INTRODUCTION TO ELECTRICITY
ELECTRICITY: Electricity is a branch
of physics which deals with study of charges. The property
of matter that is responsible for electrical phenomena is called charge.
Electricity is further divided into
two sub branches. They are:
1)
Static electricity: the
study of charges which are at rest.
And
2)
Current electricity: study
of charges under motion. This is what we are also calling as electric current.
In terms of electric current materials
are classified into three types:
1)
Conductors: the materials
which can able to allow electric current to pass through them. Eg: all metals,
water with salts and minerals, etc.
2)
Insulators: the materials
which cannot able to allow electric current to pass through them. Eg: wood,
plastic, rubber, etc.
3)
Semi conductors: the
materials which cannot allow electric current freely as conductors. These
materials can able to allow only a partial current to flow slowly through them.
Eg: silicon, germanium, L.E.D, etc
Lorentz and Drude theory:
Drude and Lorentz, scientists of the 19th
century, proposed that conductors like metals contain a large number of free
electrons while the positive ions are fixed in their locations. The arrangement
of the positive ions is called lattice.
Let us understand
the behaviour of electrons in lattice space. Assume that a conductor is an open
circuit. The electrons move randomly in lattice space of a conductor as shown
in figure. When the electrons are in random motion, they can move in any
direction. Hence, if you imagine any cross section as shown in figure, the
number of electrons, crossing the cross section of a conductor from left to
right in one second is equal to that of electrons passing the cross section
from right to left in one second. It means that net charge moving along a
conductor through any cross section is zero when the conductor is in open
circuit.
When the ends of the conductor are connected to the battery through a bulb, the bulb glows because energy transfer takes place from battery to the bulb.
The electrons are
responsible for this transfer of energy. If the electrons are responsible for
transfer of energy from battery to bulb, they must have an ordered motion. When
the electrons are in ordered motion, there will be a net charge crossing
through any cross section of the conductor. This ordered motion of electrons is
called electric current.
Let us define
electric current quantitatively,
Electric current is
defined as the amount of charge crossing any cross section of the conductor in
one second.
Let Q be the charge
crossing through any cross section of the conductor in a time interval t. Then
the amount of charge crossing through that cross section in one second is Q/t.
Therefore,
Electric current = electric
charge/time interval
I = Q/t
The SI unit of
electric current is ampere denoted by A.
1 Ampere = 1 Coloumb/1
Second
1 A = 1 C/s
What causes the electrons to
set in ordered motion?
When a conductor is
connected to a battery in a closed circuit, then immediately an electric field
is set up throughout the circuit which makes electrons to accelerate in ordered
motion.
During this motion,
i)
each electron is
attracted to a nearby lattice ion and collides with it.
ii)
During collision the energy of the electron is released in the
form of heat.
iii)
The electron comes to a temporary halt.
iv)
Then this electron is once again accelerated by electric field.
v)
Then electron again moves and collides with next lattice ion,
releases energy and comes to halt. Again field accelerates electron.
vi)
Like this an electron continuously collides with lattice ions
during its motion.
vii)
The same process is applied for all the electrons in the
circuit.
NOTE: The direction of
electric field is opposite to the direction of electron.
We have two types of currents during the flow of electron in a circuit.
ELECTRONIC CURRENT:
The current produced in the direction of flow of electron is called electronic
current.
CONVENTIONAL
CURRENT: The current produced in opposite direction to the direction of
electron is called conventional current.
We consider the direction of current as the direction of conventional current because an electron flows from low potential to high potential. So, conventional current flows from high potential to low potential.
Drift speed of electron:
Consider a
conductor with cross sectional area A. Assume that the ends of the conductor
are connected to a battery to make the current flow through it. Let vd be
the drift speed of the charges and n be the number of charges present in the
conductor in an unit volume (charge density). The distance covered by each
charge in one second is vd. Then the volume of the conductor for
this distance is equal to Avd . The number of charges contained in
that volume is equal to nAvd. Let q be the charge of each carrier.
Then the total charge crossing the cross sectional area at position D in one
second is nqAvd. This is equal to electric current.
Hence,
Electric current I
= nqAvd ………………. (1)
Therefore, vd =
I/nqA ………………. (2)
We know that the
charge carriers in a conductor are electrons. The magnitude of electric charge
‘e’ is 1.602 x 10-19C.
Let us calculate
the drift speed of electron in a copper wire carrying a current of 1A and cross
sectional area A = 10-6 m2. The electron density of
copper that was found experimentally is n = 8.5x1028m–3.
Substituting these values in equation (2) with q = e, we get,
vd =
1/(8.5x1028x10-6x1.6x10-19)
vd =
7x10-5m/s = 0.07mm/s
This shows that the
electrons are moving very slowly.
Why does a bulb glow immediately when we switch on?
When we switch on
any electric circuit, irrespective of length of the connecting wire (conductor)
an electric field is set up throughout the conductor instantaneously due to the
potential difference of the source (battery) connected to the circuit. This
electric field makes all the electrons to move in a specified direction
simultaneously.
thank you
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