GUJARAT TECHNOLOGICAL UNIVERSITY
B.E. Sem- 1st Regular Examination January 2011
Subject code: 110005 Subject Name: Elements of Electrical Engineering Date: 07/ 01 /2011 Time: 10.30 am - 01.00 pm Total Marks: 70
1. Attempt all questions. 2. Make suitable assumptions wherever necessary. 3. Figures to the right indicate full marks. Q.1 Three resistances Rab, Rbc and Rca are connected in star. Obtain their equivalent for delta resistances. (b) A 100V, 60W bulb is connected in series with a 100V, 100W bulb and the combination is connected across the 200V mains. Find the values of resistance that should be connected across the first bulb, so that each bulb may get proper current at the proper voltage. (a) Compare electric circuit and magnetic circuit by their similarities and dissimilarities. (b) Derive an expression for the energy stored in an inductor of self inductance ‘L’ henry carrying the current of ‘I’ amperes. OR (b) A mild steel ring of 30 cm mean circumference has a cross-sectional area of 6 cm2 and has a winding of 500 turns on it. The ring is cut through at a point so as to provide an air-gap of 1 mm in the magnetic circuit. It is found that a current of 4 A in the winding, produces a flux density of 1 T in the air-gap. Find (i) the relative permeability of the mild steel and (ii) inductance of the winding. ( µ0 = 4π ×10−7 ) (a) State coulomb’s law of electrostatics. (b) Define capacitance. Derive an expression for the total capacitance of a group of capacitance when (i) they are connected in series (ii) they are connected in parallel. (c) A series combination having R = 2 M Ω and C = 0.01 µF is connected across d.c. voltage source of 50V. Determine capacitor voltage and charging current after 0.02 s, 0.04 s and 0.06 s. OR (a) Explain the variations of watt-meter readings for 3-phase power measurement by two watt-meter method as power factor takes the values of unity, 0.5, between 0.5 & 0 and...