Current Electricity — Class 12 Physics
(NCERT) Detailed Notes
1. Introduction to Current Electricity
Electricity is the flow of electric charge. Current electricity refers to the continuous
flow of electric charges (usually electrons) through a conductor. This flow is
caused by an electric field produced by a battery or any other power source.
Key idea: Charges move because of a potential difference (voltage).
2. Electric Current (I)
Definition: Electric current is the amount of charge passing through a cross-
section of a conductor per unit time.
𝑑𝑞
𝐼=
𝑑𝑡
𝑞
If q coulombs pass through in t seconds, current I= .
𝑡
Measured in Amperes (A), where 1 A=1 C/s
Example:
10
If 10 C passes through a wire in 2 seconds, current I= =5 A
2
3. Drift Velocity and Current Density
Inside a conductor, electrons move randomly but when an electric field is applied,
they acquire a small average velocity called drift velocity vd
Drift velocity is usually very small (~10−4 m/s)
Current depends on how many electrons pass per second and their drift
velocity.
I=𝑛𝐴𝑒𝑣𝑑
, Where:
n = number of free electrons per unit volume (electron density)
A = cross-sectional area of the wire
e = charge of an electron (1.6×10−19 𝐶)
𝑣𝑑 = drift velocity
Current Density (J):
Current per unit area,
1
J= = 𝑛𝑒𝑣𝑑
𝐴
Unit: Ampere per square meter (A/𝑚2 )
4. Ohm’s Law and its Limitations
Ohm’s Law: At constant temperature, the current through a conductor is directly
proportional to the voltage applied across it.
V=IR
Where R is resistance.
Resistance R is constant for ohmic materials like metals.
Graph of V vs. I is a straight line for ohmic conductors.
Limitations:
Not valid for all materials (e.g., diodes, semiconductors).
Resistance may change with temperature.
5. Resistance and Resistivity
Resistance is the opposition to the flow of current.
𝐿
R=ρ
𝐴
(NCERT) Detailed Notes
1. Introduction to Current Electricity
Electricity is the flow of electric charge. Current electricity refers to the continuous
flow of electric charges (usually electrons) through a conductor. This flow is
caused by an electric field produced by a battery or any other power source.
Key idea: Charges move because of a potential difference (voltage).
2. Electric Current (I)
Definition: Electric current is the amount of charge passing through a cross-
section of a conductor per unit time.
𝑑𝑞
𝐼=
𝑑𝑡
𝑞
If q coulombs pass through in t seconds, current I= .
𝑡
Measured in Amperes (A), where 1 A=1 C/s
Example:
10
If 10 C passes through a wire in 2 seconds, current I= =5 A
2
3. Drift Velocity and Current Density
Inside a conductor, electrons move randomly but when an electric field is applied,
they acquire a small average velocity called drift velocity vd
Drift velocity is usually very small (~10−4 m/s)
Current depends on how many electrons pass per second and their drift
velocity.
I=𝑛𝐴𝑒𝑣𝑑
, Where:
n = number of free electrons per unit volume (electron density)
A = cross-sectional area of the wire
e = charge of an electron (1.6×10−19 𝐶)
𝑣𝑑 = drift velocity
Current Density (J):
Current per unit area,
1
J= = 𝑛𝑒𝑣𝑑
𝐴
Unit: Ampere per square meter (A/𝑚2 )
4. Ohm’s Law and its Limitations
Ohm’s Law: At constant temperature, the current through a conductor is directly
proportional to the voltage applied across it.
V=IR
Where R is resistance.
Resistance R is constant for ohmic materials like metals.
Graph of V vs. I is a straight line for ohmic conductors.
Limitations:
Not valid for all materials (e.g., diodes, semiconductors).
Resistance may change with temperature.
5. Resistance and Resistivity
Resistance is the opposition to the flow of current.
𝐿
R=ρ
𝐴
