Electrical theory and calculations
Scope
Electrical-theory questions test Ohm law, series and parallel circuits, power, AC impedance, and low-voltage voltage drop. You need to calculate with consistent units and recognize when a nominal rating is not the actual operating point.
Core rules
- Ohm law: V = I x R; power: P = V x I = I squared x R = V squared / R.
- Series: current is common; resistances add; voltage divides. Parallel: voltage is common; reciprocal resistances add; current divides.
- In AC, impedance includes resistance and reactance; use RMS values for power and voltage drop unless asked for peak.
- Voltage drop in a two-wire circuit uses the round-trip conductor length, not one-way length.
- Transformer VA rating limits total load; connected VA must not exceed the rating after allowing for continuous duty.
Worked example
A 24 V source feeds a 4 ohm resistor in series with a parallel pair of 12 ohm and 24 ohm resistors. Parallel equivalent: 1/Rp = 1/12 + 1/24 = 3/24, so Rp = 8 ohms. Total R = 4 + 8 = 12 ohms. Current = 24 / 12 = 2 A. Voltage across the parallel pair = 2 x 8 = 16 V. The 12 ohm branch carries 16/12 = 1.33 A; the 24 ohm branch carries 0.67 A.
Traps
Adding parallel resistances directly; using one-way length for voltage drop; mixing VA with W at a non-unity power factor; and assuming a 24 V transformer holds 24 V under load.