ELECTRICITY
Electricity
is a sort of energy like sound, light, magnetism etc. Now a days
electrical energy is used in almost each and every field of human life. In order to understand it we should first study the atomic structure of materials.
electrical energy is used in almost each and every field of human life. In order to understand it we should first study the atomic structure of materials.
2.1. ATOMIC STRUCTURE
(a) Introduction — All substances found in
solid, liquid or gaseous state are called matter. Matter is composed of small
particles called molecules. A molecule is the smallest part of a substance
which contains all physical and chemical properties of the substance and which
has a free existence. A molecule can be further subdivided into smaller
particles called atoms. An atom is the smallest particle of a substance which
can take part in chemical actions and which has no free existence. Actually
speaking, atoms are so small in size that it is not possible to see them with
naked eyes or even with the help of magnifying-lens.
Matter
composed of only one kind of atoms is called element and that composed of more
than one kind of atoms is called compound.
(b) Structure — An atom consists of a
central part called nucleus. There are protons and neutrons in the nucleus. In
the space around nucleus, electrons revolve in circular or elliptical orbits.
(i) Proton — Unit positive charge is
called proton. It has a charge of 1.6 x 10-19 coulombs. All protons obtained
from different elements are quite identical. Protons rest in the nucleus and
they do not move at all. The mass of a proton is 1845 times of the mass of an
electron.
(ii) Neutron — It is electrically neutral.
Neutrons rest in the nucleus and they also do not move at, all. Its mass is
almost equal to that of a proton. All neutrons obtained from different elements
are quite identical.
(iii) Electron — Unit negative charge is called
an electron. It has a charge of 1.6 x 10 1' coulombs which is equal in
magnitude to that of a proton but opposite in nature. All electrons obtained
from different elements are quite identical. Electrons have two different
motions — one is orbital motion due to which the revolve round the nucleus and
the second one is spin motion due to which they rotate at their own axis like
a top.
(c) Laws Governing Atomic
Structure
(i)
A normal atom is electrically neutral. Therefore,
No. of electrons = No. of protons
It
is so because of equal magnitude of charge in an electron and a proton.
(ii)
The number of electrons or protons present in an atom is know unit- number of
the element.
(iii)
The atomic mass or atomic weight of an element is a number which is equal to
the sum of protons and neutrons present in one atom of that element.
where in is the no. of orbit. Thus,
the maximum numbers of electrons in K, L, M, N, O, P, Q orbit are
2,8,18,32,18,8,2 respectively. (The rule 2n2 can be use upto 4 orbits only)
(v) For starting a new orbit its
preceding orbit should be properly filled with electrons according 2n2 law.
(vi) The last
orbit cannot have more than 8 electrons and its preceding orbit can not have
more than 8 electrons.
2.2. ION AND IONISATION
1. Ion :- An atom remains no longer neutral if it gains
or loses one or more electrons. Hence, a charged atom is called an ion. An atom
having a deficit of electrons is termed as positive ion or cation and an atom having
a surplus of electrons is termed as negative ion or anion.
2. Ionisation - The conversion of atoms into ions is known as ionisation.
2.3. ELECTRIC CURRENT
The flow
of electrons or charges through a conductor is called electric current. The
conventional flow of current takes place from the body at higher potential to
the body at lower potential, hence it takes place from positive to negative.
According to new concept, only electrons are capable to move from one atom to
another or from one terminal to the other in conductors. Therefore, current is
the flow of electrons. Electrons are capable to move from negatively charged
body to the positively charged body. Hence this new concept is known as flow of
electrons or electronic current. Its symbol is I and it is measured in Amperes.
There
are following two main types of current:
I. Direct Current
— The current
whose magnitude and direction remain fixed is termed as direct current. The
current obtained by a battery, generator etc. is D.C'.
2. Alternating
Current — The
current whose magnitude and direction remain alternating at a definite rate is
termed as alternating current. The current obtained by an alternator,
oscillator etc. is A.C. Pulsating, oscillatory and interrupted currents are its
various kinds.
Advantages of
A.C. in Comparison to D.C.
(i)
A.C.
can be developed upto kilovolts level after generation at low voltage level.
(ii)
A.C.
can he transformed from low to high and high to low voltages without considerable
power loss.
(iii)
A.C.
can he distributed from one station to another at low cost.
(iv)
Power
consumption with A.C. is lower than that with D.C., hence it is economical for
the consumer.
