We rely on electricity in different areas of our daily lives. Most
of the modern instruments and equipment’s are run by electricity. We have come
to depend so much on electricity that it is difficult to imagine what would be
life without electricity. In the previous chapter we have discussed about static
electricity. In this chapter different quantities related to current
electricity e.g. electric current, resistance, electromotive
force and potential difference will be described. In addition, the direction of
electricity, conductor, insulator and semiconductor, electric circuit, Ohm’s
law, fixed and variable resistance, dependence
of resistance, series and parallel combination of resistance, electric
power, system loss of electricity and load shedding, safe and effective
use of electricity will be discussed.
Production of current electricity from static electricity
Electric currents
When two bodies of different potential are connected by a
conducting wire, electrons flow from the body of low potential to that of
higher potential. This flow of electron continues until the potential difference
between the two bodies becomes zero. If by any process the potential difference
between the two objects is maintained, then this flow of electron goes on
continuously. This continuous flow of electrons is electric currents.
The amount of charge that flows in unit time through any cross
section of a conductor is called electric currents. If through any cross
section of a conductor, the quantity of charge Q flows in time t,
then the electric current will be I = Q ÷ t
Unit: The unit of
electric current is ampere. If an amount of charge 1 C flows in 1 second
through any cross section of a conductor, then the quantity of electric current
produced is called 1 A. [But this is not the fundamental definition of ampere.
It is given in chapter Physical
Quantities, section Units of measurement]
I = 1C ÷ 1s = 1Cs-1 = 1A
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Electric currents
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In an isolated charged conductor, the charge stays on its surface
and do not move. Such type of charges is called electrostatic charge. However,
if we can provide a conducting path, the charges will flow instead of being
bound on the conductor. When it happens, we say an electric current is
produced.
How electric current is produced from moving charges is described
in terms of the circuit as shown in figure
Electric currents. At the start of the experiment, two plug keys K1 and K2 are
taken out and the two metal plates A and B are uncharged by touching with hand.
Now, if the plug K1 is closed, the high voltage source will be connected to the
two metal plates.
Next, switch on the high voltage source to charge up the two metal
plates positively and negatively by an equal amount. Now, key K1 is removed and
key K2 is plugged in to provide a continuous conducting path linking the positively
and negatively charged metal plates to the galvanometer. Here, the galvanometer
is a device that can detect the existence of flow of current. It would be
observed that the pointer in the galvanometer is seen to deflect momentarily to
one side and then quickly return to its initial position.
The galvanometer’s deflection shows that an electric current is produced.
How this electric current is produced? The current is caused by the flow of
electrons from the negatively charged plate B through the galvanometer and then
to positively charged plate A. The positive charges of plate A are neutralized
by the incoming negatively charged electrons. As a result, the transient current
which is detected by the galvanometer is produced due to the discharge of the
two metal plates.
Direction of electric current and direction of electron flow
When current electricity was invented first, it was assumed that
the electricity was produced due to the flow of positive charges. This positive
charge flows from higher potential to lower potential. So, the direction of
conventional current is taken to be from higher potential to lower potential or
from positive plate to negative plate of an electric cell. But we know that
actually electric current is the flow of negative charges or of electrons, so
the actual direction of electric current is from lower potential to higher potential.
That is from negative plate to positive plate of an electric cell. Therefore,
the actual direction of electric current is opposite to that of conventional
current. The arrow demonstrated in the diagram is indicating the direction of
conventional current.
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Direction of electric current and direction of electron flow
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Electric circuit symbols
The complete path through which electric current can flow is
called electric circuit. When two plates of a cell are joined to the two ends
of a resistor or an electric devices an electric circuit is formed.
We have to draw simple and clear circuit diagrams to study current
electricity. Symbols that are used to represent common electrical devices that
are employed to draw electric circuits are shown in table.
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Electric circuit symbols
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Conductor, insulator and semiconductor
We know, electric current is the flow of charges through a material.
This electric current can move very easily through some substance. There are
some mediums through which electricity cannot move at all. Solid materials are
classified into three groups depending on their electricity conduction. For
example: (1) conductor (2) insulator (3) semiconductor.
Conductor
The materials through which electric current can flow very easily
are called conductors. Electrons can flow freely within these materials. In
metal wires the charges are carried by electrons. So, the metallic materials
are good conductors of electricity. Copper, silver, aluminum etc. are good
conductors. Due to this reason, metallic wires are used as electric connectors.
