School of Electrical & Electronic Engineering
EEE105/3: Circuit Theory I
Tutorial #1
Chapter 1: Basic Concepts
Question 1
How many electrons pass a fixed point in a 100-watt light bulb in 1 hour if the applied constant voltage is 120 V?
Question 2
The average current in a typical lightning thunderbolt is 2 × 104 A and its typical duration is 0.1 s (Williams, 1988). The
voltage between the clouds and the ground is 5 × 108 V. Determine the total charge transmitted to the earth and the
energy released.
(Reference: Williams, E. R. ‘The Electrification of Thunderstorms’, Scientific American, Nov. 1988. pp. 88 – 89.)
Question 3
A unit of power used for electric motors is the horsepower (hp), equal to 746 watts. How much energy does a 5-hp
motor deliver in 2 hours? Express the answer in MJ.
Question 4
A conductor has a constant current of 5 amperes. How many electrons pass a fixed point on the conductor in 1 minute?
Question 5
A certain circuit element has the current and voltage as follows:
𝑖 = 10𝑒 −5000𝑡 A
𝑣 = 50(1 − 𝑒 −5000𝑡 ) V
Find the total energy transferred during 𝑡 ≥ 0𝑠.
Question 6
The voltage and current at the terminals of an automobile battery during a charge cycle are shown in Figure 1. Based
on that figure:
(i)
Calculate the total charge transferred to the battery.
(ii)
Calculate the total energy transferred to the battery.
v (V)
12
9
15
t (ks)
i (A)
15
10
6
4
12
15
t (ks)
Figure 1
Question 7
The voltage and current at the terminals of an electrical circuit element is given by:
𝑣 = 𝑡(1 − 0.025𝑡)𝑉, 0 < 𝑡 < 40𝑠;
𝑖 = 4 − 0.2𝑡 𝐴, 0 < 𝑡 < 40𝑠
Find:
(i)
At what time is the power being delivered to the circuit element maximum?
(ii)
What is the maximum power delivered?
(iii)
At what time is the power being extracted from the circuit element maximum?
(iv)
What is the maximum power being extracted?
Question 8
A model of circuitry that distributes power to a typical home is shown in Figure 8 with voltage polarities and current
directions defined for all circuit components (i.e. a to h). The results of circuit analysis give the voltage and current
values for all circuit components, which are summarized in Table 8.
(a)
Propose one analysis or method to determine whether the values given are correct.
(b)
By using the proposed analysis or method in (a), determine whether the values given are correct.
(c)
If there is incorrect value, identify ONE component that was possibly given with incorrect voltage polarities or
current direction. Then, suggest a correct voltage polarities or current direction.
Figure 8
Table 8
Component
Voltage, 𝑣 (V)
Current, 𝑖 (A)
a
120
-10
b
120
9
c
10
10
d
10
1
e
-10
-9
f
-100
5
g
120
4
h
-220
-5