Electrical Principles and Circuit Fundamentals
Electrical Principles and Circuit Fundamentals
This domain tests Ohm's law, power, series and parallel networks, Kirchhoff's laws, magnetism and basic circuit faults. Expect calculation questions and simple circuit reasoning rather than memorised definitions.
Core rules
- Ohm's law: V = I × R. Power: P = V × I = I²R = V²/R.
- Series: one path; current is common; resistances add; voltages divide in proportion to resistance.
- Parallel: voltage is common; currents add; 1/Rt = 1/R1 + 1/R2 + ...; total resistance is less than the smallest branch.
- Kirchhoff: current into a node equals current out; voltage rises equal voltage drops around a loop.
- An open circuit has infinite resistance and no current; a short has near-zero resistance and excessive current.
- Conventional current flows positive to negative; electron flow is opposite. Collapsing coil fields induce voltage, the basis of relays, ignition and alternators.
Worked example
A 12 V battery feeds a 6 Ω lamp in series with a 4 Ω resistor. Total resistance = 10 Ω, so I = 12/10 = 1.2 A. Resistor drop = 1.2 × 4 = 4.8 V; lamp drop = 1.2 × 6 = 7.2 V. Check: 4.8 + 7.2 = 12 V. Lamp power = 7.2 × 1.2 = 8.64 W. If the lamp were instead rated 12 V, 24 W and connected directly, its normal current would be 2 A and resistance 6 Ω.
Traps
Adding parallel resistances directly; assuming series components have equal voltage; using battery EMF instead of the actual voltage across a component in P = V²/R; ignoring earth return resistance; confusing a short with an open. Check units and whether the question asks for total or branch current.