Basic Electrical Engineering
Basic Electrical Engineering is the foundational domain of RRB JE CBT 2. It tests circuit laws, network theorems, AC/DC fundamentals and electromagnetic induction, usually through short numericals and conceptual one-liners.
Circuit laws and network theorems
- KCL: the algebraic sum of currents at a node is zero; KVL: the algebraic sum of voltages around a closed loop is zero.
- Thevenin reduces a linear network to Vth in series with Rth; Norton uses In in parallel with Rn, with Rn = Rth and In = Vth/Rth.
- Superposition applies to linear circuits and to voltages/currents, never directly to power.
- Maximum power transfer to a resistive load occurs at RL = Rth, giving Pmax = Vth²/(4Rth).
AC/DC fundamentals and induction
- RMS value of a sinusoid = peak/√2; average over a full cycle is zero.
- XL = 2πfL and XC = 1/(2πfC); impedance combines resistance and reactance as a complex quantity.
- Power triangle: P = VI cosφ, Q = VI sinφ, S = VI.
- Faraday’s law gives e = −N dφ/dt; Lenz’s law fixes the induced EMF direction so as to oppose the change.
Worked example
A 12 V source feeds a 4 Ω series resistor and a 6 Ω shunt resistor; a load is connected across the 6 Ω resistor. Remove the load: Vth = 12 × 6/(4 + 6) = 7.2 V, and Rth = 4 || 6 = 2.4 Ω. For maximum power transfer, RL = 2.4 Ω and Pmax = 7.2²/(4 × 2.4) = 51.84/9.6 = 5.4 W.
Traps that cost marks
Mixing RMS with average for a rectified wave, applying superposition to power, ignoring dependent sources when finding Rth, and forgetting that a 6 Ω shunt changes the open-circuit voltage are common errors.