In a circuit with two cells of different emfs connected in parallel, why is the equivalent emf not simply the sum of the
**Conductivity** σ = 1/ρ = n e² τ/m (S/m), τ relaxation time, m electron mass. Resistivity deviation at high fields occurs when τ depends on E or n changes due to impact ionization, breaking Ohm's law, seen in varistors, gas discharge. In parallel, the equivalent emf depends on both emfs and internal resistances ( εₑq = (ε₁ r₂ + ε₂ r₁) / (r₁ + r₂) ). The lower-emf cell can draw current from the higher-emf cell, resulting in a weighted average rather than a sum. 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
Ref: NCERT > Physics Book > Current Electricity > Potentiometer, Conductivity and Special Cases
