CURRENT ELECTRICITY, CBSE CLASS XII, PHYSICS NOTES
CBSE CLASS XII, CURRENT ELECTRICITY, PHYSICS NOTES-(PART I)
CURRENT ELECTRICITY
CBSE Class XII Physics Chapter 3rd CURRENT ELECTRICITY Notes
Study Material and Notes of Chapter 3 CURRENT ELECTRICITY Class XII Physics
ACCORDING TO THE CBSE SYLLABUS 2025-26.
CURRENT ELECTRICITY
Current
is defined as the rate of flow of charge.
S.I.
unit of Electric Current is Ampere (A)
The
directed rate of flow of electric charge through any cross-section of a
conductor is known as electric current.
Where,
n = number of charged particles constitute the current
Current is a scalar quantity.
I is in the direction of flow of positive charge and opposite to the direction
of flow of negative charge.
SI unit of current is ampere and is represented by A.
CURRENT DENSITY (j)
The current density at a point in a conductor is the ratio of the current at that point in the conductor to the area of cross-section of the conductor at that point provided the area is held normal to the direction of flow of current.
FLOW OF ELECTRIC CHARGE IN
METALLIC CONDUCTORS
Among
the solids, all metals are good conductors of electricity. The cause of
conductance is free electrons.
IN CASE OF A SOLID
CONDUCTOR
(i.e.
Cu, Fe, Ag, etc) atoms are tightly bound to each other. There are large number
of free electrons in them.
IN CASE OF A LIQUID
CONDUCTOR
Like electrolytic solution, there are positive and negative charged ions which can move on applying electric field.
DRIFT VELOCITY
It is defined as the average velocity
with which the free electrons move towards the positive end of a conductor
under the influence of an external electric field applied.
ELECTRIC CURRENT AND DRIFT VELOCITY
Electric current in terms of drift velocity
I = neaVd
Where,
n
= number density of free electrons.
e = electronic charge.
a = cross-sectional area.
Vd = drift velocity of an electron.
CURRENT
DENSITY AT ANY POINT OF CONDUCTOR
j = nevd
Where,
j is a vector quantity.
MOBILITY
The ratio of drift velocity of electrons and the applied electric field is known as mobility.
S.I. unit is [m2 s-1 V-1 ].
OHM’S LAW
Ohm’s Law at constant temperature, the potential difference V across the ends of a given metallic wire (conductor) in an circuit (electric) is directly proportional to the current flowing through it.
The variation of current w.r.t. applied potential difference is shown with the help of following graph.
V = IRwhere, R = resistance of conductor
No effect of V and I on R because as V increase, I increase but R remains the same.
RESISTANCE
Resistance of a Conductor Mathematically, it is the ratio of potential difference applied across the ends of conductor to the current flowing through it.
=> R = V/I
SI unit is ohm (Ω).
Resistance can also be written as,
R =ρ
L/A
where,
L = length of the conductor,
A = area of cross-section
ρ = constant, known as resistivity of the material. It depends upon nature of the material.
RELATIONSHIP BETWEEN RESISTIVITY AND RELAXATION TIME
Specific resistance or resistivity (ρ) depends on
the material of conductor, not on the length and cross-sectional area (A) i.e.,
geometry of conductor.
At constant volume of conductor, if length increases, area decreases and vice-versa.
TEMPERATURE COEFFICIENT OF RESISTANCE
The change in resistance per ohm for change in temperature of t0C from 00C is called temperature of resistance coefficient of resistance of 00C.
( Rt – R0 ) ᾲ R0 ……………………………(1)
( Rt – R0 ) ᾲ Δ t …………………………
(2)
( Rt – R0 ) ᾲ Δ t R0
( Rt – R0
) = α Δ t R0
Rt = R0 + α Δ t R0
α = Rt – R0
/ Rt = R0
Rt = R0 ( 1 + α Δ t )
α = temperature coefficient of resistance
Δ t = Change in
temperature (t – t0 )
CONDUCTIVITY
It is defined as the reciprocal of resistivity of a conductor.
It is expressed as,
σ =
1/ρ
SI unit is mho per metre (Ω-1 / m).
SUPERCONDUCTIVITY
The resistivity of certain metal or alloy drops to zero when they are cooled below a certain temperature is called superconductivity.
RELATIONSHIP BETWEEN CURRENT DENSITY (J), ELECTRIC FIELD (E) AND CONDUCTIVITY (σ ) IS
j = σ E
Some Important Units
(i) Resistance Ohm ( Ώ )
(ii) Resistivity Ohm – metre ( Ώ – m)
(iii) conductivity (I / R ) = Mho or Ώ – 1 or Siemen
(S)
(iv) current density A / m2
If a conductor is stretched or compresses
to n times of original length, then
l’ = nl => R’ = n2R
where, R’ = new resistance and R = original resistance.
COMBINATIONS OF RESISTANCE
There are two types of resistance combinations.
SERIES COMBINATION
In this combination, different resistances are connected end to end.
Equivalent resistance can be obtained as the formula,
Req.= R1 + R2 + ………………. +Rn
The total resistance
in the series combination is more than the greatest resistance in the circuit.
PARALLEL COMBINATION
In this combination, first end of all the resistances are connected to one point and last end of all the resistances are connected to other point. Equivalent resistance can be obtained by the formula
1/ Req = 1 / R1
+ 1 / R2 + ……….+ 1 / Req.
The
total resistance in parallel combination is less than the least resistance of
the circuit.
If n identical resistors each of resistance (r) are connected in
(i)
series combination, Req = nr
(ii) parallel combination, Req = r/n
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