Sarswati Institute of Engineering & Management
Assignment: Chapter-1 (DC Circuits)
Subject: Elements of Electrical Engg.
1. State and explain factors affecting resistance of a conductor. ( Jan – 2010)
2. Define temperature co-efficient of resistance. How does the resistance of different
materials vary with temperature? Prove that Rt= R0(1 + α t), Rt2 = R t1 [1 + α1 (t2-t1)],
α t = α0/(1+ α0t) and α 2 = 1/ [(1/ α1) + (t2-t1)], where notation has usual meanings.
( June 2010, Dec – 2008 Jan – 2010, June – 2009, March – 2009)
3. Explain the method of transforming a star network of resistances into delta network and
vice versa. (Dec – 2008)
4. State and explain Ohm’s law, KCL and KVL. ( Jan – 2010, June – 2009, March – 2009,
Oct – 2009))
5. The resistance of tungsten filament of a lamp is 20 Ω at the room temperature of 20º C.
What is the operating temperature of the filament if the resistance temperature co-
efficient of tungsten is 0.005/º C at 20º C. The base of the lamp is marked 120 Volt, 50W.
(March – 2009)
6. The field winding of d.c. motor connected across 230 V supply takes 1.15A at room
temp. of 20°C. After working for some hours the current falls to 0.26 A, the supply
voltage remaining constant. Calculate the final working temperature of field winding.
Resistance temperature coefficient of copper at 20°Cis 1/254.5.
7. A piece of resistance wire, 15.6 m long and of cross-sectional area 12 mm2 at a
temperature of O°C, passes a current of 7.9 A when connected to d.c. supply at 240 V.
Calculate (a) resistivity of the wire (b) the current which will flow when the temperature
rises to 55°e. The temperature coefficient of the resistance wire is 0.00029Ω/Ω/0C.
8. The field winding of a generator has a resistance of 12.7 Ω at 180 C and 14.3 Ω at 50o C.
Find: (i) temperature coefficient of resistance at 0oC (ii) resistance at 0oC (iii)
temperature coefficient at 18o C ( Oct – 2009)
9. A lamp rated 100V, 75 W is to be connected across 230 V supply. Find the value of
resistance to be connected in series with the lamp. Also find the power loss occurring in
the resistance. ( Jan – 2010)
10. For the network shown below, determine the current supplied by the battery using star-
delta transformation.(All resistances are in ohms) (Dec -2008)
11. Calculate the current flowing through the 10 Ω resistor of circuit shown below, by using
any method. Values of resistors are in ohm. ( Jan -2010)
12. Find the resistance between the terminals A and B in the network shown below ( June
2010, June - 2009)
13. A carbon colour coded resistor has first ring of orange, second of blue, third of red and
fourth of golden colour. Find the specifications of the resistor. ( June 2010)
14. Draw Wheatstone’s bridge network ABCD as follows: Resistance between terminals A-
B, B-C, C-D, D-A and B-D are 10, 30, 15, 20 and 40 ohms respectively. A 2 volt battery
of negligible resistance is connected between terminals A and C. Determine the value and
direction of the current in the 40 Ω resistor. ( March – 2009)
15. Determine the current in 10 Ω resistance in the network shown below using star-delta
transformation
4Ω 34 Ω 30 Ω
8Ω 12 Ω 30 Ω 15 Ω
12 Ω 10 Ω 15 Ω
180 V
17. Determine the current by the source in the circuit shown below.
18. In the circuit shown below calculate (a) current I (b) current I1and (c) VAB .All resistances are
in ohms.
19. In the circuit show in Fig. calculate the value of the unknown resistance R and the current
flowing through it when the current in branch OC is zero.
20. Determine the branch currents in the network shown below when the value of each branch
resistance is in ohm.
Please submit this assignment by 4th
March, 2011.