Advantages
of D.C. in Comparison to A.C.
1.
D.C. is essential for the operation of various electronic equipments.
2.
D.C. is essential for electroplating.
3.
D.C. is better for electric arc welding.
2.4. TYPES OF ELECTRIC CURRENT
The flow
of current through various substances can take place in the following three
ways:
1. Conduction
Current — The
flow of current through metallic conductors is called conduction current, Metals,
whose atoms have only one or two electrons in their outermost orbit are said to
have free electrons. If a small amount of e.m.f. is applied on the conductor
then the free electrons start to move from one end to another in the metal by
passing on through its atoms and it is known as conduction current.
2. Displacement
Current — This
type of current flows through insulators. Insulators do not have free electrons
and tnus the flow of conduction current through them is not possible. But on
applying an e.m.f. on the insulators, the electrons presents in the outermost
orbit of the o it atoms try to move towards positive terminal f the body. This
action tends them to incline towards positive terminal. Now, if the direction
of applied e.m.f. is changed then electrons also change their direction of inclination
and it is known as displacement current. The flow of current through a
capacitor is of displacement type.
3. Convection
Current – When
a flow of current is set up between two electrodes placed in a vacuum or gas or
liquid filled tube, it is known as convection current. Electrodes or positive
and negative ions set into motion constitute the current. These charge carriers
move from on electrodes to the other. Chemical cells, spark gaps, thermionic
tubes etc. Are the examples of convection current.
2.5. ELECTROMOTIVE FORCE,
E.M.F.
1. E.M.F. — The force which sets up a flow
of current in a circuit or a conductor is called e.m.f. It is produced by an
electrical energy generating device. Its symbol is E and its unit is volt.
2. Potential — The electrical status of a
body which decides, the direction of flow of electric current is
called its potential. If the body is at positive potential, the current will
flow from the body towards the earth and if the body is at negative potential,
the current will
flow from the earth towards the body.
3. Potential
Differences —
The flow of current through a resistor or load up a difference of potentials
between its two terminals which is called potential difference (p.d.) Its
symbol is V and its unit is volt.
4. Difference
between P.D. and E.M.F. —
If the magnitude of (current flowing in a circuit is zero then P.D. will also
be zero, whereas, e.m.f. will not be zero.
2.6. RESISTANCE
The
natural property of a substance of producing hindrance to the flow of current
through it is known as resistance. All substances have little or more
resistance. Its symbol is R and its
unit is ohm.
2.7. CONDUCTANCE
The
natural property of a substance of producing relaxation to the flow of current
through it is known as conductance. Its symbol is G and its unit is mho (℧) or Simen (S) which is
reciprocal of ohm.
2.8. BASIC ELECTRICAL TERMS
1 Ampere. It is the unit of current. Its
symbol is A. If one coulomb of charge is passed through a point in a circuit in
one second, the magnitude of current will be one ampere. The charge of 6.28 X
1018 electrons equals one coulomb. Hence
where, I
= current, coulombs/second or amperes
q = charge, coulombs
t = time, seconds.
2. Volt — It is the unit of p.d. and
e.m.f. Its symbol is V. If the work done in transferring a charge of one
coulomb from one point to another in a circuit is one joule then the p.d.
between the two points will he one volt. Hence
where, V = p.d. or e.m.f.,
joules/coulomb or volts
W = work done, joules
Q = charge, coulombs.
3. Ohm — It is the unit of resistance.
Its symbol is Ω (omega). If a current of one ampere flowing through a conductor
or resistor, produces a p.d. of one volt then the resistance of the conductor
will be one ohm. Hence
Where, R= Resistor, volts/ampere or
obms
V= p.d., volts
I = current,
amperes.
4. Mho – It is
the unit of conductivity. It is reciprocal of ohm, Its symbol is ℧
or S (omegha reversed or simen).
Where, R = resistance, ohms
G = conductance, mhos.
2.9
POWER AND ENERGY
1. Electrical
Power - The
rate of doing work by an electrical machine is called power. Its symbol is P.
Its mechanical unit is joules/second and its electrical unit is watt.
Where, P = electrical power, watt
W = work, joules
t = time, second
∴ W = Q.V
(According to topic 2.8)
Or W = I.t.V
Therefore,
Or
Large
Units of Electrical Power
(i)
Kilowatt,
kW = 1000 watts
(ii)
Horse
power (British), H.P. = 746 watts
(iii)
Metric
horse power, M.H.P. = 735.5 watts
2. Mechanical Energy – The capacity of doing work is
called energy. It is of following two types:
(i)
Potential Energy. P.E.
