Electricity -1
Electric Current: The flow of charge (i.e. the ordered
directional motion of charged particles) constitutes electric current. Atoms of
all substances are built up of positive charged particles called protons and
negative charged particles called electrons. If particles interact with one
another through forces that decrease slowly with increase in distance and
exceed the forces of universal gravitation many times, they are said to have an
electric charge and are called charged particles. There can be
particles without any electric charge, but an electric charge does not exist
without a particle. In metals the carriers of charge are electrons. An atom
consists of a central nucleus made up of protons and neutrons. Neutrons are
particles that have no electrical state, neither positive nor negative. Around
this
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nucleus
there are a number of electrons revolving in different orbits. In the normal
state, the number of electrons in an atom equals the number of protons, thus
balancing the positive and negative charges, and hence the atom is electrically
neutral. The number of protons in an atom is called the Atomic Number.
(E.g. Atomic number of hydrogen is 1, oxygen is 8, copper is 29). The revolving
electrons are held to the nucleus by an attractive force. In conductors they
are easily displaced and can move from one atom to another. A conductor[2005] or wire is a
material which contains movable electric charges, enabling electric current to
flow. SWG [2005] Standard Wire Gauge; a notation for the diameters of metal
wires or thickness of metal sheet ranging from 16 mm to 0.02 mm or from 0.5
inch to 0.001 inch. When a potential difference is applied between the ends of a conductor,
the haphazard
movement of charges causes a steady flow along the conductor
and it is this moving stream of electrons that constitutes the electric
current. The electron movement is impeded by collision with the molecules
giving rise to a certain opposition to the flow of current. This is called resistance.
The flow of current is from positive to negative while the electrons flow from
negative to positive. In insulators the electrons are firmly held and hence if
a potential difference is applied, little or no electrons flow and hence no
current flows.
Effects of electric current: We cannot observe directly
the motion of particles in the conductor. However, the presence of electric
current is manifested in effects and phenomenon accompanying the current.
First, a current carrying conductor gets heated. Second, the current can change
the chemical composition of substance (e.g. in electrolysis, copper gets
separated from copper sulphate solution). Third, the current exerts a force on
neighbouring conductors and magnetized bodies. This is called the magnetic
effect of current.
Types of charges: 1.When atom loses one electron it becomes a
positive charge. 2. Electron has a negative charge.
Quantitatively, electric current is defined as the
rate of flow of charge. (I = q/t, where ‘I’ is the
current and ‘q’ is the charge that has passed through a given area in time ‘t’)
Charge: The quantity of electricity residing on an
electrostatically charged body. The unit of charge is ‘Coulomb’. (1 coulomb = 6.29 x 1018 electrons)
Current: The rate of flow of electric charge is current.
The unit of current is Ampere.[2009,2006] Flow of one coulomb charge in one second is
one ampere. (1Amp = 1coulomb/sec)
Voltage: Electromotive force is the
force that starts and maintains flow of electrons in a conductor. Volt
is the unit of electromotive force i.e. a measure of the electrical pressure. Volt is the pressure of
electricity, Ampere (amp for short) is the flow of electricity. The voltage at any point is
known as the potential of the point. The difference of the electrical voltage
between any two points is called the Potential Difference.[2009,2006]
The electrical P.D. is 1 Volt if 1 Joule of work is done in
moving a unit charge (i.e.1 coulomb) from one point to another.
Resistance: Is the property of material by which it opposes the flow of current through
it. The electron movement i.e. the current, is impeded by collision with the
molecules giving rise to a certain opposition to the flow of current. It can be
seen that the resistance depends on the length of the conductor viz. more the
length, more the resistance. Similarly if the cross-section of the conductor is
more, the less will be its
resistance to
current. Thus resistance R = r(L/A),
L is length of wire, A is area of cross-section, r (row) is the resistivity
or specific resistance, which is constant depending on the material of
conductor. The resistance is measured in Ohms.[2005] The ohm is defined as a resistance between
two points of a conductor when a constant potential difference of 1 volt,
applied to these points, produces in the conductor a current of 1 ampere, The
resistance of a conductor varies with temperature.
