of charge flow. SI unit: ampere (A), a base SI quantity. Conventional current flows from high to low potential (positive charge flow direction), opposite to electron drift. Current density J = I/A = nev_d, where n = number density of free electrons (~10^28 /m^3 for copper), e = 1.6 x 10^(-19) C, v_d = drift velocity. The drift velocity v_d = eE*tau/m_e, where tau is the relaxation time (~10^(-14) s) and E is the applied electric field. Typical v_d ~ 10^(-4) m/s — electrons drift slowly but the electric signal propagates at nearly c because the field establishes almost instantaneously throughout the conductor.