UNIT
8: ELECTROSTATICS, ELECTRIC CURRENT
AND ELECTRONIC DEVICES
 8.1
Electrostatics.
8.1.1
Basic Electrostatics Phenomena.
(8 Periods)
 Charging by friction.
Types of charges.
- use of electroscope to detect charge.
Charge by induction.
Distribution of
charge outside and inside a hollow conductor
at constant potential.
Principle of the
Van der Graaf generator.
Applications
- lightning conductor, electrostatic screening,
paint spraying, and dust extraction.
Assessment objectives
By the end of this topic, the student should be
able to:
- Distinguish between a conductor and a non conductor.
- Perform an experiment to show that there are two
types of charges.
- Explain gharging by electrostatic induction.
- Explain the attraction of an uncharged material by
a charged body.
- Describe the structure and action of a Gold Leaf
Electroscope.
- Explain how a gold leaf electroscope can be charged
negatively or
positively.
- Describe how a gold leaf electroscope can be used
to determine the
charge on a body
- Describe Faraday's Ice Pail experiment and state
the conclusions that
can be deduced from it.
- Perform and describe an experiment to show the distribution
of charge
on a charged conductor of different shapes.
- Explain corona discharge (action at points) and give
an example of its
application.
- Describe the structure and operation of the Van de
Graaf generator.
8.1.2
The Electric Field
Electric
fields and electric field lines.
Force between
point charges.
Electric Field
Intensity.
- electric field intensity of a point charge.
- electric field intensity between charged
parallel metal plates.
Electric Potential
- relationship between electric potential
and electric field intensity.
- equipotential surfaces and electric field
lines.
- electric potential at a point in the electric
field of a point charge.
Assessment objectives
By the end of this topic, the student should be
able to:
- State Coulomb's law of electrostatics.
- Calculate the force between two point charges.
- Calculate force on a point charge due to a number
of charges.
- Define electric field intensity, state its units
and draw sketch diagrams to show the electric field
patterns for different charge configuration.
- State the expression for the electric field intensity
at a point charge.
- Calculate the electric field intensity at a point
due to a number of point charges.
- Derive and use the relation between electric potential
and electric field intensity.
- Compare Coulomb's law with Newton's law of graviation.
8.1.3 Capacitors
Capacitor and capacitance , the farad
Polarization of dielectrics
Parallel
plate capacitor
Factors
which affect capacitance
Series
and parallel arrangement of capacitors
Energy
stored in a charged capacitor
Assessment Objectives
By the end of this topic the student should be able to:
- Define the capacitance of a capacitor
- State the factors which determine capacitance of
a capacitor
- Explain the action of a dielectric using the molecular
theory
- Explain what is meant by dielectric constant (relative
permitivity) and dielectric strength.
- Perform and describe experiments to investigate the
dependence of capacitance of a parallel plate capacitor
on the area, A, of the plates,the separation,d, of the
plates and
the nature of the dielectric material between the plates
using a gold leaf electroscope.
- Perform and describe an experiment to measure dielecric
constant of dielectric material.
- State and use the law of coservation of charge
- Derive and use expressions for effective capacitance
of capacitors in series and in parallel.
- Derive and use the expression for energy stored in
a charged capacitor.
END OF S6 TERM 1
Electric Current
Electric
current as flow of charge I = Q
t
the ampere, the Coulomb, electric potential difference, the
volt.
Electric
power
- resistance and Ohm's law
- electric energy, kWh
- Power delivered to an ohmic circuit element
- Interule conversion of electrical energy with
forms
Simple d.c
circuits
- e.m.f of a source of electrical energy
- internal resistance
- conservation of charge at a junction in a circuit
- resistors in series and parallel.
- potential divider
- mechanism of metallic conduction, current density
j
= nev
- mechanism of the heating effect of an electric
current
- temperature coefficients of resistance
- electrical resistivity, p: the relation
R = pl
A
Measurement of resistance and voltage
- the Wheat-stone bridge and its applications including
measurement
of temperature coefficient of resistance
- the potentiometer and its applications including
measurement
of voltage, current, thermocouple,
e.m.fs,
comparison of resistances.
Assessment Objectives:
By the end of this topic the student should be able to:
- Define an electric current and state its unit
- State the charge carriers in different types of
conductors
(metals, ionized gases, electrolytes,
semi-conductors)
- Explain the mechanism of electric conduction in metals.
- Derive and use the relation between current and the
drift
velocity of electrons in metals I = nAVde
- Explain the causes of electrical resistance in metals
and
identify the factors which determine resistance of a
metallic conductor.
- Define the term electical resistivity and state its
unit
- Explain the effect of temperature on resistance.
- Define temperature coefficient of resistance and
state its unit.
- State Ohm's law and give examples of ohmic and non-ohmic
conductors,
and draw sketch graphs to show their I-V characteristic
curves.
- Perform and describe an experiment to verify Ohm's
law for metallic
conductors.
- State and use the law of conservation of current
at a junction.
- Derive and use expressions for effective resistance
of resistor in series
and
in parallel.
