UNIVERSITY EXAMINATIONS: 2011/2012
THIRD YEAR EXAMINATION FOR THE BACHELOR OF
SCIENCE IN INFORMATION TECHNOLOGY
BIT 1103 BASIC ELECTRICITY
DATE: AUGUST, 2012 TIME: 2 HOURS
INSTRUCTIONS: Answer Question ONE and any other TWO
The following constants might be of help to you
QUESTION ONE
a) Define an Ampere. (2 Marks)
b) How many electrons are required to flow through a conductor so as to create a current
of 60mA in 2 minutes. (3 Marks)
c) Define conductance and state its SI unit. (2 Marks)
d) An iron box consumes 3.6KJ when connected to 250V supply for 30 minutes. Find
the power rating of two such boxes. (3 Marks)
e) Briefly explain any two defects of a simple cell. State the remedy of each (6 Marks)
f) A cell has an internal resistance of 0.06Ω and an e.m.f of 2V. Calculate its terminal
p.d if it delivers a current of 5A (3 Marks)
g) Calculate the cross-sectional area, in mm2 of a copper wire, 40m in length and having
a resistance of 0.25ohms. Take the resistivity of copper to be 2.0 x18-8 (3 Marks)
h) A 1µF capacitor is required to store 4J of energy. Find the p.d to which the capacitor
must be charged. (4 Marks)
i) 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) State Faraday’s laws of electromagnetic induction (2 Marks)
b) State any 3 factors that influence the magnitude of the force on a current-carrying
conductor subjected in a magnetic field. (3 Marks)
c) A conductor 2m long carries a current of 800A at 300
to a magnetic field having a
density of 0.5T. Calculate the force exerted on the conductor. (3 Marks)
d) Calculate the e.m.f generated on the axle of a car travelling at 80km/hr assuming the
length of the axle is 2m and the vertical component of the earth’s magnetic field is 40
microteslas. (5 Marks)
e) Explain the principle of operation of an a.c. generator (7 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
where
i is the final current value
I is the initial current value
t is time of growth
T is time constant
(7 Marks)
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) Show that the effective resistance Reff of two resistors R1 and R2 in parallel circuit
connection is given by the formula (6 Marks)
Calculate
i. The effective resistance of the circuit (4 Marks)
ii. The voltage drop across each resistor (5 Marks)
iii. The current through each resistor (5 Marks)
QUESTION FIVE
(6 Marks)
b) Three capacitors have capacitances each of 6µF, 10µF, and 14µF are all connected
in parallel in an electric circuit. The circuit is supplied by a voltage of 20V
i. Find the pd across each capacitor (3 Marks)
ii. The charge stored on each capacitor. (3 Marks)
c) A capacitor consists of 2 metal plates each 25cmx25cm spaced 6mm apart. The
space between the metal plates is filled with glass 4mm thick and a paper layer
2mm 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)
d) List and describe any two types of capacitors (2 Marks)
