Electrical Engineering: Circuits, Phasors and Thevenin Reasoning
What this section tests
The Electrical Engineering portion of the GIKI Test checks whether you can apply first- and second-year circuit theory quickly and without sign errors: Ohm's and Kirchhoff's laws, series/parallel reduction, source transformation, Thevenin and Norton equivalents, RC/RL transients, single-phase AC with phasors and impedance, ideal op-amp basics, diode circuits, and combinational logic.
Core rules
- KCL: currents into a node sum to zero; KVL: voltages around a closed loop sum to zero. Fix a sign convention before writing equations and never change it mid-problem.
- Power in a resistor is I²R = V²/R; it is always dissipated, never negative.
- For AC, X_L = ωL, X_C = 1/(ωC), Z = R + jX. Magnitudes add in quadrature, not linearly.
- Thevenin: V_th is the open-circuit output voltage; R_th is the resistance seen with independent sources killed (voltage sources shorted, current sources opened).
- Maximum power transfer occurs at R_load = R_th, giving P = V_th²/(4R_th).
Worked example
A 12 V source feeds a 4 Ω series resistor and a 6 Ω shunt resistor; the output is taken across the 6 Ω. Open-circuit output: V_th = 12 × 6/(4+6) = 7.2 V. Kill the source (short it): R_th = 4 ∥ 6 = 24/10 = 2.4 Ω. Attach a 2.4 Ω load: V_load = 7.2 × 2.4/(2.4+2.4) = 3.6 V - exactly half of V_th, the signature of a matched load.
Traps
Mixing rms and peak values (V_rms = V_peak/√2); adding 3 Ω + j4 Ω as 7 Ω instead of 5 Ω; forgetting an ideal ammeter has zero resistance and an ideal voltmeter infinite resistance; assuming a diode conducts before checking bias polarity.