Chapter 3 — Current Electricity

Chapter 3 of the Class 12 Physics NCERT textbook, "Current Electricity", covers the flow of electric charges through conductors, Ohm's law (V = RI), resistivity, drift velocity of electrons, Kirchhoff's rules, and circuit analysis tools such as the Wheatstone bridge.

• What electric current really is — The chapter moves from static charge to charge in motion, defining current as charge flowing per unit time and connecting the everyday flow in wires to the drift of free electrons under an applied field.
• Ohm's law and resistance — It builds Ohm's law from the microscopic picture, showing how resistance arises from a material's resistivity and geometry, and how temperature shifts resistivity differently for metals and semiconductors.
• Real cells and power — Sources are treated realistically through EMF and internal resistance, explaining terminal voltage and how electrical energy converts to heat as power dissipated in a circuit.
• Analysing complex circuits — Kirchhoff's junction and loop rules give a systematic method for networks that Ohm's law alone can't solve, applied in tools like the Wheatstone bridge for measuring unknown resistances.

1. 01Electric current is defined as the net charge flowing per unit time across a cross-section: I = Q/t (steady) or I = lim(ΔQ/Δt) as Δt→0 (instantaneous).
2. 02Ohm's law states V = RI, where resistance R = ρl/A; resistivity ρ depends on material and temperature but not on the conductor's dimensions.
3. 03Drift velocity of electrons under an electric field E is vd = –eEτ/m, where τ is the relaxation (average collision) time, giving conductivity σ = ne²τ/m.
4. 04Resistivity of metals increases with temperature (ρT = ρ0[1 + α(T – T0)]), while resistivity of semiconductors decreases with increasing temperature.
5. 05For a cell of EMF ε and internal resistance r connected to external resistance R, current I = ε/(R + r) and terminal voltage V = ε – Ir.
6. 06Kirchhoff's rules — Junction Rule (sum of currents entering a junction equals sum leaving) and Loop Rule (algebraic sum of potential changes around any closed loop is zero) — enable analysis of complex circuits; the Wheatstone bridge balance condition is R1/R2 = R3/R4.