Basic Electrical Principles and Circuit Fundamentals
Basic Electrical Principles and Circuit Fundamentals
This section tests numerical fluency with Ohm's law, series and parallel networks, Kirchhoff's laws and single-phase AC quantities, plus the practical meaning of power factor in vocational power work.
Core rules
- V = IR; P = VI = I²R = V²/R; energy in kWh = kW × hours.
- Series: one current path, R = R1 + R2 + …, voltages divide in proportion to resistance.
- Parallel: same voltage, 1/R = 1/R1 + 1/R2 + …, currents divide.
- KCL: currents into a node equal currents out. KVL: algebraic sum of voltages round a closed loop is zero.
- AC: V_rms = V_peak/√2. A 230 V supply has a peak of about 325 V.
- X_L = 2πfL, X_C = 1/(2πfC), Z = √(R² + (X_L − X_C)²), cos φ = R/Z.
Worked example
A 6 Ω and a 3 Ω resistor are connected in parallel across 12 V. Find the total current. Product-over-sum: R_p = (6 × 3)/(6 + 3) = 18/9 = 2 Ω. I = V/R_p = 12/2 = 6 A. Branch check: 12/6 = 2 A and 12/3 = 4 A; 2 + 4 = 6 A, confirming KCL.
Traps that cost marks
- Treating 230 V as a peak value. It is RMS; the peak is about 325 V and peak-to-peak about 650 V.
- Adding parallel resistances directly, or forgetting to invert the reciprocal at the end.
- Writing P = VI for an AC motor without cos φ, which overstates true power.
- Mixing milliamps and amps, or ohms and kilo-ohms, inside one calculation.
- Believing a capacitor or inductor consumes real power; it shifts phase only.