Insulator
The materials through which electric current cannot flow are
called insulators. Therefore, the materials where electrons are not free to
move about are the insulators. For example: Plastic, rubber, wood, glass etc.
There are no free electrons inside insulating materials. Electrons do not flow
easily through plastic type materials. As a result plastics are insulator for
electricity. Due to this, the handles of screwdrivers and pliers used by
electricians are covered with plastic type materials. In addition, the copper
wires which we use in our daily needs are covered with plastic.
Semiconductor
The materials whose current conduction capacity lies between that of
conductors and insulators in normal temperature are called semiconductors. For
example, germanium, silicon etc. The current conduction capacity of
semiconductor can be increased by adding suitable impurities.
Electromotive force and potential difference
Electromotive force
Electrical energy
is needed to produce electric current in a circuit. The electromotive force of
an electrical energy source is defined as the work done by the source or the energy
spent by the source in driving a unit positive charge from one point of the
circuit to the same point by traversing the complete circuit along with the
source.
If the work done is W J in bringing Q C of charge in
a complete circuit, then the work done in bringing 1 C of charge is W ÷ Q.
Therefore the electromotive force of the source is, E = W ÷ Q
Unit: The SI
unit of electromotive force is JC-1 or volt (V).
The devices which can transform some other forms of energy into
electrical energy they only have electromotive force. For example: cell,
generator, etc. An electric cell converts chemical energy into electrical
energy and a generator coverts mechanical energy into electrical energy. The
electromotive force of a cell is the sum of the potential differences which
develops in different parts of the circuit along with the cell.
Potential difference
The electricity flows through a conductor due to the potential
difference between the two terminals. The potential difference between any two
points is defined as the amount of work done to carry unit positive charge from
one point to another of a circuit. When a dry cell is used in a torch, the
electrical energy provided by the dry cell is converted into light and heat
energy. The conservation of energy is
maintained in this process of transformation of energy. The amount of energy
converted across the light bulb for migration of unit positive charge is the
potential difference between the two terminals of the bulb. Therefore, the
potential difference between the two points of a circuit is defined as the
amount of electrical energy converted to other forms of energy (e.g. - heat,
light) when unit positive charge migrates between the two points. If W is the
amount of electrical energy converted to other forms for migration of Q amount
of charge, then the potential difference between the two points is
V = W ÷ Q
The SI unit for potential difference is the same as that for
electromotive force. That is volt (V). The potential difference between
the two points will be 1 V if 1 J of electrical energy is converted to
other forms when 1C positive charge flows between the two points.
Experiment: Measure the potential difference between the
two terminals of a dry cell. This is the electromotive force. Now connect this
cell to the bulb and again measure the potential difference between the two
terminals of the cell.
The voltmeter reading is the potential difference between the two
ends of the bulb or of resistance during the current flow. Now compare the
values of the measured electromotive force and potential difference. You will
observe that the value of E is larger than that of V.
Relationship between potential difference and electric current- Ohm’s law
We know, if there is a potential difference between the two terminals
of a conductor, current flows through it. The quantity of this electric current
depends on the potential difference between the two ends of the conductor, the
conductor itself and the temperature of it. George Simon Ohm has discovered the
law regarding the relationship between the electric current that flows in a
conductor and the potential difference between the two terminals of it- which
is known as Ohm’s law.
Ohm’s law
The current passing through a conductor at constant temperature is
directly proportional to the potential difference between the two ends of the
conductor.
By proportionality it means- if the potential difference between
the two ends is doubled, the current flowing through the conductor will be doubled.
Again, if the potential difference between the two terminals is made one third,
the current passing through the conductor will be one third.
Assume AB is a conducting wire. The potential of its two terminals
are VA and VB [Figure 1] respectively. If VA > VB, The
potential difference between the two terminals of the conductor will be V = VA
- VB.
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| Figure 1 |
Now at constant temperature, if the current passing through the
conductor is I, then according to ohm’s law,
I ∝ V
=I ÷ V = R = constant
This constant is called the resistance
of the conductor at that temperature.
Or, I = V ÷ R
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| Figure 2 |
Graph of V-I is shown in figure 2
End
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