Why does the emf of a cell depend on its chemical composition rather than its size?
**Drift velocity** v_d = I/(n e A), I current (A), n number density of conduction electrons (m⁻³) ≈8.5×10²⁸ m⁻³ for copper, e =1.6×10⁻¹⁹ C, A cross-sectional area (m²). Typical v_d ≈10⁻⁴ m/s for 1 A in mm² wire, slow despite fast signal propagation due to electric field establishment. Emf is the potential difference generated by chemical reactions between electrodes and electrolyte, determined by the materials’ electrochemical properties, not physical dimensions like size. Applying I = n e A v_d, R = ρ l/A, R_t = R₀[1+αΔT], Kirchhoff's ΣI=0, ΣV=0, R_eq series/parallel, V = ε - I r and P = I²R,
Ref: NCERT > Physics Book > Current Electricity > Electric Current, Drift Velocity and Mobility
