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Tuesday, April 19, 2011

DC ELECTRIC CIRCUITS - Day 4

Question 1: EE Board April 1981, EE Board October 1984
     Two (2) 115-V incandescent lamps A and B are connected in series across a 230-V source. If lamp A is rated 75 watts and lamp B is rated 50 watts, determine the current drawn by the series connection.
A. 0.52 A
B. 0.64 A
C. 0.48 A
D. 0.57 A


Question 2:
     An arc lamp takes 10 A 50 volts. A resistance R is to be place in series so that the lamp may burn correctly from a 110 V supply. Find the power wasted in this resistor.
A. 800 watts
B. 600 watts
C. 700 watts
D. 900 watts


Question 3:
     Conductor “x” of a certain material and a given cross section has a resistance of 0.1 ohm per meter and a temperature coefficient of 0.005 per 0C. Conductor “y” of another material and a given cross section has a resistance of 0.5 ohms per meter and a temperature coefficient of 0.001 per 0C. Is desired to make a coil having a resistance of 500 ohms and a temperature coefficient of 0.002 by using suitable lengths of the two wires connected in series. Calculate the required length of wire “x”
A. 1225 m
B. 1250 m
C. 1240 m
D. 1210 m


Question 4: EE Board April 1997
     If a resistor rated at 5 watts and 6 volts are connected across a battery with an open circuit voltage of 6 volts. What is the internal resistance of the battery if the resulting current is 0.8 A?
A. 0.30 ohm
B. 0.26 ohm
C. 0.23 ohm
D. 0.03 ohm


Question 5:
     A high voltage DC transmission line delivers 1000 MW at 500 kV to an aggregate load over a distance of 900 km. Determine the voltage at the sending end. Assume the loop resistance of the line to be 1mΩ per km.
A. 510.5 kV
B. 505.2 kV
C. 507.7 kV
D. 503.6 kV


Question 6: EE Board April 1989
     It is required that a loading of 3 kW be maintained in a heating element at an initial temperature of 20 0C, a voltage of 220 V is necessary for the purpose. After the element has settled down to steady state, it is found that a voltage of 240 volts is necessary to maintain the 3 kW loading. The element resistance temperature coefficient is 0.0006 per degree centigrade at 20 0C. Calculate the final temperature of the heating element.
A. 345.43 0C
B. 326.42 0C
C. 336.84 0C 
D. 318.48 0C


Question 7:
     A 200-W, 110-V incandescent lamp has a filament having a temperature coefficient of resistance equal to 0.005 at 0C. If the normal operating temperature of the bulb is 2500 0C, how much current will the bulb draw at the instant it is turned on. Assume a room temperature of 20 0C.
A. 29.42 A
B. 18.37 A
C. 22.31 A
D. 28.21 A


Question 8:
     The power drawn by a resistive coil made of copper wire is 220 W at 100 V and 20 0C. Calculate the power consume by the  coil at 115 V and 100 0C. The temperature coefficient of resistance at 20 0C is 0.00393.
A. 175 W
B. 211 W
C. 183 W
D. 225 W


Question 9:
     An electric water heater has a rating of 1 kW, 230 V. The coil used as the heating element is 10 m long and has a resistivity of 1.724 x 10-6 ohm-cm. Determine the required diameter of the wire in mils?
A. 2.43 mils
B. 2.52 mils
C. 3.21 mils
D. 1.35 mils


Question 10:
     When two resistors A and B are connected in series, the total resistance is 36 ohms. When connected in parallel, the total resistance is 8 ohms. What is the ratio of the resistance Ra to resistance Rb . Assume Ra <Rb.
A. 0.5
B. 0.4
C. 0.8
D. 0.6

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