Basic Electricity Question Paper
Basic Electricity
Course:Bachelor Of Science In Information Technology
Institution: Kca University question papers
Exam Year:2009
UNIVERSITY EXAMINATIONS: 2008/2009
FIRST YEAR EXAMINATION FOR THE DEGREE OF BACHELOR OF
SCIENCE IN INFORMATION TECHNOLOGY
BIT 1103: BASIC ELECTRICITY
DATE: AUGUST 2009 TIME: 2 HOURS
INSTRUCTIONS: Answer question ONE and any other TWO questions
QUESTION ONE
a) Give your understanding of the phrase “electrical circuit” and explain circumstances under which
the circuit is an “open circuit” and when it is a “short circuit”. [4 Marks]
b) Energy can exist in many forms (electrical energy, mechanical energy, chemical energy, etc).
Identify any THREE reasons why electrical energy is often one of the most popular forms.
[3 Marks]
c) The following are commonly used components in an electric circuit. Give your understanding of
each component.
?? Resistor
?? Capacitor
?? Inductor
[3 Marks]
d) A resistor is marked as follows:
?? 1st band – Brown,
?? 2nd band – Black,
?? 3rd band – Orange
2
?? 4th band – Silver.
Determine the value of the resistor in kilo-ohms. [4 Marks]
e) Define the resistivity of a resistor and show its units. [3 Marks]
f) (i). State Ohm’s Law, and sketch the relationship between the voltage across and current through a
resistance of value R. [3 Marks]
(ii). Sketch the electrical circuit diagram that can be used to obtain the results needed to draw a
graph of Ohm’s Law. Clearly indicate the meters and how they are connected in the electrical
circuit to measure the needed parameters. [5 Marks]
g) A dc power supply is connected, through a switch, across an inductor, which has a resistance r and
inductance L.
(i). Show the above information diagrammatically
(ii). Upon closing the switch, sketch the variation of the current with time in the inductor.
[5 Marks]
QUESTION TWO
a) Three resistors of values 3O, 6O and 9Oare connected in series to an 18 V dc supply. The supply
has an internal resistance of 0.1O. For the arrangement, determine the following:
(i). Current in each resistor;
(ii). Voltage drop across each resistor;
(iii). Power consumed in each resistor
(iv). Explain why the voltage drops in (ii) do not add up to the supply voltage
[9 Marks]
b) The resistors as given in Question 2(a) are now connected all in parallel to the same power supply
given. Determine:
(i). Current in each resistor;
(ii). Voltage drop across each resistor;
(iii). Power consumed in each resistor
[11 Marks]
QUESTION THREE
a) Three resistors (R1, R2 and R3) made from the same material have dimensions shown in Table Q3..
3
Table Q3. Resistance dimensions
R1 R2 R3
Cross-sectional area
A
A 2A
Length l 2l l
The current through resistor R1 is 10 A for a given voltage V. For the same voltage separately applied
across R2 and R3, determine the currents flowing. [8 Marks]
(b). (i). Give statements of the two Kirchhoff’s Laws. [2 Marks]
(ii). Illustrate your knowledge of Kirchhoff’s Laws by finding all the currents in the circuit given.
Fig. Q3
15 V
10O 12O
7O 8O
I2 I1
I3
[10 Marks]
QUESTION FOUR
A capacitor of capacitance C and a resistor of resistance R are connected in series as shown in Fig. Q4.
Fig. Q4
R V
C
(i) When the supply is switched on, sketch the variation of the voltage across the capacitor, VC,
with time. [4 Marks]
(ii) State the condition for which current in the circuit will become zero. [2 Marks]
(iii)Give an expression for the time constant of the circuit. [1 Mark]
(iv) If C is 1 µF, R is 10O, and V is 20 V dc, determine the time constant of the circuit and the
time at which the voltage across the capacitor will be 15 V. [10 Marks]
4
(v) Explain why a capacitor should never be charged by directly connecting it across the supply
without a resistor as shown in Fig. Q4. [3 Marks]
QUESTION FIVE
a) Sketch the waveform showing the variation of voltage with time for an alternating current (ac)
power supply. [3 Marks]
b) An alternating current (ac) circuit has a power supply of 220 V connected in series with a
resistance of 10O. For the circuit, determine the following:
(i). Maximum current, minimum current, average current, and root mean square (rns) current.
(ii). the power consumed by the resistor and the power generated by the supply. [14 Marks]
c) Briefly explain why electric power is almost universally generated, transmitted and distributed as
ac and not dc. [3 Marks]
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