Electric current I = Q/t is the rate of flow of charge. SI unit: ampere (A). One ampere = one coulomb per second. It is a fundamental SI base quantity.

Drift velocity vd = I/(neA) = eEτ/m is the average velocity of free electrons under an applied field. Typical value: ~10^{-4} m/s. Despite this small value, current is instantaneous because the electric field propagates at ~c, causing all electrons to drift simultaneously.

Relaxation time τ (~10^{-14} s) is the average time between electron-lattice collisions. Higher temperature → shorter τ → lower vd → higher resistance (for metals).

Ohm's law V = IR applies to ohmic conductors at constant temperature. The V-I graph is a straight line through the origin with slope = R. Non-ohmic conductors (diodes, filament bulbs) have curved V-I graphs.

Resistance R = ρl/A. Resistivity ρ is a material property; resistance R depends on both material and geometry. Dimensional formula: R has [ML^{2}$$T^{-3}$$A^{-2}]; ρ has [ML^{3}$$T^{-3}$$A^{-2}].

Temperature effects: Metals have α > 0 (R increases with T). Semiconductors have α < 0 (R decreases with T). Alloys (manganin, constantan) have α ≈ 0. Formula: R = $R_{0}$(1 + α$\Delta T$).

Power P = VI = $I^{2}$R = $V^{2}$/R. Use $I^{2}$R for series (constant I); use $V^{2}$/R for parallel (constant V). Key NEET fact: P_parallel = $n^{2}$ × P_series for n identical resistors on the same battery.

EMF and terminal voltage: For a real cell, V = ε − Ir (discharge). The difference Ir is lost internally. At open circuit, V = ε. During charging, V = ε + Ir.

Series combination: R_eq = $R_{1}$ + $R_{2}$ + …; same current, voltage divides. Parallel combination: 1/R_eq = 1/$R_{1}$ + 1/$R_{2}$ + …; same voltage, current divides. OPPOSITE of capacitor rules.

Kirchhoff's Laws: KCL (ΣI = 0) based on charge conservation; KVL (Σ$\Delta V$ = 0) based on energy conservation. Sign convention: −IR with current; +IR against current; +ε from − to +; −ε from + to −.

Wheatstone bridge: Balance when P/Q = R/S → Ig = 0. Metre bridge: R/S = l/(100 − l). Interchange of R and S: l′ = 100 − l.

Potentiometer: Measures true EMF (null method, no current at balance). ε_{1}/ε_{2} = l_{1}/l_{2}; r = R(l_{1} − l_{2})/l_{2}. Always check: V_wire ≥ ε_test.