Glossary (20–30 Terms)

Term	Definition
Electric Current (I)	Rate of flow of charge: I = Q/t; SI unit: ampere (A)
Drift Velocity (vd)	Average velocity of free electrons under applied field; ~10^{-4} m/s
Relaxation Time (τ)	Average time between successive electron-lattice collisions
Resistance (R)	Opposition to current flow; R = V/I = ρl/A; unit: $\Omega$
Resistivity (ρ)	Intrinsic material property; ρ = RA/l; unit: $\Omega$ ·m
Conductivity (σ)	Reciprocal of resistivity; σ = 1/ρ; unit: S/m
Current Density (J)	Current per unit area; J = I/A = σE; unit: A/ $m^{2}$
Ohm's Law	V = IR (for constant R at constant T)
Ohmic Conductor	One where R is constant; V-I graph is linear
Non-ohmic Conductor	R varies with current/voltage; V-I graph is curved
EMF (ε)	Work done per unit charge by cell's internal source; unit: V
Terminal Voltage (V)	V = ε − Ir (discharge); V = ε + Ir (charge)
Internal Resistance (r)	Resistance inside the cell to current flow
Kirchhoff's Current Law (KCL)	ΣI = 0 at any junction (charge conservation)
Kirchhoff's Voltage Law (KVL)	Σ $\Delta V$ = 0 around any closed loop (energy conservation)
Temperature Coefficient (α)	Rate of change of R with temperature; α = (R− $R_{0}$ )/( $R_{0}$$\Delta T$ )
Wheatstone Bridge	Circuit to precisely measure unknown resistance using null method
Metre Bridge	Practical Wheatstone bridge with a 1-metre uniform wire
Potentiometer	Device to compare EMFs or measure internal resistance using null method
Null Method	Measurement technique relying on zero-deflection (no current from source)
Potential Gradient (k)	Potential drop per unit length of potentiometer wire; k = V_wire/L
Joule Heating	Heat produced = $I^{2}$ Rt; conversion of electrical energy to thermal energy
Short Circuit	Connection with R ≈ 0; terminal voltage → 0; current = ε/r
Balance Point	Position on potentiometer/metre bridge wire where galvanometer reads zero
Sensitivity (bridge)	Maximum when all bridge arms are equal (P = Q = R = S)
Manganin	Alloy with very small α; used for standard resistors
NTC Thermistor	Semiconductor temperature sensor; resistance decreases with temperature
Reciprocity Theorem	Interchanging battery and galvanometer in balanced Wheatstone bridge preserves balance