Class 12 Physics Revision Notes
Chapter 1: Electric Charges and Fields
CBSE BOARD EXAM HIGH-YIELD FORMULAE QUICK REVISION
1. Electric Charge
Electric charge is an intrinsic property of elementary particles of matter which gives rise to electric force
between various objects.
• SI Unit: Coulomb (C)
• Types: Positive (deficiency of electrons) and Negative (excess of electrons).
Basic Properties of Charge
• Additivity of Charges: Total charge of a system is the algebraic sum of all individual charges. If a system
contains charges q1, q2, q3..., then Qtotal = q1 + q2 + q3 + ...
• Charge is Conserved: The total charge of an isolated system remains constant. Charge can neither be
created nor destroyed; it can only be transferred from one body to another.
• Quantization of Charge: All free charges are integral multiples of a basic unit of charge denoted by e
(charge of an electron/proton = 1.6 × 10-19 C).
q = ± ne (where n = 1, 2, 3...)
2. Coulomb's Law
The electrostatic force of attraction or repulsion between two stationary point charges is directly proportional
to the product of the magnitudes of charges and inversely proportional to the square of the distance between
them.
F = k · ½q1 q2½ / r2
Where k is the electrostatic force constant. For vacuum or air:
k = 1 / (4πε0) ≈ 9 × 109 N·m2·C-2
Here, ε0 is the permittivity of free space = 8.854 × 10-12 C2·N-1·m-2.
Coulomb's Law in Vector Form
The force exerted by charge q1 on q2 is given by:
Class 12 CBSE Physics • Chapter 1: Electric Charges and Fields Page 1
, F21 = [1 / (4πε0)] · [q1q2 / r2] · r̂12
Significance: Since r̂12 = -r̂21, it implies that F12 = -F21. This proves that Coulomb's Law obeys Newton's
Third Law of Motion.
Principle of Superposition
When a number of charges are interacting, the total force on a given charge is the vector sum of the forces
exerted on it due to all other charges individually, unaltered by the presence of other charges.
F1 = F12 + F13 + F14 + ...
3. Electric Field
The space around a charge up to which its electrical influence can be felt is called its electric field.
Electric Field Intensity (E)
Force experienced per unit positive test charge (q0) placed at that point:
E = F / q0
• SI Unit: N/C or V/m (It is a vector quantity pointing away from positive and toward negative charge).
• Electric Field due to a Point Charge Q:
E = [1 / (4πε0)] · [Q / r2]
Properties of Electric Field Lines
1. Start from positive charges and terminate on negative charges.
2. The tangent drawn at any point on a field line gives the direction of the electric field intensity at that point.
3. Two field lines can never intersect each other because if they do, there will be two tangents (directions)
at the point of intersection, which is physically impossible.
4. Electric field lines do not form closed loops because the electrostatic field is conservative in nature.
4. Electric Dipole
A system of two equal and opposite charges separated by a small distance (2a).
Class 12 CBSE Physics • Chapter 1: Electric Charges and Fields Page 2
Chapter 1: Electric Charges and Fields
CBSE BOARD EXAM HIGH-YIELD FORMULAE QUICK REVISION
1. Electric Charge
Electric charge is an intrinsic property of elementary particles of matter which gives rise to electric force
between various objects.
• SI Unit: Coulomb (C)
• Types: Positive (deficiency of electrons) and Negative (excess of electrons).
Basic Properties of Charge
• Additivity of Charges: Total charge of a system is the algebraic sum of all individual charges. If a system
contains charges q1, q2, q3..., then Qtotal = q1 + q2 + q3 + ...
• Charge is Conserved: The total charge of an isolated system remains constant. Charge can neither be
created nor destroyed; it can only be transferred from one body to another.
• Quantization of Charge: All free charges are integral multiples of a basic unit of charge denoted by e
(charge of an electron/proton = 1.6 × 10-19 C).
q = ± ne (where n = 1, 2, 3...)
2. Coulomb's Law
The electrostatic force of attraction or repulsion between two stationary point charges is directly proportional
to the product of the magnitudes of charges and inversely proportional to the square of the distance between
them.
F = k · ½q1 q2½ / r2
Where k is the electrostatic force constant. For vacuum or air:
k = 1 / (4πε0) ≈ 9 × 109 N·m2·C-2
Here, ε0 is the permittivity of free space = 8.854 × 10-12 C2·N-1·m-2.
Coulomb's Law in Vector Form
The force exerted by charge q1 on q2 is given by:
Class 12 CBSE Physics • Chapter 1: Electric Charges and Fields Page 1
, F21 = [1 / (4πε0)] · [q1q2 / r2] · r̂12
Significance: Since r̂12 = -r̂21, it implies that F12 = -F21. This proves that Coulomb's Law obeys Newton's
Third Law of Motion.
Principle of Superposition
When a number of charges are interacting, the total force on a given charge is the vector sum of the forces
exerted on it due to all other charges individually, unaltered by the presence of other charges.
F1 = F12 + F13 + F14 + ...
3. Electric Field
The space around a charge up to which its electrical influence can be felt is called its electric field.
Electric Field Intensity (E)
Force experienced per unit positive test charge (q0) placed at that point:
E = F / q0
• SI Unit: N/C or V/m (It is a vector quantity pointing away from positive and toward negative charge).
• Electric Field due to a Point Charge Q:
E = [1 / (4πε0)] · [Q / r2]
Properties of Electric Field Lines
1. Start from positive charges and terminate on negative charges.
2. The tangent drawn at any point on a field line gives the direction of the electric field intensity at that point.
3. Two field lines can never intersect each other because if they do, there will be two tangents (directions)
at the point of intersection, which is physically impossible.
4. Electric field lines do not form closed loops because the electrostatic field is conservative in nature.
4. Electric Dipole
A system of two equal and opposite charges separated by a small distance (2a).
Class 12 CBSE Physics • Chapter 1: Electric Charges and Fields Page 2