Ohm’sLaw: The ratio of the potential
difference V between any two points in a conductor and the current I flowing
through it is constant and it equals the resistance R between the two points. R
= V/I. (resistance R is expressed in ohms,
voltage V in volts, current I in amps.)(Temperature of the conductor being
constant)
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Resistors: These are devices used in
electric circuits and offer resistance to current flow. They are of various
types like Carbon Resistor, Metal Film resistor, Carbon film resistor, Wire
wound resistor, Variable resistor etc.
Insulating
Materials
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Solids
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Gases
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Paper and press board
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Air
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Fibrous material
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Nitrogen
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Resins and polymers
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Hydrogen
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Natural and synthetic
rubbers
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Argon
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Glass
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Helium
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Mica
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Methane
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Asbestos
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Propane
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Ceramics
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Carbon dioxide
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Insulators: Very high resistance materials. When the
resistance offered to flow of current is very high, it almost totally impedes
the flow of current. These materials are called insulators. A good insulation
material should have high resistance, good thermal conductivity, high
di-electric strength, high mechanical strength, low dissipation factor. The
insulating materials generally used are solid or gases (including vacuum). Air
is the most important of all di-electric gases because it occurs free in
nature. Air is a reliable insulating material when voltages are not very high.
Leakage currents are very low in air as compared to other insulating materials.
Insulating materials can withstand temperatures depending upon their thermal
properties. The classification of insulating materials as per their thermal
properties is as follows:
Class
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Limiting temp.
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Material
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Y
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900C
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Cotton, silk, paper
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A
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1050C
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Impregnated paper, resins
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E
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120
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Mica, fibre glass
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B
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1300C
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Impregnated Mica, Fibre glass,
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F
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1550C
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Polyester epoxy
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H
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1800C
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Composite materials mica, fibre
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C
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Above 1800C
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Mica, Teflon, glass, ceramics
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PVC:[2005] Polyvinyl chloride, commonly
abbreviated PVC, is a thermoplastic polymer,is commonly used as the
insulation on electric wires
Force, Work, Power, Energy:
Force: When you push or pull some
object you exert a force on it. Thus force is an agent that produces or tends
to produce, destroys or tends to destroy, motion. A force
is any influence that causes a free body to undergo a change in speed, a change
in direction, or a change in shape. The SI unit of force is Newton, which is the force
required to give a mass of 1kg an acceleration of one meter per sec2.
(In CGS the unit is Dyne)
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Work: It is the amount of
energy transferred by a force acting through a distance in the direction of the
force. The SI unit of work is the joule (J), which is defined as the work done
by a force of one newton acting over a distance of one meter (1Newtom meter)
Power:.[2005,2006] It is the rate of doing work. The electric
unit of power is Watt,[2009,2005] defined as the power expended when one Joule
of work is done in one second. (1 Watt = 1 Joule / sec. = 1 Nm/sec.) (4.186J=1cal.)
So, Power P = Work / time = V x I x t /t = V x I Watts. From Ohm’s Law
V = I x R, hence P = I x I x R = I2R Watt.
Energy is an indirectly
observed quantity. It is often understood as the ability a physical system has
to do work on other physical systems. Since work is defined as a force acting
through a distance (a length of space), the total work done in a given time is
called Energy. Its unit is watt-sec. In general terms, energy is stated in
kilowatt-hours (kWh). (1 unit of electrical energy = 1 kWh.) .[2009,2004] (1
kWh = 3.6 x 106 Joules
= 860 k cal.)
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Question
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Solution
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In
a house, the consumption is as below:
2
nos. 20 W CFL are lighted for 8 hours per day.
2
– 60 W bulbs are lighted for 5 hours per day.
Calculate
the energy consumed in 30 days.
