CBSE Class 12 Physics: Current Electricity — Complete Notes 2026

Drift Velocity and Ohm's Law

• Free electrons in metal drift due to electric field; drift velocity vd = eEτ/m (τ = relaxation time)
• Current I = nAevd; resistance R = V/I; Ohm's law valid for ohmic conductors
• Resistivity ρ = m/(ne²τ); increases with temperature for metals

Combination of Cells

• Cells in series: EMF = nε, internal resistance = nr; used when high EMF needed
• Cells in parallel: EMF = ε, internal resistance = r/n; used when high current needed
• Terminal voltage: V = ε − Ir (discharging); V = ε + Ir (charging)

Kirchhoff's Laws

• KCL: sum of currents at a junction = 0 (conservation of charge)
• KVL: sum of potential drops around a closed loop = 0 (conservation of energy)
• Apply to solve complex circuits; assign current directions; set up simultaneous equations

Wheatstone Bridge

• Balanced condition: P/Q = R/S; no current through galvanometer
• Slide wire bridge (metre bridge): X = R × (100−l)/l; used to find unknown resistance
• Sensitivity: condition for balanced bridge; near-balanced condition produces small galvanometer deflection

Potentiometer

• Long uniform wire; potential drops uniformly with length
• Comparison of EMFs: ε₁/ε₂ = l₁/l₂
• Internal resistance measurement: r = R(l₁/l₂ − 1) where l₁ = EMF balance length, l₂ = terminal voltage balance length

Power Dissipation in Circuits

• P = I²R = V²/R; maximum power transferred when external R = internal r
• Efficiency of cell: η = R/(R+r); maximum power transfer reduces efficiency to 50%
• Joule heating: used in heating elements; high resistivity material needed

CBSE Exam — Current Electricity

• Derive expression for drift velocity in terms of current and number density
• A potentiometer wire AB 4m long; find balancing length conditions — numerical
• Prove that metre bridge is more accurate when resistance is near 50 Ω