Electrical Circuits
What this domain tests
This domain tests your grip on series and parallel networks, Kirchhoff's laws, power distribution in a circuit, and basic AC terms such as RMS value, frequency and power factor.
Core rules
- Series: same current through every element; voltages add; R = R1 + R2 + R3.
- Parallel: same voltage across every branch; currents add; 1/R = 1/R1 + 1/R2 + 1/R3.
- Kirchhoff's current law: current into a node equals current out. Voltage law: the algebraic sum of voltages round a closed loop is zero.
- Power in a resistor: P = I²R = V²/R = VI.
- For a sine wave, V(rms) = V(peak)/√2. A 230 V supply has a peak of about 325 V.
- Cells in series add voltage; cells in parallel add current capacity, not voltage.
Worked example
A 6 Ω and a 3 Ω resistor are connected across 12 V. Compare series and parallel. Series: R = 6 + 3 = 9 Ω; I = 12/9 = 1.33 A; total P = 12 × 1.33 = 16 W. Parallel: R = (6 × 3)/(6 + 3) = 2 Ω; total current = 12/2 = 6 A; branch currents 12/6 = 2 A and 12/3 = 4 A; total P = 72 W. The parallel circuit draws far more current because its equivalent resistance is smaller - a common exam point.
Traps that cost marks
Adding parallel resistances directly; assuming series elements share the same voltage; mixing peak and RMS values in one calculation; forgetting that the 6 Ω branch carries the smaller current because it has the larger resistance.