P.E. = m.g.h
(ii)
Kinetic Energy. K.E.
Where, m = mass, kg
g = acceleration due to
gravity, meters/second2 (m/s2)
h = height, meters
v = velocity of the
body, meters/second.
3. Electrical Energy – The capacity of an electrical
machine of doing work is called electrical energy. Its symbol is E and its
units arc joule and watt-hour. Their symbols are J and Wh respectively.
Hence,
electrical energy= work done
or E = Joule or watt hours
IWh = 3600 joules
I kWh = 1000 Wh = 3,6 x
106 joules
Unit. Commercial unit of electric
consumption is 'Board Of Trade unit (i.e., B.O.T.) unit) or only 'unit'.
1 unit = 1 kWh
= 1000
watt-hours = 3.6 x 106 joules
or
Example 2.1. Calculate the
electric consumption of a heater of 1.5 kilowatt capacity used half an hour
daily for one month.
Solution. Given : Wattage = 1.5 kW = 1500 watts
Example 2.2. Calculate the
horse-power of an electric motor if it can fill up a tank of 5000 litres
capacity in half an hour. The height of the tank is 15 metres and the
efficiency of the motor is 75%.
Let g = 10m/s2.
Solution.
Capacity of the tank = 5000
litres
Mass of water. m = 5000 kg
Height of tank, h = 15 m
gravitational acceleration, g
= 10m/s2
2.10. CONDUCTORS, INSULATORS AND SEMI-CONDUCTORS
1. Conductors – Substances through
which a flow of current, i.e., a flow of free electrons can be set up easily
are called conductors. The number of free electrons present in the substances
decides its conductivity. Most of the metals are good conductors. The atoms of
these substances have either only 1-2 electros o a shortage of 1-2 electrons in
their outermost orbit.
I. Properties of Conductors:
(i) They should have a low specific resistance.
(ii) They should be
mechanically rigid.
(iii) They should be easily available and should
not be much costly.
(iv) If the conductor is a
metal then it should be ductile.
(v) The resistance of most of the conductors
(except carbon) increases with an increase in their temperature.
(vi) A conduction path for
the flow of current can be prepared by connecting conducting wires or strips.
II.
Use of Conductors:
Sl. No.
|
Name of Conductor
|
Uses
|
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
|
Silver
Copper
Brass
Aluminum
Iron
Lead
Tin
Zinc
Eureka
Nichrome
Tungsten
Carbon
|
In sensitive measuring instruments, tiny
capacitors, C.B. points etc.
In electric wires and cables, winding
wire, transformer, choke, motor, generator etc.
In electrical accessories.
In electrical wires and cables, winding
wire, capacitors, shielding etc.
In telephone wire, chassis and body of
equipments.
In underground cables, solder.
In solder and anti-corrosive plating on
various metals.
In Lechlanche and dry cells and in
galvanizing iron sheets, wires etc.
In resistors.
In heating elements.
In bulbs, thermionic tubes etc.
In resistors, brushes of electric
machine, electrodes, etc.
|
III. Note – Out of the
above-mentioned metals brass, eureka and nichrome are alloys. The percentage of
their constituent metals may vary in accordance with their application.
Generally their composition is as follows:
(i)
Brass – Copper 67%, Zinc 33%.
(ii)
Eureka – Nickel 60%, Copper 40%.
(iii)
Nichrome – Nickel 80%, Chromium 20%.
2. Insulators — Substances, through which a flow
of current or liberation and diversion in any direction of free electrons not
easy, are called insulators. Some insulators have 4 and some other have 8 electrons
in their outermost shell, Substances
having 8 electrons in their outermost shell are very good insulators, In
chemistry, such elements are called inert elements and
they do not combine with any other element
I.
Properties of Insulators — Good insulators should have the following properties:
(i) Permanent — An insulator should have a
permanent nature and its property should not finish by a change in physical conditions.
(ii) High Break-down Voltage — An insulator should have a
high value of break-down voltage. "The magnitude of voltage for a
substance at which a flow of current can be set up through one win thick sheet
of the substance is called its break down voltage-. It is also known as
dielectric strength. It is measured in kilo volts/mm.