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2-20
W CFL consume (2x20x8x30)W in 30 days
= 9600 W-hour =
9.6 kWh
2-60
W bulbs consume (2x60x5x30)W in 30 days
= 18000 W-hour = 18.0 kWh
Total energy = (9.6 + 18.0) = 27.6 kWh
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A
kitchen heater draws 100A at 220V supply. Find cost of using heater for 6
hours every day
for
30 days. The cost of 1 unit (i.e.1 kwh) is Rs.4.00
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Power
= V x I watts =
220 x 100 = 22000 W = 22 kw
Total
usage hours = 6 x 30 = 180 hours
Total
consumption = 22 kw x 180 hrs. = 3960 kwh
Hence
cost = 3960 x 4 = Rs.15840/-
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An
electric kettle of 500 W, 230 V takes 15min. to bring 1 kg of water from 15
oC to boiling at 100 oC. Find the heat efficiency of the
kettle.
(Given
Sp. Heat of water = 1 in MKS units
i.e.1
kcal / kg) and
1kcal
= 4.2 x 103 Joule
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Heat
reqd. by water = Mass x Sp.Heat x Rise in
temp.
= 1 x 1 x (100-15) = 85 kcal.
Heat
generated by electricity = W x t watt-sec
= 500x15x60 watt-sec. or
Joule (1watt-sec= 1 Joule)
= (500 x 15 x 60)/4.2 x 103 kcal. (1kcal = 4.2 x 103 Joule
Thermal
efficiency = (Heat reqd.)/(Heat
Generated)
= (85 x 4.2 x 103 )/(500x15x60)
x 100 = 79.33%
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[2005]
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Direct
Current: In
case of direct current (DC) it is seen that the voltage or current remains
constant throughout the time of flow. There will be two wires, one of them will
be positive and the other will be negative that can be earthed. The voltage
obtained by Dry cell battery and DC generator is DC type.
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Alternating Current (AC): In case of Alternating
Current, the voltage or current becomes positive and negative alternatively.
One positive and one negative loop form a complete cycle. The number of cycles
per second is called frequency. The unit of frequency is Hz (Hertz) or
cycles per second (cps). In India the electric supply frequency is 50 Hz
i.e. the alternating quantity goes through 50 complete cycles in 1 sec. This
wave shape of the AC is called a sine wave.
One way to express the
intensity, or magnitude (also called the amplitude), of an AC
quantity is to measure its peak height on a waveform graph. This is known as
the peak or crest value of an AC waveform:
INSTANTANEOUS value
of an alternating voltage or current is the value of voltage or current at one
particularinstant. The
value may be zero if the particular instant is the
time in the cycle at which the polarity of the voltage is changing. It may also
be the same as the peak value, if the selected instant is the time in the cycle
at which the voltage or current
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stops increasing and starts decreasing. There are
actually an infinite number of
instantaneous values between zero and the peak value.
AVERAGE VALUE: of an alternating current
or voltage is the average of all the INSTANTANEOUS values during ONE
alternation. Since the voltage increases from zero to peak value and decreases
back to zero during one alternation, the
average value must be some value between those two
limits. You could add series of instantaneous values of the alternation
(between 0° and 180°), and then divide the sum by the number of instantaneous
values used.
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The
computation would show that one alternation of a sine wave has an average value
equal to 0.637 times the peak value. The formula for average voltage is Eavg = 0.637 x Emax, where Eavg is average
voltage of one alternation, and Emax is the maximum or peak voltage. Similarly,
the formula for average current is Iavg = 0.637 x Imax where Iavg is the
average current in one alternation, and Imax is the maximum or peak current. Do
not confuse the above definition of an average value with that of the average
value of a complete cycle. Because the voltage is positive during one alternation
and negative during the other alternation, the average value of the voltage
values occurring during the complete cycle is zero. The average value is the
value that usually determines the voltage or current indicated on a test meter.
There are some meters that will read the Root Mean Square RMS value, these are
called "True RMS meters".
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The PEAK
TO PEAK value is the vertical distance between the top and bottom of the
wave. It will be measured in volts on a voltage waveform, and may be labelled VPP
or VPK−PK. In a current waveform it would be labelled IPP
or IPK−PK as I (not C) is used to represent current.
The AMPLITUDE
of a sine wave is the maximum vertical distance reached, in either direction
from the centre line of the wave. As a sine wave is symmetrical about its
centre line, the amplitude of the wave is half the peak to peak value, as shown
in Fig above.
