Download GTU BE/B.Tech 2019 Winter 1st And 2nd Sem New And Spfu 2110005 Elements Of Electrical Engineering Question Paper

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GUJARAT TECHNOLOGICAL UNIVERSITY

SEMESTER- I & II (NEW) EXAMINATION — WINTER 2019

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Subject Code: 2110005 Date: 11/01/2020

Subject Name: Elements of Electrical Engineering

Time: 10:30 AM TO 01:00 PM Total Marks: 70

Instructions:

1. Question No. 1 is compulsory. Attempt any four out of remaining Six questions.

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2. Make suitable assumptions wherever necessary.
3. Figures to the right indicate full marks.

Q.1 (a) Objective Question (MCQ) Mark 07

1. A circuit contains two unequal resistances in parallel
   [a] current is same in both
   

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   [b] large current flows in larger resistance
   [c] potential difference across each is same
   [d] smaller resistance has smaller conductance
2. Which medium has least dielectric strength?
   [a] Air [b] Quartz [c] Glass [d] Paraffin wax

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3. Which of the following is a vector quantity?
   [a] Magnetic potential [b] Magnetic field intensity
   [c] Flux density [d] Relative permeability
4. If the current and voltage are out of phase by 90°, the power is
   [a] Minimum [b] Maximum [c] Zero [d] 1.1 *V*I

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5. In ac circuits, the ac meters measure
   [a] RMS values [b] Peak values
   [c] Mean value [d] Mean square value
6. Which of the following statement is not correct?
   [a] A primary cell is an electrochemical cell
   

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   [b] After charging, a primary cell can be again put to use
   [c] Dry cell is a primary cell
   [d] Leclanche cell is used in experiments, where constant supply of current is not needed.
7. Candela is the unit for
   [a] Light flux [b] Brightness
   

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   [c] Luminous efficiency [d] Luminous intensity

(b) 07

1. A resistor is to be connected across a 45 V battery to provide 1 mA of current. The required resistance with a suitable wattage rating is
   [a] 45 O, 10W [b] 4.5 O, 2 W
   [c] 450 kO, 20W [d] 45 kO, ½ W

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2. The capacitance reactance of an ac circuit containing 35uF capacitance at a frequency of 60 Hz will be
   [a] 50 O [b] 60 O [c] 75 O [d] 100 O
3. A property of a material which opposes the creation of magnetic flux in it is known as
   [a] magneto motive force [b] reluctance
   [c] permeance [d] reluctivity

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4. The number of cycles that an alternating current (AC) completes in one second, will be
   [a] 1/1000 Hz [b] 50 Hz [c] 100 Hz [d] 1000 Hz
5. In an ac source R=36 O, frequency = 50 Hz and L = 0.12 H, then phase difference between current and voltage is nearly
   [a] 90° [b] 60° [c] 45° [d] 75°
6. In a lead acid battery, during charging
   

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   [a] anode becomes whitish in colour [b] voltage drops
   [c] specific gravity of acid increases [d] the cell gives out energy
7. In a fluorescent tube circuit, choke acts as
   [a] starter [b] power factor improving device
   [c] source of heat [d] current limiting device

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Q.2 (a) Two wires A & B made up of same material, wire B has twice the length of wire A and having half the diameter to that of A. Calculate the ratio Rb/Ra. 03

(b) Why domestic appliances are connected in parallel? Give comparison with series circuit? 04

(c) Three resistances Ra, Rb and Rc are connected in delta. Obtain expressions for their equivalent star resistances. 07

Q.3 (a) State and explain Coulomb’s laws. 03

(b) Discuss B-H curve of a ferro-magnetic material and explain the following. (i) Magnetic saturation (ii) Hysteresis (iii) Residual Magnetism (iv) coercive force. 04

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(c) A capacitor of 0.1 µF is charged from a 100 V battery through a series resistance of 1 kO. Find 07

1. The time for the capacitor to receive 63.2% of its final charge.
2. The charge received in this time
3. The final rate of charging
4. The rate of charging when the charge is 63.2% of the final charge.

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Q.4 (a) Give comparison between magnetic and electric circuits. 03

(b) Derive an expression for the equivalent capacitance of a group of capacitors when they are connected (i) in parallel (ii) in series. 04

(c) A ring composed of three sections. The cross-sectional area is 0.001 m² for each section. The mean arc length are l_a = 0.3 m, l_b = 0.2 m, l_c = 0.1 m. An air-gap length of 0.1 mm is cut in the ring. µ_r for sections a, b, c are 5000, 1000 and 10,000 respectively. Flux in the air gap is 7.5 X 10^-4 Wb. Find (i) mmf (ii) exciting current if the coil has 100 turns. (iii) Reluctance of the sections. 07

Q.5 (a) Define: (i) r.m.s. value (ii) Form factor (iii) Amplitude factor 03

(b) Distinguish between (i) apparent power (ii) active power and (iii) reactive power in ac circuits. 04

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(c) What is resonance in AC circuit? Elaborate resonance in case of series R-L-C circuit with the help of phasor diagram and sketch the resonance curve. 07

Q.6 (a) List out the merits of two watt meter method. 03

(b) A balanced 3-phase, star connected load is connected across 400 V, 50 Hz supply. The line current is 25 A. Power factor of load is 0.707 (lag). Assuming that the system is 3-wire and power is measured using two wattmeters, find the readings of wattmeters. 04

(c) Obtain the relationship between line and phase values of current in a three phase, balanced, delta connected system. 07

Q.7 (a) List out the different types of illumination schemes. 03

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(b) Discuss the Lead acid battery with charging & discharging equations. 04

(c) Define term ‘earthing’. State all the methods of earthing and explain any one of them. 07

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