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Summary CLASS 12 PHYSICS CHAPTER3 CURRENT ELECTRICTY CBSE
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ALL THE PHYSICS CLASS12 NOTES CHAPTER 3 CURRENT ELECTRICITY NCERT ALL THE SHORT NOTES BEST FOR CLASS12
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1Parishram (2024)
FORMULA SHEET_(Current Electricity)
➢ Electric current - The rate of flow of charge through a cross section of some region of a metallic wire (or an
electrolyte) is called the current through that region.
➢ If the rate of flow of charge is not constant then the current at any instant is given by the differential limit:
I = dQ/dt.
If a charge Q flows through the circuit for time t, then
I = Q/t.
➢ The S.I unit of current is called ampere (A) (coulomb/second).
➢ In metallic conductors the current is due to the motion of electrons whereas in electrolytes and ionized gases,
both electrons and positive ions move in opposite direction. The direction of current is taken as the direction in
which positive charges move.
➢ In conduction although the current is only due to electrons, the current was earlier assumed to be due to positive
charges flowing from the positive of the battery to the negative. The direction of current therefore is taken as
opposite to the flow of electrons.
➢ Electromotive force - The emf () of the source is defined as the work done per unit charge in taking a positive
charge through the seat of the emf from the low potential end to the high potential end. Thus,
= W/Q.
When no current flows, the emf of the source is exactly equal to the potential difference between its ends. The
unit of emf is the same as that of potential, i.e., volt.
➢ The average flow of electrons in the conductor not connected to battery is zero i.e., the number of free electrons
crossing any section of the conductor from left to right is equal to the number of electrons crossing the section
from right to left. Thus, no current flows through the conductor until it is connected to the battery.
➢ Drift velocity of free electrons in a metallic conductor - In the absence of an electric field, the free electrons
in a metal move randomly in all directions and therefore their average velocity is zero. When an electric field is
applied, they are accelerated opposite to the direction of the field and therefore they have a net drift in that
direction. However, due to frequent collisions with the atoms, their average velocity is very small. This average
velocity with which the electrons move in a conductor under a potential difference is called the drift velocity.
➢ If E is the applied field, e is the charge of an electron, m is the mass of an electron and is the time interval
between successive collisions (relaxation time), then the acceleration of the electron is
a = eE/m
➢ Since the average velocity just after a collision is zero and just before the next collision, it is a, the drift velocity
must be
eE
vd =
m
➢ If I is the current through the conductor and n is the number of free electrons per unit volume, then it can be
shown that I = nAevd.
, 2
➢ The mobility of a charge carrier is defined as the drift velocity per unit electric field.
= vd /E
➢ Current density (J)
I
(i) J= nevd [ I = nevd area]
area
(ii) S.I. unit of J = Am–2.
(iii) Current density is a vector quantity. Its direction is that of the flow of positive charge at the given point
inside the conductor.
(iv) Dimensions of current density [M0L–2T0A1]
➢ Current carriers: The current is carried by electrons in conductors, ions in electrolytes and electrons and holes
in semiconductors.
➢ Ohm’s law Physical conditions (such as temperature) remaining unchanged, the current flowing through a
conductor is proportional to the potential difference across its ends. i.e. V I or V = RI.
➢ The constant of proportionality R is called the resistance of the conductor. This law holds for metallic conductors.
➢ According to Ohm’s law, the graph between V and I is a straight line. Ohm’s law is not valid for semiconductors,
electrolytes and electronic devices etc. These are called non-ohmic or non-linear conductors.
➢ The resistance of a conductor is a measure of the opposition offered by the conductor to the flow of current.
This opposition is due to frequent collision of the electrons with the atoms of the conductor.
➢ The resistance of a conductor is directly proportional to its length l and inversely proportional to the area of
cross-section A.
l
R=
A
where the constant depends on the nature of the material. It is called the resistivity (or specific resistance) of
the material. The S.I. unit of resistance is ohm () and resistivity is ohm-metre (m).
➢ The resistivity of material of a conductor is given by
m
=
ne 2
where n is number of free electrons per unit volume and is the relaxation time of the free electron. Its value
depends on the nature of the material of the conductor and its temperature.
➢ The inverse of resistance is called conductance G.
G = 1/R.
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