Questions
8–12 questions in major PSU papers
Difficulty
Medium-Hard
Importance
Core — never skip
Overview
Electric Circuits forms the fundamental backbone of electrical engineering, testing your ability to simplify complex networks and solve for specific variables. Mastery of these concepts is non-negotiable for PSU exams as it provides the foundation for Power Systems, Machines, and Electronics. Focus on speed and accuracy in solving Mesh and Node equations.
Network Laws and Circuit Analysis
Circuit analysis revolves around Kirchhoff's laws and systematic reduction techniques to determine branch currents and node voltages. Proficiency here ensures you can handle both simple DC circuits and complex linear networks with ease.
- KCL: Algebraic sum of currents meeting at a node is zero (Conservation of Charge)
- KVL: Algebraic sum of voltages around a closed loop is zero (Conservation of Energy)
- Mesh Analysis: Uses KVL for planar circuits with 'n' meshes
- Nodal Analysis: Uses KCL at each principal node
Network Theorems
These theorems are designed to simplify large, complex circuits into an equivalent single-source representation. In exams, identifying which theorem reduces the circuit fastest is the key to managing your time.
- Thevenin's Theorem: V_th in series with R_th
- Norton's Theorem: I_sc in parallel with R_n
- Superposition Theorem: Applicable only to linear circuits; one source at a time
- Maximum Power Transfer Theorem: R_load = R_thevenin
- Millman's Theorem: For parallel voltage sources
AC Circuits and Resonance
AC analysis extends DC principles into the phasor domain using impedance (Z). Understanding the behavior of R, L, and C components at varying frequencies is vital for both numerical and conceptual questions.
- Impedance: Z = R + j(X_L - X_C)
- Power Factor: cos(phi) = R/Z
- Series Resonance: f_r = 1 / (2 * pi * sqrt(LC))
- Quality Factor (Q): (1/R) * sqrt(L/C)
- Bandwidth: BW = f_r / Q
Transient Analysis
Transients describe the circuit's behavior during the switching interval before reaching a steady state. Aspirants must memorize the standard step-response equations for common circuits.
- Time constant (RC): tau = RC
- Time constant (RL): tau = L/R
- General form: x(t) = x(infinity) + [x(0+) - x(infinity)] * e^(-t/tau)
- Inductor current cannot change instantaneously
- Capacitor voltage cannot change instantaneously
Formula Sheet
V = IR
P = V^2 / R = I^2 R
R_eq (series) = R1 + R2
1/R_eq (parallel) = 1/R1 + 1/R2
X_L = 2 * pi * f * L
X_C = 1 / (2 * pi * f * C)
P_avg = V_rms * I_rms * cos(phi)
V_th = V_oc
R_th = V_oc / I_sc
i_L(t) = I_f + (I_i - I_f) * e^(-Rt/L)
Exam Tip
Always verify the unit of the answer; PSU papers often include distractors where the magnitude is correct but the unit (e.g., mH vs H) is wrong.
Common Mistakes
- Confusing the impedance of inductors and capacitors, specifically mixing up X_L and X_C formulas.
- Forgetting to deactivate dependent sources when calculating R_thevenin or R_n.
- Applying Superposition theorem to circuits with non-linear elements like diodes or transistors.
More Revision Notes
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