UNIVERSITY EXAMINATION 2017/2018
SCHOOL OF ENGINEERING, ENERGY AND BUILD ENVIRONMENT
DEPARTMENT OF ENERGY AND ENVIRONMENTAL ENGINEERING
BACHELOR OF TECHNOLOGY IN COMPUTER AND ELECTRONIC SYSTEMS
REGULAR
UNIT CODE: BEG2108
DATE: AUGUST 2018
UNIT TITLE: CIRCUIT THEORY I
SPECIAL/SUPPLEMENTARY
TIME: 2 HOURS
INSTRUCTION TO CANDIDATES
Attempt ALL questions in section A and any other TWO questions in section
B.
Calculators and Tables may be used.
DO NOT WRITE ANYWHERE ON Question Paper.
SECTION A (Compulsory – 30 Marks)
QUESTION ONE (COMPULSORY) Ω
a) With the aid of an electric circuit diagram define the Ohm’s Law and Kirchhoff’s
voltage Law.
(6 Marks)
b) A piece of wire of cross-section area of 2mm2 has a resistance of 300Ω. Find the:i)
the resistance of a wire of the length and material if the cross-section area is
5mm2,
ii) The cross-sectional area of a wire of the same length and material of resistance
750Ω.
(6 Marks)
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c) With the aid of schematic diagram of an inductance, deduce the expression for the
stored energy in that inductance.
(6 Marks)
d) Define the parallel resonance and Q-factor of an R-L-C circuit. Sketch the circuit
diagram.
(6 Marks)
e) Deduce the expression for power in a three-phase system with a balanced load.
(6 Marks)
SECTION B (ATTEMPT ANY TWO QUESTIONS, ALL QUESTIONS CARRY EQUAL
MARKS)
Question Two
a) A coil having a resistance of 1Ω and a reactance of 1 Ω is shunned by a
resistance of 1 Ω.The combination is connected in series with an inductive
reactance of 1 Ω. Sketch the circuit digram. Draw to scale a complete phasor
diagram for the arrangement and determine the applied voltage necessary to
give a current of 1 A in the coil and show that it leads this current by 90o.
(12 Marks)
b) In a series R-L circuit the p.d. across the resistance R is 12 volts and the p.d.
across the inductance L is 5 volts. Find the supply voltage and the phase angle
between current and voltage.
(8 Marks)
Question Three
a) For a series RC circuit, show the relationship between the voltage and current
waveforms Support your answer with the relevant mathematical equations.
(8 Marks)
b) A circuit having a resistance of 12 Ω an inductance of 0.15 Ω and a capacitance
of 100 in series, is connected across a 100v, 50 Hz supply. Calculate the:i)
Impendance.
ii)
Supply current.
iii)
Voltage across R, L, and C.
iv)
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Phase difference between the current and the supply voltage.(12 Marks)
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Question Four
a) With the aid of a phasor diagram deduce the expressions for the phase and line
voltages for a star connected load.
(6 Marks)
b) In a three phase four-wire system the line voltage is 415 V, 50 Hz and noninductive loads of 10kW, 8kW, 5kW are connected between the line and neutral
in clockwise, Red, Yellow and Blue phases respectively. Calculate the :i)
Current in each line
ii)
Current in the neutral conductor.
(14 Marks)
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