BT-104 Basic Electrical and Electronics Engineering - Jun 2025 Question Paper

Under each question, How to answer lists the points an answer should cover, outlined by CampusPrep from the syllabus. Check the details against your textbook.

1. 7 Marks
   aState and explain Kirchhoff's current and voltage law.
   How to answer

   • Statement of Kirchhoff's Current Law (algebraic sum of currents at a junction is zero)
   • Mathematical formulation for KCL: $\sum I = 0$ with sign convention
   • Statement of Kirchhoff's Voltage Law (algebraic sum of voltages around a closed loop is zero)
   • Mathematical formulation for KVL: $\sum V = 0$ or $\sum IR = \sum E$
   • Simple illustrative circuit diagram demonstrating both laws
2. 7 Marks
   bFor the circuit shown in Fig (i) find the current in $2\Omega$ resistor.

   How to answer

   • Apply nodal analysis at junction C (with reference node at bottom rail A-F-E)
   • Write KCL equation: $(V_C - 35)/12 + V_C/2 + (V_C - 40)/4 = 0$
   • Solve for node voltage $V_C$
   • Calculate current $I_{2\Omega} = V_C / 2$
3. 7 Marks
   aState and explain Superposition theorem.
   How to answer

   • Formal statement of Superposition theorem for linear bilateral circuits
   • Conditions: applicable only to linear networks with independent sources
   • Procedure: considering one independent source at a time while replacing other independent voltage sources with short circuits and current sources with open circuits
   • Summing individual branch currents/voltages algebraically
4. 7 Marks
   bFind the current through 5-ohm resistance in Fig (ii) using mesh current analysis.

   How to answer

   • Identify three independent mesh currents $I_1$, $I_2$, and $I_3$ clockwise
   • Set up KVL equations for Mesh 1 (left), Mesh 2 (middle with $5\Omega$), and Mesh 3 (right)
   • Formulate matrix system or solve simultaneous equations for $I_2$
   • State the resulting current through the 5 ohm resistor
5. 7 Marks
   aDefine the following with respect to alternating quantity. i) R.M.S. value ii) Peak value iii) Average value iv) Instantaneous value
   How to answer

   • Definition and heating effect equivalent of RMS value: $V_{rms} = \sqrt{\frac{1}{T}\int_0^T v^2 dt} = \frac{V_m}{\sqrt{2}}$
   • Definition of Peak value (maximum amplitude $V_m$ reached during a cycle)
   • Definition of Average value over a half-cycle: $V_{avg} = \frac{1}{T/2}\int_0^{T/2} v dt = \frac{2V_m}{\pi}$
   • Definition of Instantaneous value (value of voltage/current at any specified instant $t$)
6. 7 Marks
   bA 4 ohm resistor is connected to a 10 mH inductor across a 100 V, 50 Hz voltage source. Find the input current, voltage across resistor and inductor, power factor of the circuit and real power consumed in the circuit.
   How to answer

   • Calculate inductive reactance: $X_L = 2\pi(50)(0.010) = 3.1416\,\Omega$
   • Calculate total impedance: $Z = \sqrt{4^2 + 3.1416^2} = 5.086\,\Omega$
   • Determine circuit current: $I = 100 / 5.086 = 19.66\text{ A}$
   • Find $V_R = I \cdot R = 78.65\text{ V}$ and $V_L = I \cdot X_L = 61.77\text{ V}$
   • Compute power factor: $\cos\phi = 4 / 5.086 = 0.786\text{ lagging}$
   • Compute real power: $P = I^2 R = (19.66)^2 \cdot 4 = 1546.5\text{ W}$
7. 7 Marks
   aExplain Faraday's laws of electromagnetic induction.
   How to answer

   • Statement of Faraday's first law: whenever magnetic flux linking a circuit changes, an EMF is induced
   • Statement of Faraday's second law: magnitude of induced EMF is proportional to rate of change of flux linkages
   • Mathematical expression: $e = -N \frac{d\phi}{dt}$
   • Significance of the negative sign (Lenz's law)
8. 7 Marks
   bExplain the role of resistance, capacitance and inductance in an electric circuit.
   How to answer

