Table of Contents

## WBUT Question Papers CS

## Basic Electrical & Electronics Engineering – I B Tech

## Sem 1st

PART – I (Marks – 35)

GROUP – A (Multiple Choice Type Questions)

- Choose the correct alternatives for any five of the following:

i) What percentage of the maximum power is delivered to a load if load resistance is 10 times greater than the Thevenin resistance of the source to which it is connected?

a) 25% c) 35.39%

b) 40% d) 33.06%

ii) Kirchhoff’s voltage law is used for

a) loop analysis c) finding out equivalent resistance

b) node analysis d) none of these

iii) Hysteresis loss in a transformer can be reduced by using

a) laminated core c) oil

b) silicon Steel core d) none of these

iv) For a coil with N-turns the self-inductance will be proportional to

a) N c) N^{2}

b) 1/N d) 1/N^{2}

v) In series R-L-C circuit, the power factor at resonance is

a) unity c) 0.5

b) zero d) 0.75

vi) If the peak value of a sine wave is 100 volts, then its rms value will be

a) 70.7 V c) 100 V

b) 63.6 V d) 88 V

GROUP – B (Short Answer Type Questions)

Answer any two of the following –

- State Biot-Savart law applicable to electro-magnetism.
- Determine the current through the 15 Q resistor in the given network by Norton’s Theorem.

ion

————– VyV—————- ■*

- An air-coiled toroidal coil has 450 turns and a mean diameter of 300 mm and cross-sectional area of 300 mm
^{2}. Determine the self-inductance of the coil and the average voltage induced in it when a current of 2 A is reversed in 40 ms.

GROUP -C (Long Answer type Questions)

Answer any two of the following –

- a) The equation of an alternating current i = 62.35 sin 323t A. Determine its

i) | maximum value |

ii) | frequency |

iii) | rms value |

iv) | average value |

v) | form factor |

b) Define phasor diagram, the phenomenon of resonance in a circuit containing an inductance, a capacitor and a resistor is series.

- a) Define self and mutual inductance. What do you mean by co-efficient of coupling?

b) A conductor having a length of 80 cm is placed in a uniform magnetic field of 2 Wb/m^{2} (Tesla). If the conductor moves with a velocity of 50 msec^{-1}, find the induced e.m.f. when it is (i) at right angle (ii) at an angle of 300 and (iii) parallel to the magnetic field.

8. a) Find the current through 5Q resistor using Thevenin’s theorem in figure below:

b) A horse-shoe electromagnet is required to lift a load of 200 kg. The electromagnet is wound with 500 turns and the length of the magnetic path is 50 cm and the cross-section of each arm is 25 cm^{2}. Find the current in the coil.

USE SEPARATE ANSWER-BOOK TO ANSWER PART – II QUESTIONS.

State the assumption. [^_{r} = 400]

PART – II (Marks – 35)

GROUP – A (Multiple Choice Type Questions)

- Choose the correct alternatives for any five of the following:

i) If the filtered load current is 10 mA, which of the following has a current of 10 mA?

a) Half-wave rectifier c) Bridge Rectifier

b) Full wave rectifier d) Impossible to say

ii) The density of majority carriers in a p-type semiconductor depends on the

a) donor impurity atom concentration

b) intrinsic atom concentration

c) doping technique

d) acceptor impurity atom concentration

iii) Avalanche breakdown is primarily depends on the phenomenon of

a) particle collision c) ionization

b) impurity doping d) direct rupture of covalent bond

iv) Transistor having high input impedance and low output impedance is in

a) CE mode c) CC mode

b) CB mode d) none of these

v) P and a of a BJT are related as

a) a = P + 1/P c) P = a / 1 + a

b) P = a / 1 – a d) a = P / P – 1

vi) When the temperature changes, the Q-point is shifted due to

a) changes in Icbo

b) changes in VCC

c) changes in the value of circuit resistance

d) none of these

GROUP -B (Short Answer Type Questions)

Answer any two of the following –

- Compute the value of R
_{e}in figure shown below, if the stability factor (w.r.t. I_{CO}) is equal to 4.5. Given that V_{CC}= 10V, V_{CE}= 4.5V, V_{BE}= 0.7V, R_{1}= 22kQ, R_{2}= 4.7kQ, P of the transistor = 49. Assume I_{C}to be approximately equal to I_{E}.

- What is your concept about an ideal diode? How does it differ from an actual one?
- Why do the carriers in a semiconductor diffuse? Obtain expressions for hole and electron diffusion currents and define diffusion constants.
- What is the position of the Fermi Level in an intrinsic semiconductors? How does its position changes when i) donors ii) acceptors are added to semiconductors?
- Explain Ebers Moll model for an ideal P-N-P transistor.

GROUP -C (Long Answer Type Questions)

Answer any two of the following-

- a) What do you mean by band gap of a semiconductor?

b) Describe the physical mechanism for Avalanche breakdown.

c) Why does a pure semiconductor behave like an insulator at absolute zero?

- a) Establish the expressions for current gain, voltage gain, input resistance, output resistance of CE amplifier of CE amplifier in terms of h parameter.

b) Determine the values of I_{B}, I_{C}, and V_{CE} for the circuit shown below. Take P = 50 for silicon transistor. Also find the stability factor for this circuit.

- a) A silicon diode having resistance RF = 30 Q is used for half wave rectification. The input ac voltage is V
_{i}= 6 sin wt volt and load resistance 500 Q. Find (i) dc output voltage (ii) ac input power and (iii) efficiency of the rectifier.

b) Explain physically how a p-n junction diode can be used as a rectifier.