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Basic Electricity Question Paper
Basic Electricity
Course:Bachelor Of Science In Information Technology
Institution: Kca University question papers
Exam Year:2011
UNIVERSITY EXAMINATIONS: 2010/2011
THIRD YEAR EXAMINATION FOR THE DEGREE OF BACHELOR OF
SCIENCE IN INFORMATION TECHNOLOGY
BIT 1103: BASIC ELECTRICITY
DATE: APRIL 2011 TIME: 2 HOURS
INSTRUCTIONS: Answer question ONE and any other TWO questions
Electronic charge, e =1.6x10-19C
1 Coulomb of charge, C =6.24x 1018electrons
1 eV = 1.6x10-19 Joules
Permativity of free space = 8.85 x 10-12 C2/Nm2
QUESTION ONE
a) Define electric current and state its SI unit. [2 Marks]
b) How many electrons are required to flow through a conductor so as to create a current of 40µA
in 4 minutes. [3 Marks]
c) Calculate the conductance of a conductor when a p.d of 40KV produces a current of 200mA
through the conductor. [3 Marks]
d) An electric heater consumes 1.8MJ when connected to 250V supply for 30 minutes. Find the
power rating of the heater. [3 Marks]
e) From the concept of the structure of an atom, briefly explain the origin of charge [3 Marks]
f) Briefly explain how the Lenchlanche cell works [5 Marks]
g) A cell has an internal resistance of 0.02O and an e.m.f of 2V. Calculate its terminal p.d if it
delivers a current of 5A [3 Marks]
h) Calculate the cross-sectional area, in mm2 of a copper wire, 40m in length and having a
resistance of 0.25 Ohms. Take the resistivity of copper to be 2.0 x18-8 [3 Marks]
2
i) A 1µF capacitor is required to store 4J of energy. Find the p.d to which the capacitor must be
charged. [3 Marks]
j) A 750 turn coil of inductance 3H carries a current of 2A. Calculate the flux linking the coil and
the e.m.f induced in the coil when the current collapses to zero in 20s. [4 Marks]
QUESTION TWO
a) Define capacitance of a capacitor and state its SI unit. [2 Marks]
b) show that the effective capacitance Ceff of 3 three capacitors C1, C2 and C3 all arranged in
series is given by
Ceff
1 =
1
1
C
+
2
1
C
+
3
1
C
[6]
c) Three capacitors have capacitances each of 2µF, 4µF, and 8µF are all connected in series in an
electric circuit. The circuit is supplied by a voltage of 10V
i) Find the pd across each capacitor [3 Marks]
ii) The charge stored on each capacitor. [3 Marks]
d) A capacitor consists of 2 metal plates each 400mmx400mm spaced 6mm apart. The space
between the metal plates is filled with glass 5mm thick and a layer 1mm thick. The relative
permativities of glass and paper are 2 and 8 respectively
Calculate:
i) The capacitance, neglecting any fringing flux [3 Marks]
ii) The electric field strength in each dielectric in kV per mm due to a p.d of 10kV
between the metal plates. [3 Marks]
QUESTION THREE
a) Define inductance of an inductor and state its SI unit [2 Marks]
b) Show that the current growth i in an LR circuit is given by
i = I (1-et/T)
where
i is the final current value
I is the initial current value
t is time of growth
T is time constant [7 Marks]
3
c) A coil having a resistance of 4 Ohms and a constant inductance of 2H is switched across a 20V
d.c supply. Calculate
i) The time constant T [3 Marks]
ii) The final value of the current [3 Marks]
iii) The value of the current 3 seconds after the switch is closed. [3 Marks]
d) State any two factors that affect the energy stored in an inductor [2 Marks]
QUESTION FOUR
a) State Ohms law [2 Marks]
b) Show that the effective resistance Reff of three resistors R1, R2 and R3 in parallel circuit
connection is given by the formula [6 Marks]
Re ff
1
=
1
1
R
+
2
1
R
+
3
1
R
c) The figure below shows a circuit arrangement of resistors
20V
Calculate
i) The effective resistance of the circuit [4 Marks]
ii) The voltage drop across each resist [4 Marks]
iii) The current through each resistor [4 Marks]
25O
15O
40O
10O
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