   • Role of Resistance ($R$): opposes current flow, dissipates electrical energy as heat ($P=I^2 R$), $v=iR$
   • Role of Inductance ($L$): opposes changes in current, stores energy in magnetic field ($E = \frac{1}{2} L I^2$), $v = L \frac{di}{dt}$
   • Role of Capacitance ($C$): opposes changes in voltage, stores energy in electric field ($E = \frac{1}{2} C V^2$), $i = C \frac{dv}{dt}$
   • Phase relationships between voltage and current for each element in AC circuits
9. 7 Marks
   aExplain the working principle of a single-phase transformer.
   How to answer

   • Basic construction: primary winding, secondary winding, and laminated iron core
   • Principle of mutual induction: alternating current in primary produces alternating flux in the core
   • Induced EMF in primary ($E_1$) and secondary ($E_2$)
   • EMF equation and voltage transformation ratio: $\frac{E_2}{E_1} = \frac{N_2}{N_1}$
10. 7 Marks
    bIn a 25 kVA, 2000/200 power transformer the iron and copper losses are 350W and 400W respectively. Calculate the efficiency at full load.
    How to answer

    • Assume unity power factor ($\\cos\phi = 1.0$) or state assumed power factor (e.g., 0.8 lagging)
    • Calculate full load output power: $P_{out} = 25000 \times 1 = 25000\text{ W}$
    • Calculate total losses: $P_{losses} = P_i + P_{cu} = 350 + 400 = 750\text{ W}$
    • Calculate efficiency: $\eta = \frac{25000}{25000 + 750} \times 100\% \approx 97.09\%$ (at unity pf)
11. 7 Marks
    aExplain with schematic diagram different parts of D.C. machines.
    How to answer

    • Draw cross-sectional schematic diagram of a DC machine showing stator, rotor, and poles
    • Functions of stationary parts: Yoke/Frame, Poles, Field winding
    • Functions of rotating parts: Armature core, Armature winding, Shaft
    • Function of Commutator and Carbon brushes
12. 7 Marks
    bExplain the principle of three phase induction motor.
    How to answer

    • Generation of Rotating Magnetic Field (RMF) of constant magnitude ($1.5 \Phi_m$) rotating at synchronous speed $N_s = \frac{120f}{P}$
    • Cutting of RMF flux lines by stationary rotor conductors inducing EMF by Faraday's law
    • Production of rotor currents and resulting Lorentz force ($F = BIl$)
    • Rotor rotates in the same direction according to Lenz's law; definition of slip $s = \frac{N_s - N}{N_s}$
13. 7 Marks
    aDraw and explain the V-I characteristic of diode.
    How to answer

    • Sketch V-I characteristic curve showing forward and reverse bias quadrants
    • Forward bias operation: barrier potential reduction, knee/cut-in voltage ($0.7\text{ V}$ for Si, $0.3\text{ V}$ for Ge), exponential current rise
    • Reverse bias operation: reverse saturation current ($I_0$), minority carrier conduction
    • Breakdown mechanism (Zener/avalanche) at reverse breakdown voltage
14. 7 Marks
    bExplain the working principle of common base transistor.
    How to answer

    • Circuit diagram of BJT in common base (CB) mode (e.g. NPN or PNP)
    • Biasing conditions: emitter-base junction forward biased, collector-base junction reverse biased
    • Mechanism of charge carrier injection from emitter through thin base to collector
    • Relation between currents: $I_E = I_B + I_C$ and current gain $\alpha = I_C / I_E$
15. 14 Marks
    Write short notes on any two of the following: i) Logic gates ii) R-S flip flop iii) Compare electric and magnetic circuit iv) B-H curve
    How to answer

    • For Logic gates: define basic gates (AND, OR, NOT) and universal gates (NAND, NOR) with symbols and truth tables
    • For R-S flip flop: logic diagram using NAND/NOR gates, truth table, and states (set, reset, no change, invalid)
    • For Compare electric and magnetic circuit: tabular comparison of EMF vs MMF, current vs flux, resistance vs reluctance, Ohm's law vs Hopkinson's law
    • For B-H curve: plot of magnetic flux density $B$ vs magnetic field intensity $H$, hysteresis loop, retentivity, and coercivity