(iii) Mechanical Strength — An insulator
should he capable to bear mechanical pressure and vibrations.
(iv) Temperature Tolerance — An insulator should he capable
to hear variations in atmospheric temperature, humidity etc. Further, it should
he capable to hear high temperature without any change in its physical or
chemical structure.
(v) Least Electric Absorbent — The electric absorption
capability of an insulator should be least so that no unwanted consumption of
electricity may result.
(vi)
Dielectric Constant —
The dielectric constant of an insulator
should he high enough. Dielectric strength of some principal insulators is
shown in the following table:
Table:
Dielectric Strength of Some Principal Insulators
S.No.
|
Name of Insulator
|
Dielectric Strength
kV/mm, 50Hz
|
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
|
Dry wood
Slate
Paper
Marble
Shellac
Cotton
Asbestos
Fibre
Glass
Parraffin Wax
Porcelain
Transformer Oil
Empire Cloth
Rubber
Resin
Bitumen
Bakelite
Mica
Ebonite
Vulcanised
rubber
|
0.4
to 0.6
1
to 2
1
to 10
2
to 6
3
3
to 4
3
to 4.5
5
5
to 22
8
8
to 12
10
to 16
10
to 20
10
to 25
12
14
17
to 21
20
to 40
30
to 40
30
to 50
|
3.
Semiconductors – Substances which are neither good conductors nor good
insulators are called semiconductors. The no. of free electrons in such
substance is quite low in comparison to that in conductors, hence their
resistance is quite high, e.g. germanium, silicon, carbon, boron etc.
If
a little quantity of any other substance is mixed in such substance as an
impurity then their resistance is reduced. These substances are used for making
diodes, transistors etc.
2.11. EFFECTS
OF CURRENT
The electric current has following
three main effects:
1. Heating Effect – A current
carrying conductor because hot, it means heat is developed in it. The
resistance of the conductor opposes the flow of current and as a result
electrical energy is converted into heat energy.
H = I2 . R. t
Where, H = heat, joules.
I = current, amperes.
R = resistance, ohms.
t = time, seconds.
If
heat is to be calculated in calories then,
Or
Where, J = joules constant 4.2
Uses. The above principle is used in
electric bulb, heater, iron, radiator, welding etc.
2.Magnetic Effect – A magnetic field is developed
around a current carrying conductor. If a magnetic compass is placed in this
field then it rests in a definite direction which is different from north-south
direction. According to Biot Savart law the field intensity at a point in the
magnetic field developed due to flow of electric current:
newton/ampere-meter
Where, B = magnetic field intensity, N/A-m
I = current, amperes
d = the distance of the point
from the conductor, meters.
Uses. The above principle is used in
electric bell, electromagnet, electrical measuring instruments, electric fan,
motors, generators etc.
3.Chemical Effect – A flow of current through an
inorganic chemical solution decomposes the solution into its constituents. This
action is called electrolysis.
Note. Decomposition by the flow of
electric current is possible only in case of electrolytes and not in all
solutions.
The mass of a substance deposited or liberated
at an electrode on account of electrolysis is determined by the following
formula:
where, m = mass of substance deposited
or liberated at a electrode, kg
I = current, amperes
t = time, seconds.
Z = electro chemical
equivalent, kg/coulomb.
Uses. The above principle is used in
electroplating, electrotyping, extraction and purifications of metals, cells
etc.
Note. Besides three above stated
effects of electric current there are some other effects also such as – effects
on human body, due to which we experience an electric shock and the same is
used in the treatment of certain diseases, ray effects which is used in
preparing X-rays photo of human body and the same is used in electronics for
many purpose (e.g., in picture tube of TV).
2.12 STANDARD WIRE GAUGE
It is a small disc type measuring
instrument ment for the measurement of diameter of a metallic wire and
thickness of sheets. The disc which is made of steel has a number of slats made
along its periphery. The diameter of each slats is marked in S.W.G. as well as
in mm.
S.W.G. is a british system of diameter
measurement in which number 8 corresponds to 1/8”, no. 16 to 1/16” and no. 32
to 1/32”. The measurement can measure diameter from 0 to 36 numbers. The
diameter of a wired in inches can be determine with the help of a table in
accordance to the number indicated by the instruments.
Gauges are available which show the
diameter directly in mm.
 |
| Fig 2.1 Standard Wire gauge |
