Electrical Circuits and Network Principles
What the domain tests
Circuit and network principles test your ability to reduce a circuit to an equivalent resistance, apply Kirchhoff's laws, and calculate voltage, current and power in series and parallel networks. You should also know how meters and internal resistance affect a measurement.
Core rules
- Series: same current; RT = R1 + R2 + ...; voltages add; VT = V1 + V2.
- Parallel: same voltage; 1/RT = 1/R1 + 1/R2 + ...; branch currents add.
- Kirchhoff's current law: current into a node equals current out. Kirchhoff's voltage law: algebraic sum of voltages around a loop is zero.
- Voltage divider: Vout = Vin × R2/(R1 + R2) for two series resistors, with output across R2.
- Ammeters go in series and should have low resistance; voltmeters go in parallel and should have high resistance.
Worked example
A 12 V supply is connected across a 6 Ω resistor in parallel with a 3 Ω resistor. Equivalent resistance: Rp = (6 × 3)/(6 + 3) = 18/9 = 2 Ω. Total current: IT = 12/2 = 6 A. Branch currents: 12/6 = 2 A and 12/3 = 4 A, which sum to 6 A. Total power: P = V²/Rp = 144/2 = 72 W, or 12 × 6 = 72 W. Checking branch power: 24 W + 48 W = 72 W.
Traps that cost marks
Do not add parallel resistances directly. Do not place an ammeter across a component or a voltmeter in series. In voltage-divider calculations, identify which resistor the output is across. Real meters and supply internal resistance alter results; only ignore them when the question says ideal.