Electrical Principles
Ohm's law and power
You must be fluent with V = IR, P = VI, and their combinations P = I²R and P = V²/R. General questions often require a two-step calculation: find current, then power, or find resistance from power and voltage. Practice unit conversions: mA to A, kΩ to Ω, and dB calculations.
AC, reactance, and impedance
Know that capacitive reactance falls as frequency rises (Xc = 1/(2πfC)) and inductive reactance rises with frequency (Xl = 2πfL). Impedance is the vector combination of resistance and reactance. Resonance occurs when Xl = Xc, leaving a purely resistive impedance. Know the meaning of RMS versus peak: RMS = peak × 0.707 for a sine wave.
Decibels and power ratios
Know the decibel rules: 3 dB is a factor of 2 in power, 10 dB is a factor of 10, and 20 dB is a factor of 100. A 3 dB loss halves power; a 3 dB gain doubles it. For voltage, 6 dB is a factor of 2. These shortcuts answer many exam questions quickly.
Worked example
A 12 V supply delivers 2 A to a resistor. P = VI = 24 W. The resistance is R = V/I = 6 Ω. If the same resistor carried 1 A, power would be I²R = 6 W - one quarter, illustrating that power scales with the square of current.
Common traps
Mixing peak and RMS values. Forgetting that reactance changes with frequency. Misapplying 3 dB as a voltage ratio instead of a power ratio.