UNIVERSITY OF TECHNOLOGY
ELECROMECHNICAL DEPARTMANT
SYSTEMS BRANCH
FIRST YEAR ELECTROMECHNICAL
ENGINEERING
BASICS ELECRICAL ENGINEERING
Dr. Sameir Abd Alkhalik Aziez
Question Sheet (1)
1.Give the dimensions (units) for each of the fallowing quantities :- rate of
flow of charge ; rate of loss of energy; rate of gain of energy; rate of doing
work; energy loss per unit charge; energy gain per unit charge; work per unit
charge.
2. Find, the current resulting from the "steady flow of (a) 90 C in 6 s; (b) 900
C in 20 minutes; and (c) 4x1023 electrons in 5 hours. (15 A, 0.75 A, 3.56 A)
3. The current through a given section is steady at 30 pA. How long does a
charge of 12 μC take to cross the section? (111 hours)
4. A light bulb draws a steady current of 0.5 A; what is the charge transported
through the bulb in 1 minute? (30 C)
5. A 2000 Kg lift rises 50 m in 25 s; what is the average power? (39.2 KW)
6. Find the voltage between points a and b if 24 J are required to move, from a
to b, a charge of (i) 3 C; (ii) -4 C; (iii) 20 x 1019 electrons.
(8 V, a low;
6 V, a high; 0.749 V, a high)
7. What is the current drawn by a 220 V,
60 W light bulb? What is the
energy it consumes in 1 s? In 1 minute? In 1 hour?
(272.7 mA; 60 J; 3.6 KJ; 216 KJ)
-1-
8. A 6220 V, 60 W light bulbs is switched on for 5 hours every day. What is
the monthly cost if the electrical rate is 0.01 ID/KWh (Iraqi dinare per kilo
watt – hour)? (0.09 ID = 90 fils )
9. Find the voltage across a light bulb if a 0.5 A current flowing through it for
4 s causes the light bulb to give off 480 J of light and heat? (240 V)
10. Batteries are usually rated by their voltage and their Ampere – hour: a 12
V, 45 Ah car battery can deliver 4.5 A for 10 hours, or 9A for 5 hours, or 15 A
for 3 hours, etc. What is the energy stored in this battery? (1.9 MJ)
11. A small transistor radio works at 6 V , and draws a current of 20 m A . It is
supplied by 4 batteries, each rated at 1.5 V, 80 mAh. (i) How many joules can
the batteries supply ? (ii) what is the average power consumed by the radio ?
(iii) how long can the radio be operated ? (1.728 KJ; 120 mW; 4 hours)
12. How long does it take a 100 W electric light to consume energy worth 5
ID if the electric rate is 0.015 ID/KWh ? How long does it take a 2 KW
heater ? (3333 hours = 140 days ; 167 hours = 7 days)
13. If the electric rate is 20 fils/KWh, how many joules does a dinar buy ?
(180 MJ)
14. A metal sphere is moved at a uniform speed a distance of 8 m. The sphere
is charged with 250 μC, and its motion is opposed by a uniform force of 450
nN. Find the energy gained by the charge, and the potential difference
between the initial & final points.( 3.6 μJ ; 14.4 mV )
-2-
15. Complete the following table using suitable units :resistance
current
voltage
power
1
?
4A
12 V
?
2
20 ohms
3.8 A
?
?
3
4700 ohms
?
7.3 V
?
4
?
6.3 A
240 V
?
5
?
42 μA
8.4 V
?
6
4.7 M - ohms
?
140 V
?
7
22 K – ohms
?
180 V
?
8
50 K – ohms
2.1 mA
?
?
9
2.2 M – ohms
?
17 KV
?
10
35 ohms
91.5 A
?
?
11
?
31 KA
240 V
?
12
?
30 A
?
72 KW
13
?
500 A
75 mV
?
14
0.33 M – ohms
?
?
2W
15
?
?
220 V
75 W ( domestic lamp )
?
12 V
5 W ( car lamp )
16
16. A 330 ohm resistor is rated at 0.5 W. What is the maximum allowable
voltage ?
(12.85 V)
17. A 3.3 MΩ resistor is rated at 2.5 mW. What is the maximum allowable
current? voltage? (27.5 μA; 90.8V)
18. A wire is 3 Km long and has a radius of 4.5 mm. The resistivity is 1.6
μΩcm , Find the resistance of the wire.
-3-
(755 mΩ )
19. A 100 m aluminum cable carries a current of 30 A at 20 °C. It is required
that the voltage drop along the wire does not exceed 6 V. What is the
minimum allowable cross-sectional area of the wire? (14.15 mm2)
20. A cable transports power from a generator to a factory 80 m away. The
factory load is 200 A. The cable core has o resistivity of 1.724 μΩcm , and a
diameter of 10 mm. Find the voltage drop in the cable. (7.0 V)
21.A coil consists of 200C turns of wire having cross–sectional area 0.8 mm2,
and resistivity 0.02 μΩm at the working temperature. The mean length per
turn is 80 cm. calculate the power dissipated in the coil when it is connected to
a 110 V dc supply. ( 302.5 W )
22. A coil of copper wire is 200 m long. A coil of aluminum wire has the
same resistance and cross–sectional area; what is its length? (121.6 m)
23. A two-wire distributor is 400 m long. It is fed from one end at 240 V.
There is a load of 200 A at a distance of 250 m from the feeding end, and a
load of 160 A at the far end. The total voltage drop must not exceed 4 %.
