Current Electricity: Visual Summary with Circuit Diagrams

ε, r r R (ext) V_terminal = ε − Ir | I = ε/(R+r) → I (clockwise)

SERIES $R_{1}$ $R_{2}$ $R_{3}$ R_eq = $R_{1}$+$R_{2}$+$R_{3}$ Same I, V divides

PARALLEL $R_{1}$ $R_{2}$ $R_{3}$ 1/R_eq = 1/ $R_{1}$ +1/ $R_{2}$ +1/ $R_{3}$ Same V, I divides

Key Comparison Table Series: R grows | Parallel: R shrinks | P_parallel = $n^{2}$ × P_series OPPOSITE rules to capacitors!

Method	Formula	What It Measures	Advantage
Wheatstone Bridge	P/Q = R/S	Unknown resistance	High precision, null method
Metre Bridge	R/S = l/(100−l)	Unknown resistance	Simple, uniform wire
Potentiometer (EMF)	ε_{1}/ε_{2} = l_{1}/l_{2}	Ratio of EMFs	No current from source
Potentiometer (r)	r = R(l_{1}−l_{2})/l_{2}	Internal resistance	True EMF measured first