Basic Electricity and Passive Components
Basic Electricity and Passive Components tests DC circuit laws, equivalent resistance, capacitor and inductor behaviour, reactance, and component ratings. Expect numerical questions rather than definitions alone.
Core rules
- Ohm's law is V = IR; power is P = VI = I²R = V²/R.
- Resistors in series add. For parallel resistors, 1/R = 1/R1 + 1/R2; two resistors give R = R1R2/(R1+R2).
- Capacitors in parallel add; in series their reciprocals add. Capacitive reactance XC = 1/(2πfC).
- Inductors in series add; in parallel their reciprocals add. Inductive reactance XL = 2πfL.
- A capacitor blocks steady DC but passes changing signals; an inductor opposes changes in current.
- For a sine wave, Vrms = Vpeak/√2 and Irms = Ipeak/√2.
Worked example
A 12 V supply is connected across a 6 Ω resistor in parallel with a 3 Ω resistor. Equivalent resistance = (6×3)/(6+3) = 2 Ω. Supply current = 12/2 = 6 A. Branch currents are 12/6 = 2 A and 12/3 = 4 A, which sum to 6 A. Total power = VI = 12×6 = 72 W, or I²R = 36×2 = 72 W.
Marks-losing traps
Do not add parallel resistors directly. Keep tolerance as a percentage of nominal value: ±10% of 100 Ω is ±10 Ω. A capacitor’s voltage rating is a maximum, not its capacitance. In AC, use rms values unless peak is requested, and remember that reactance changes with frequency. A 100 Ω resistor with ±5% tolerance can lie between 95 Ω and 105 Ω, so a calculated current may vary accordingly.