The resistivity of the conductors is 2 micro-ohm-cm at the working
temperature. Find the minimum allowable cross-sectional area of the wires?
(475 mm2 )
24. If the distributor of question 23 works 24 hours a day every day of the
year, find the yearly cost of the energy loss,
assuming an electrical rate of
0.01 ID/K.Wh. How can this cost be decreased at the design stage ? (267 ID)
25. A cylindrical iron rod is 66 cm long; its diameter is 3 mm. Find the
resistance at a temperature of 72o C.
(1.654 milli-ohms)
-4-
26. A coil of copper wire has a resistance of 38 ohms at 28 °C. Find its
resistance at 75 °C & at -15 °C.
(44.8 ohms; 31,8 ohms)
27. The voltage drop across a 500 cm long copper conductor is 400 mV. The
cross-sectional area is 2.5 mm2, and the temperature is 65 oC. Find the power
consumed in the conductor. (3,95 W )
28. A 2.5 mm2 cable is 500 m long. It is connected to a 200 V supply at one
end, and short-circuited at the other end. The resistivity is 2.60 micro-ohm-cm
at 0 oC, and 2.83 micro-ohm-cm at 20 oC. Find the short-circuit current if the
coble temperature is 50 oC.
(15.75 A)
29. The voltage and current in a copper conductor at 20 °C are 240V and 4.8A
respectively. Find the conductor temperature when it is consuming 900 W at
the same voltage. (91.3 oC)
30. A wire is 120cm long; its cross-sectional area is 1.5 cm2. The conductivity
is 32*106 mho/m at 75 oC , and 40*l06 mho/m at 35 oC. Find the resistance
of the wire at 50 °C. (21.9 milli-ohms)
31.The resistance of a coil was found to be 36 milli-ohms at a temperature of
80 oC , and 29 milli-ohms at 15 oC. At what temperature will the resistance be
32 milli-ohms?
(42.9 oC)
32. A conductor is 160 cm long; its cross-sectional area is 0.8 mm2. The
conductivity is 50*106 mho/m at 60 oC;
the temperature coefficient of
resistance is 3.4*l0-3 (°C)-1, also at 60 oC. Find the resistance at 85 °C.
(43.4 milli-ohms)
-5-
33. Constantan wire of 0.2 mm diameter is wound around a plastic cylinder of
1.0 cm diameter to make a wire-wound resistor. How many turns are needed
to obtain a resistance of 50 ohms if the resistivity is49 micro-ohm-cm?
(102 turns)
34. A voltage of 1.5 V is applied to a 7.5 KΩ, ± 5 % resistor. What is the
expected current? ( 190.5 – 210.5 μA )
35. A generator has an emf of 115 V, and an internal resistance of 0.1 ohms.
Find the terminal voltage when it supplies a current of 50 A. (110 V)
36. A current source has o constant current of 5 A, and an internal resistance
of 2 ohms. Find the terminal current when the voltage is 4 V. (3 A)
37. A battery has an open circuit terminal voltage of 28 V, and a short-circuit
current of 200 A. Find the load current when the terminal voltage is 18 V.
(71.4 A)
38. The terminal voltage of a d.c. source was found to be 150 V when it was
supplying a load resistor of 25 K-ohms. The short-circuit current is 10 mA.
Find the terminal voltage and current for a load resistor of 35 K-ohms.
(181 V, 5.17 A)
39. The no load voltage of a small battery is 1.5 V. When a 2 K-ohm resistor
is connected across the battery terminals, the current is 0.4 mA. Find the load
voltage when a 900 ohm resistor is connected to the battery. (0.51 V)
40. a d.c. source was tested, and was found to supply 3 A at 120 V, and 5 A at
100 V. What current will it supply to a 65 ohm resistor?
-6-
(2 A)
41. A d.c. source was tested, and was found to supply 0.93 A to a 33 ohm
load, and 0.55 A to an 82 ohm load. Find the voltage that would appear across
a 63 ohm load resistor.
(41.17 V)
42. When connected to a certain load resistor, the terminal voltage and current
of a d.c. source were found to be 150 V and 2 A reactively. The open circuit
voltage of the source is 200 V. Determine the terminal voltage and current
when the source is loaded by a 50 ohm resistor.
(133.3 V, 2.67 A)
43. When a voltmeter is connected across a given battery, it reads 2.5 V.
When an ammeter is connected across the battery, it reads 500 mA. The
resistance of the voltmeter is 0.2 K-ohms, and the resistance of the ammeter
is 300 m-ohms. Find the battery parameters: E, Io, Ro, and Go.
(2.56 V, 531 mA, 4.82 ohms, 207 m-mho)
44. In the figure shown, when the switch is in position 1, the ammeter reading
is 2.5 A; when in position 2, the voltmeter reading is 150 V.
Find the source
parameters E, Io, Ro, and Go if (a) the two meters are ideal, and if (b) the
ammeter and voltmeter resistances are 3 ohms and 2 K-ohms respectively.
(Ro = 60 ohms, 58.76 ohms)
-7-