The PEAK value of the
wave is the highest value the wave reaches above a reference value. The
reference value normally used is zero. In a voltage waveform the peak value may
be labelled VPK or VMAX (IPK or IMAX
in a current waveform).
(If the sine wave being
measured is symmetrical either side of zero volts (or zero amperes), meaning
that the dc level or dc component of the wave is zero volts, then the peak
value must be the same as the amplitude, that is half of the peak to peak
value.)
The PERIODIC TIME (symbol T) is the time, in
seconds taken for one complete cycle of the wave. It can be used to find the
FREQUENCY of the wave ƒ using the formula T =1/ƒ.
Thus if the periodic time of a wave is 20ms (or 1/50th.
of a second) then there must be 50 complete cycles of the wave in one second. A
frequency of 50 Hz. Note, if the periodic time is in seconds then the frequency
will be in Hz.
The RMS or ROOT MEAN
SQUARED value is the value of the equivalent direct (non varying) voltage
or current which would provide the same energy to a circuit as the sine wave
measured. That is, if an AC sine wave has a RMS value of 240 volts, it will
provide the same energy to a circuit as a DC supply of 240 volts. It can be
shown that the RMS value of a sine wave is 0.707 (i.e. 1/Ö2) of the peak value.
VRMS = VPK x 0.707 and IRMS
= IPK x 0.707. (Peak value of a sine wave is equal to 1.414 x the
RMS value.)
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Phase
difference is the difference, expressed in electrical
degrees or time, between two waves having the same frequency and referenced to
the same point in time. (See fig. above)
(Phase in waves is the fraction of a wave cycle which has elapsed relative to an
arbitrary point.)
AC Phases: Phases in AC circuits are of 3 types: Single-phase
(1f), Two-phase (2f) &
Three-phase (3f).
A DC circuit has two wires through which the current in the
circuit flows from a source of electricity through a load and back to the
source. A single-phase AC circuit also has two wires connected to the
source of electricity. However, unlike the DC circuit in which the direction of
the electric current does not change, the direction of the current changes many
times per second in the AC circuit. The 230volt electricity supplied to our
homes is single-phase AC electricity and has two wires - an "active /
live" and a "neutral / earth".
Electrical
phase is measured in degrees, with 360° corresponding to a complete cycle. A
sinusoidal voltage is proportional to the cosine or sine of the phase.
Three-phase, abbreviated 3φ, refers to
three voltages or currents that that differ by a third of a cycle, or 120
electrical degrees, from each other. They go through their maxima in a regular
order, called the phase sequence.
In India, the standard
supply for domestic consumption is Single-phase (1-f), 230V, 50 Hz, AC while for commercial applications the supply is
Three-phase
(3-f),
440V, 50 Hz, AC.
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In India, the standard
supply for domestic consumption is Single-phase (1-f), 230V, 50 Hz, AC while for commercial applications the supply is
Three-phase
(3-f),
440V, 50 Hz, AC.
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Concept of Phase
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Advantages of polyphase
(i.e. more than one phase)
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Comparison of AC and DC
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In
1-f if fault is in one line,
power becomes zero which is undesirable.
In
1-f motor, torque is
pulsating, but in 3-f the torque is
rotating and uniform
A 3-f transmission requires
less conductor copper or aluminium.
It is easy to synchronise 3-f alternator.
3-f motors occupy less
space, take less current, are light in weight and cheaper.
From 3-f supply we can use 1-f, 2-f or 6-f supply.
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AC
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DC
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Cannot
be used for electrolysis or electroplating
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Only
DC can be used for electrolysis or electroplating
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It
cannot be used directly for battery charging
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Only
DC can be used for battery charging
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AC
can be easily transformed to high or low voltage
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DC
voltage cannot be easily transformed to high or low
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Normally
DC appliance will not be damaged if used on AC
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Appliance
marked for use on AC will be damaged if used on DC
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Inductor
offers opposition to current flow
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Inductor
offers little opposition to current flow
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Capacitor
does not prevent current flow
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Capacitor
prevents current flow.
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Connection
arrangement of three-phase system:
There are two
connection arrangements of the three-phase system. 1. Star connection 2.
Delta connection
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