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UNIVERSITY OF TECHNOLOGY
ELECROMECHNICAL DEPARTMANT
SYSTEMS BRANCH
Dr. Sameir Abd Alkhalik Aziez
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) 4x10
23
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 10
19
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)
FIRST YEAR ELECTROMECHNICAL
ENGINEERING
BASICS ELECRICAL ENGINEERING
Question Sheet (1)
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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 )
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15. Complete the following table using suitable units :-
resistance
current
voltage
power
1
?
4 A
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
?
?
2 W
15
?
?
220 V
75 W ( domestic lamp )
16
?
12 V
5 W ( car lamp )
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. (755 mΩ )
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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 mm
2
)
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 mm
2
,
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 mm
2
)
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 72
o
C. (1.654 milli-ohms)
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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 mm
2
, and the temperature is 65
o
C. Find the power
consumed in the conductor.
(3,95 W )
28. A 2.5 mm
2
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
o
C, and 2.83 micro-ohm-cm at 20
o
C. Find the short-circuit current if the
coble temperature is 50
o
C. (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
o
C)
30. A wire is 120cm long; its cross-sectional area is 1.5 cm
2
. The conductivity
is 32*10
6
mho/m at 75
o
C , and 40*l0
6
mho/m at 35
o
C. 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
o
C , and 29 milli-ohms at 15
o
C. At what temperature will the resistance be
32 milli-ohms? (42.9
o
C)
32. A conductor is 160 cm long; its cross-sectional area is 0.8 mm
2
. The
conductivity is 50*10
6
mho/m at 60
o
C; the temperature coefficient of
resistance is 3.4*l0
-3
(°C)
-1
, also at 60
o
C. Find the resistance at 85 °C.
(43.4 milli-ohms)
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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?
(2 A)
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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, I
o
, R
o
, and G
o
.
(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, I
o
, R
o
, and G
o
if (a) the two meters are ideal, and if (b) the
ammeter and voltmeter resistances are 3 ohms and 2 K-ohms respectively.
(R
o
= 60 ohms, 58.76 ohms)