- List sources of e.m.f
- Explain what is meant by e.m.f, E, and internal resistance,
r, of a cell
- Explain how the e.m.f and iunternal resistance of
a cell change with time
and
use.
- Derive and use the expression P = I2R
- Convert energy in joules into kWh.
- Convert electrical energy to other forms of energy.
- Derive the condition for maximum power dissipation
in the external
resistance
and the expression for efficiency, h.
- Derive and use the condition for balance of Wheatstone
bridge
- Perform and describe an experiment to compare resistances
using simple
metre
bridge
- Perform and describe an experiment to determine the
resistivity, p, and
temperature
coefficient of a resistance of a wire using a metre bridge.
- Explain why the Wheatstone bridge network is not
suitable for comparison of two very high or very low
resistances.
- Solve problems on simple bridges including calculations
of end-corrections.
- Explain the principle of a slide wire Potentiometer
- Perform and describe an experiment to calibrate a
slide wire potentiometer.
- Perform and describe experiments to determine the
internal resistance, r, of a cell, the e.m.f, E, of
thermocouple using the slide wire potentiometer.
- Perform and describe experiments to calibrate an
ammeter and voltmeter using a calibrated slide wire
potentiometer.
- State the advantages and disadvantages of the potentiometer
over an ordinary voltmeter for measurement of voltage
- State theadvantage of using a potentiometer instead
of a Wheatstone bridge to compart resistances.
8.3
Electronic Derives
The Vacuum
diode valve
- thermionic emission
- anode current - anode voltage chracteristics
- incremental resistance of a diode
- half - wave rectification.
- full wave (bridge) rectification
The vacuum
triode
- anode current-anode voltage characteristics
- anode current -grid voltage characteristics
- anode slope resistance, mutual conductance and
amplification factor
- amplification by a triode - voltage gain,
A = yRL
Ra + RL
The p-n junction
- I-V characteristic
- half - wave rectification
- full wave rectifier using semi-conducting diodes
The transistor
- transistor characteristics
Assessment Objectives:
By the end of this topic the student should be able to :
- Explain the mechanism of thermionic emission.
- Describe the structure and operation of a vacuum
diode
- Draw a sketch graph of the anode current -anode voltage
characteristics of a thermionic diode and explain its
special features.
- Perform an experiment to obtain the I-V characteristic
of a p-n junction and explain forward bias and reverse
bias
- Explain half-wave and full-wave rectificatin and
how they can be achieved
- Draw sketch graphs of the anode current-anode voltage
and mutual characteristics of a triode.
- Define the terms anode resistance, mutual conductance
and amplification factor of a triode.
- Derive and use the expression A =
yRL for the voltage gain
Ra + RL
- Describe the structure of n-p-n and p-n-p type transistorI
- Perform experiments to obtain IB
- VBE , Ic -
VCE and
Ic - IB
characteristics of transistor.
9.1 Magnetic Effects of an Electric Current
Idea of a magnetic field as a field of force due to current
- carrying conductors or permanent magnet
Force on a current
- carrying straight wire.
- Fleming's left hand rule
- definition of magnetic flux density and the tesla
Magnetic field patterns due to an electric current in a straight
wire, circular coil and long solenoid.
Motion of a charge particle in a uniform magnetic field
Hall effect
Torque on a current carrying coil in a uniform magnetic field.
- moving coil galvanometer
- conversion of moving coil galvanometer into an
ammeter and voltmeter
Magnetic force between current carrying conductors
- definition of ampere
- simple form of current balance
Assessment objectives
By the end of this topic the student should be able to:
- Define a magnetic field
- Perform experiments to obtain the magnetic field
patterns for a bar magnet, a current - carrying
straightwire, a current - carrying circular coil, and
a current - carrying solenoid.
- Perform an experiment to determine the direction
of the force on a straight current carrying conductor
in a magnetic field.
- State, explain and use the expressions
B = m0 I ,
B = m0 NI,
and B = m0nI
2pa
2R
for the magnetic flux density at
a perpendicular distance a from a straight current carrying
wire, at the centre
of a circular coil of N turns each of
radius R and at centre of a long solenoid of n turns per
metre.
Derive and apply the expression for the
magnetic force between two long parallel current - carrying
conductor
- Derive and apply the expression for the magnetic
force between two long parallel current - carrying conductor
- Define the ampere
- Describe a simple form of a current balance.
- Recall and use the expression F =
Bqv sin q for the force on a particle of charge
q, moving in a uniform magnetic field of flux density
B.
- Describe quantitively the motion of a charge particle
in a uniform magnetic field.
- Explain the Hall Effect
- Explain how a calibrated Hall Probe can be used to
measure magnetic flux density.
- Derive and use the expresion t =
BANI sin q for the torque on a current carrying
coil in a magnetic field.
- Describe how a moving coil galvanometer can be converted
into an ammeter and into a voltmeter.
- Calculate the value of the resistor required to convert
a moving coil galvanometer into an ammeter or voltmeter.
- Describe how a moving coil galvanometer is converted
into a ballistic galvanometer.
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