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# First Term Examination

Second Semester [B.Tech] February 2004

 Paper Code : ETES – 112 Subject : Electrical Science
 Time : 1:30 Hours Maximum Marks : 30
 Note : Attempt Q.1 and any 2 more questions.
 Q1 ( a ) Define temperature coefficient of resistance and write name of two materials having negative temperature coefficient of resistance. 1 ( b ) Determine RAB of the infinite ladder network shown below if R1=2 ohm and R2=3 ohm. ``` R1 R1 R1 A----^v^v^----+----^v^v^----+----^v^v^----+ - - - - - - - | | | < < < > > > ----infinity < R2 < R2 < R2 > > > | | | B-------------+-------------+-------------+ - - - - - - -``` 2 ( c ) In circuit shown below for R=20 ohm the current I is 2 A. Find the current I when R=100 ohm. ``` +-------+ __ I +-------+ | N1 |-----(->)-----^v^v^v^-----+----->----| N2 | +-------+ 4A R | +-------+ < > 20 ohm * __ < (->) = current source | ^v^v^v^ = resistance | |(Earth)|``` 1 ( d ) Write the limitations of superposition theorem. 1 ( e ) A current i=10+20 sin (wt) is flowing through 10 ohm resistance. Calculate the energy dissipated by the resistance. 1 ( f ) Define power factor and write its significance in AC circuits. 1 ( g ) In the circuit shown below, VAB = 3 V when R=infinite and current I is 3A when R=Zero. Find current I if R=2 ohm. 2 ( h ) Why a Wheatstone bridge cannot be used for precision measurement of low resistances? 1
 Q2 ( a ) Derive the equations for balance in case of Maxwell's inductance-capacitance bridge. Draw the phasor diagram for balance conditions. 4 ( b ) Determine the current flowing through 6 ohm resistance in circuit shown below using Thevenin's Theorem. 6
 Q3 ( a ) Calculate the average and R.M.S. Value of a clipped full rectified sine wave, shown below: ``` ^ | | Em + ... ... | . . . . | ._______. ._______. Em/1.414 + / \ / \ | / \ / \ | / \ / \ | / \ / \ |/ \/ \ ----+-----+--------+----+----+-------+-----+--------> 0| pi/4 3pi/4 pi 5pi/4 7pi/4 pi |``` 3 ( b ) A coil having a resistance of 50 ohms and an inductance of 0.2 H is connected in series with a capacitor. The current is maximum when frequency is 50 Hz. Calculate the capacitance of the condenser to be connected in parallel with the above combination so that the total current drawn from the mains is minimum when frequency is 100 Hz. Draw the phasor diagram, representing all the currents at 100Hz. Take supply voltage as reference. 5 ( c ) Derive an expression for dynamic impedance in parallel resonance. 2
 Q4 ( a ) In the circuit shown in figure, the same current flows through each element. If Vx=1V and Vr=9V, compute : the power absorbed by element A power supplied by each of the two sources Does the total power supplied = total power absorbed? Is your finding reasonable? Why or Why not? ``` + +------+ - +------------| A |----------+ | +------+ | | Vr | | | Vx --- + / \ - - PD=8 V < | > Current Source I= 5*Vx | \v/ | | | | +------------------------------+``` 3 ( b ) In the circuit shown below V1=40V when R=10 ohms. Find V2 when R is zero. ``` 10 ohm V1 R V2 10 ohm +---^v^v^v^---+---^v^v^v^---+---^v^v^v^---+----+ | | | | | 60V --- + < < < | - - > 20 ohm > 20 ohm > (^) Current source 5A | < < 20 ohm < | | | | | | +-------------+-------------+-------------+----+``` 3 ( c ) A filament of 60 W, 220V, electrical bulb has 2000o C working temperature. Calculate current flowing through the filament at: Working Temperature Room Temperature (30o) Assume temperature coefficient of resistance 0.0045 peroC at room temperature. 4