# Current Electricity > Current Electricity for NEET-UG Physics — drift velocity, Ohm's law, EMF and internal resistance, Kirchhoff's rules, Wheatstone bridge and potentiometer. - Canonical URL: https://prepelephant.com/topics/neet-ug/physics/current-electricity-ncert - Exam / course: NEET-UG · Subject: Physics - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Current Electricity", PrepElephant, https://prepelephant.com/topics/neet-ug/physics/current-electricity-ncert ## Direct answer Electrons drift through copper at under a millimetre per second, yet the lamp lights the instant you flip the switch, because the electric field — not the electrons — propagates at near light speed; current is I = nAev_d. Ohm's law V = IR with R = ρl/A then governs the steady circuit, Kirchhoff's two rules extend it to networks, and the exam concentrates its marks on emf versus terminal voltage, the Wheatstone bridge and the potentiometer. ## What you must remember - Drift velocity v_d = I/(nAe); for a copper wire of 1 mm^2 carrying 1 A it is of order 10^-4 m/s — the switch responds to the field, not to the electron commute. - Resistance R = ρl/A: longer and thinner wires resist more; resistivity ρ is the material's property and rises with temperature in metals, while semiconductors fall. - Ohm's law is a material behaviour, not a law of nature; it fails for diodes, electrolytes and at extreme conditions — those are non-ohmic. - A real cell: terminal voltage V = ε - Ir while discharging, V = ε when the circuit is open, and V = ε + Ir while being charged; internal resistance wastes power I^2r. - Kirchhoff's junction rule (ΣI = 0) expresses charge conservation and the loop rule (ΣΔV = 0) energy conservation; every network problem is solved by writing both honestly. - Wheatstone bridge balances when P/Q = R/S; at balance the galvanometer current is zero and the bridge is insensitive to the galvanometer's own resistance — the metre bridge measures unknown resistance from a balance length. - The potentiometer compares emfs as ε1/ε2 = l1/l2 at null — no current drawn at balance, which is why it beats a voltmeter, which always draws some. - Electrical power P = VI = I^2R = V^2/R; the commercial energy unit is the kilowatt-hour, 1 kWh = 3.6 × 10^6 J. ## Inside a cell under load Connect a 6 V cell of internal resistance 2 Ω across a 4 Ω resistor. The circuit current is I = ε/(R + r) = 6/6 = 1 A. The terminal voltage is what the external resistor actually receives: V = IR = 4 V, confirmed by the bookkeeping V = ε - Ir = 6 - 2 = 4 V. Of the 6 W the cell generates, 4 W reaches the load and 2 W warms the cell itself — an efficiency of R/(R + r) = 2/3. Two conclusions generalise. First, terminal voltage equals emf only when I = 0, which is why a battery tester reads full voltage until loaded. Second, the load receives maximum power exactly when R = r (here 2 Ω, giving 4.5 W); beyond that matching point efficiency keeps improving while delivered power declines — the distinction between maximum power transfer and maximum efficiency that question setters enjoy probing. ## Where students slip The ammeter is a low-resistance device inserted in series (a parallel shunt protects its galvanometer), while the voltmeter is a high-resistance device placed in parallel; swapping them — or asking what an ammeter across a battery would read — is a standard trap, since an ammeter across a battery is effectively a short circuit. Second, series and parallel outcomes: students mix up "resistances add" with "conductances add"; series resistances always exceed the largest, parallel always dips below the smallest. Third, temperature: a metal's resistance rises when heated, a semiconductor's falls because carrier numbers explode — the filament lamp's non-ohmic curve in NCERT is exactly this. Fourth, in potentiometer problems the driver cell must exceed the tested emf or no null exists; and at the null the tested cell delivers nothing, which is the entire point of the null method. Finally, drift-velocity questions punish the assumption that electrons race around the circuit carrying energy like couriers; the energy travels in the field. ## Frequently asked questions ### Why does a lamp glow immediately although drift velocity is tiny? The electric field establishing itself around the circuit travels at nearly the speed of light; electrons everywhere in the filament start drifting almost at once. ### When does terminal voltage equal the emf of a cell? Only on open circuit, when I = 0. Under load V = ε - Ir, so the terminal voltage always falls below the emf while the cell delivers current. ### What is the balance condition of a Wheatstone bridge? P/Q = R/S; the galvanometer then carries no current, making the measurement independent of the galvanometer's resistance and of the cell's emf. ### Why does a potentiometer measure emf more accurately than a voltmeter? At the null point the potentiometer draws no current from the cell under test, so it reads the true emf; a voltmeter always draws some current and shows only the reduced terminal voltage. ### How do metals and semiconductors respond to heating? Metal resistivity rises (lattice vibrations scatter electrons harder); semiconductor resistivity falls, because thermal energy frees far more charge carriers than the extra scattering removes.