Basic Electrical Fundamentals
Basic Electrical Fundamentals
This domain tests your ability to apply Ohm's law, the power relations and Kirchhoff's rules to series, parallel and mixed networks, and to keep units and quantities consistent.
Core rules
- Ohm's law: V = I × R (volts, amperes, ohms).
- Power: P = VI = I²R = V²/R; energy in kWh = kW × hours.
- Series: common current; R = R1 + R2 + …; voltage divides in proportion to resistance.
- Parallel: common voltage; 1/R = 1/R1 + 1/R2 + …; current divides inversely with resistance (two resistors: product over sum).
- KCL: currents at a node sum to zero. KVL: voltages round a closed loop sum to zero.
- R = ρl/A: resistance rises with length and resistivity, falls with cross-section.
- Sine AC: RMS = 0.707 × peak, average (full wave) = 0.637 × peak, form factor 1.11.
Worked example
A 12 V supply feeds 6 Ω in series with a parallel pair of 6 Ω and 3 Ω. Pair: (6 × 3)/(6 + 3) = 2 Ω. Total: 6 + 2 = 8 Ω. Supply current 12/8 = 1.5 A. Voltage across the pair = 1.5 × 2 = 3 V, so branch currents are 3/6 = 0.5 A and 3/3 = 1 A - sum 1.5 A, satisfying KCL. Total power = VI = 12 × 1.5 = 18 W, confirmed by I²R = 1.5² × 8 = 18 W.
Mark-losing traps
- Adding parallel resistances directly instead of adding reciprocals.
- Mixing prefixes - mA with kΩ, or reporting kW where W is required.
- Using peak voltage when RMS belongs in a power calculation.
- Assuming series components split voltage equally regardless of resistance.