Electricity
Electricity is one of the most important source of energy. It has become an
indispensable part of our life. It is used in home and in industry for various
purposes, such as lighting, heating, operating machines and so on.
Potential due to a Charge
The process of depositing an electric charge, for example a positive charge on a
body is known as charging the body. Initially the body has no charge on it; it
now acquires a small amount of charge. If one wishes to deposit some more
charge of the same kind on that body, this charge will experience a force of
repulsion, due to charges already present on the body. Therefore, some work
has to be done to counter this force of repulsion. The work done on electrical
charge in the process of charging gets stored as potential energy of charges.
This potential energy is called electrostatic potential or just potential.
The electrostatic potential at any point is defined as the work done in
bringing a single positive charge from infinity to that point. The unit of
potential is Volt (V)
A potential of one volt at a point implies that the amount of work done to bring
one coulomb of positive charge from infinity to that point is one joule.
Thus if a body is given Q coulomb of charge to raise its potential by V volt,
then the amount of work done will be VQ joule
W= VQ
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Electric Current
If two objects charge to different potential are placed side by side , charges will
not move from one to the other. Now if the two are connected, using a wire or a
conductor, the flow of charges take place. Charge will flow as long as there is a
difference of potential between the two. The flow stops as soon as their
potentials become equal. This flow of electric charge is called electric current.
The potential difference (p.d.) of one volt between two points is the measure of
work done in moving one coulomb of charge across them.
p.d= Work done / charge moved= W/Q= Joule/Coulomb= volt (equation-2)
One simple way of maintaining a potential difference between the two ends of
a conductor is to connect it to battery or a dry cell. With the help of battery or a
cell, the potential difference between two electrodes is maintained by chemical
processes going on inside it, between the electrodes and the electrolytes.
What is an Electric circuit ?
Electric Circuit is an arrangement for obtaining electric current in a conductor
by connecting it to a source of electric current.
The magnitude of the current I is the charge flowing in the circuit in one
second.
I =Q/T= Charge/ time
Q is the charge in coulomb flowing in t second. If one coulomb of charge flows
through the circuit in one second, the current in the circuit will be 1 ampere,
which is the unit of the current.
When a charge flows across a p.d , the work done= electrical energy involved
=V.Q.=V. I.t
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An ammeter is an instrument used for measuring current in a circuit. Ammeter
should be connected in such a way that whole current flows through it. This
way of connecting ammeter is called series connection.
Voltmeter is an instrument used for measuring potential difference between
two points.
The picture of an ammeter and voltmeter is as under:
As compared to ammeter, voltmeter is joined as an addition to an already
completed circuit. The voltmeter is connected across a connection. This way of
connecting voltmeter is called parallel connection.
Ohm’s Law – Resistance
In an experiment that studies the variation of current with changing potential
difference, it has been found that there is direct proportion between potential
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difference
and
the
electric
current.
The graph between the current and the potential difference is a straight line.
This relationship can be represented as V= (constant) x I
The current in the circuits is not decided by the battery. But the components of
the circuit determine how much current they will draw from the given battery.
Some materials allow larger current to flow through them than others. In other
words, certain materials offer more resistance to the flow of current than others.
Electrical Resistance or Resistance is the property of a material due to which it
opposes or limits the flow of electric current through it.
The resistance of the conductor can be defined in terms of current passing
through it and the potential difference across its ends. The resistance is defined
as the ratio of potential difference V across its ends and the current I flowing
through it.
R= V/I
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Potential differences’ unit is volt and the unit of current is ampere, the unit of
resistance is volt/ampere. The unit of resistance is called the ohm, named after
George Simon Ohm, who discovered the relations V= constant X I and R= V/I
Ohm’s law states that the current in a circuit is proportional to the voltage
across it. It also states that the resistance in a circuit is the ratio of the voltage to
its current. One ohm is the resistance of a conductor through which a current of
1 ampere flows when the potential difference across its ends is one volt.
An important role is played by the resistance in electrical circuits. The parts or
components of a circuit which are to provide a path for the movement of
charges are made from materials with low resistance. These are called
Conductors. Conductors allow electricity to pass through them.
Often some parts or components of the circuit are intentionally designed for
offering high resistance. These are made of materials with a high resistance
which are called resistors. A substance that has infinitely high resistance will
not allow electricity to flow through. It is called an insulator. Rubber is an
excellent insulator.
Electric Power
Power is the rate at which work is done. In an electric circuit, the work done in
a time t is given by
When a charge flows across a p.d , the work done= electrical energy involved
=V.Q.=V. I.t
Therefore, Power P= W/t = VI
If p.d. is measured in volt and current in ampere, the unit of Power is volt
ampere, also called a watt.
1 volt x ampere= 1 Joule/ 1 coulomb x 1 coulomb/ 1 second= 1 Joule/1
second= 1 watt.
Thus unit of electric power is watt. Electric power is the rate at which the
energy is consumed by the ciruit. If an appliance of 1000 watt or one kilowatt
is connected to the circuit, it will consume 1 KJ of electric energy in one
second.
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