BT-104 Basic Electrical & Electronics Engineering - Dec 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 the steps to solve the Thevenin's Theorem. An Electric iron is rated 1000W, 240V. Find the current drawn & resistance of the heating element.
   How to answer

   • Steps to find Vth, Rth, load connection for Thevenin's theorem
   • Power formula P = V*I for electric iron
   • Calculate current I = P/V = 1000/240
   • Calculate resistance R = V^2/P
2. 7 Marks
   bFind the current through $3 \,\Omega$ resistor using superposition theorem in given figure.

   How to answer

   • State superposition theorem and steps
   • Consider 20V alone with 40V shorted, find current in 3 ohm
   • Consider 40V alone with 20V shorted, find current in 3 ohm
   • Algebraically add components for total current
3. 7 Marks
   aExplain about Star and Delta connected three phase balanced circuits.
   How to answer

   • Star connection: line vs phase voltage/current relations
   • Delta connection: line vs phase voltage/current relations
   • Balanced load phasor and power expressions
   • Comparison diagram of both connections
4. 7 Marks
   bA series RLC circuit containing a resistance of $12 \Omega$, an inductance of $0.15 \text{H}$ and a capacitor of $100 \mu\text{F}$ are connected in series across a $100 \text{V}$, $50 \text{Hz}$ supply. Calculate the total circuit impedance, the circuits' current, power factor and draw the voltage phasor diagram.
   How to answer

   • Calculate XL = 2*pi*f*L and XC = 1/(2*pi*f*C)
   • Calculate impedance Z = sqrt(R^2+(XL-XC)^2)
   • Calculate current I = V/Z and power factor cos phi = R/Z
   • Draw voltage phasor diagram with VR, VL, VC
5. 7 Marks
   aDerive expression for the coefficient of: i) Self-inductance for a solenoid coil ii) Mutual inductance for a pair of closely wound solenoid coils
   How to answer

   • Define flux linkage and self-inductance derivation for solenoid
   • Expression L = mu0*N^2*A/l
   • Define mutual inductance for coupled solenoids
   • Expression M = mu0*N1*N2*A/l and coupling
6. 7 Marks
   bDiscuss some of the magnetization characteristics of ferromagnetic materials.
   How to answer

   • Domain structure and hysteresis concept
   • B-H curve, saturation, retentivity, coercivity
   • Permeability and hysteresis loss characteristics
   • Examples of soft vs hard ferromagnetic materials
7. 7 Marks
   aWith a neat circuit diagram and explain the construction and principle of operation of DC Motor.
   How to answer

   • Construction parts: yoke, poles, armature, commutator, brushes
   • Working principle: Fleming's left hand rule and back emf
   • Neat circuit diagram labelling parts
   • Operation steps with current direction
8. 7 Marks
   bA three phase $50 \text{Hz}$, 2 pole induction motor has a full speed of 2860 rpm. What is the (i) synchronous speed (ii) slip of the induction motor (iii) rotor emf frequency?
   How to answer

   • Synchronous speed Ns = 120f/P
   • Slip s = (Ns-Nr)/Ns
   • Rotor frequency fr = s*f
9. 7 Marks
   aDefine $\alpha$ and $\beta$ with respect to BJT and derive the relationship between them. For a transistor circuit having $\alpha = 0.98$, $I_{CBO} = 5 \,\mu\text{A}$ and $I_B = 100 \,\mu\text{A}$, find $I_C$ and $I_E$.
   How to answer

   • Define alpha = IC/IE and beta = IC/IB
   • Derive relation beta = alpha/(1-alpha)
   • Calculate IC using IE/IC/IB relations with ICBO
   • Calculate IE = IC+IB
10. 7 Marks
    bDraw the circuit of an S-R flip flop using NAND gates. Explain it.
    How to answer

    • Circuit using 2 cross-coupled NAND gates with S,R inputs
    • Truth table for set, reset, hold and forbidden states
    • Explanation of active-low operation
    • Advantages / memory function
11. 7 Marks
    aDetermine the equivalent resistance between terminals A and B in the given figure.

    How to answer

    • Identify series-parallel resistor network between A-B
    • Combine series branches and parallel 10 ohm, 20 ohm etc.
    • Stepwise reduction to single equivalent
    • Final Rab value
12. 7 Marks
    bWhen three balanced impedances are connected in delta across a 3 phase 400 V, 50 Hz supply the line current drawn is 20 A at a lagging p.f. of 0.3. Determine the value of the impedance connected in each phase.
    How to answer

    • Delta line current Il = sqrt(3)*Iph relation
    • Calculate phase current Iph = Il/sqrt(3)
    • Phase impedance Zph = Vph/Iph with Vph=Vl for delta
    • Express as magnitude and angle from p.f. 0.3 lagging
13. 7 Marks
    aState and Prove Demorgan's theorem.
    How to answer

    • State two De Morgan's laws with Boolean expressions
    • Truth table proof for both laws
    • Logic gate equivalence diagrams
    • Application in digital simplification
14. 7 Marks
    bExplain about open circuit and short circuit test.
    How to answer

    • Purpose and connection of OC test for iron losses
    • Purpose and connection of SC test for copper losses
    • Circuit diagrams and measurement procedure
    • Parameter determination from test data
15. 14 Marks
    Write technical short note on any **two** of the following: i) Voltage and current sources ii) Applications of DC machine iii) Half and Full adder circuits
    How to answer

    • Select any two topics from the three options
    • Give definition, diagram/circuit and working for each
    • Mention applications and key points
    • Concise technical presentation