Electrostatics Class 12 Chemistry Notes | Complete Guide with MCQs

Electrostatics is the branch of physics and chemistry that deals with the study of electric charges at rest. It focuses on the forces, fields, and potentials produced by stationary charges. In Class 12 Chemistry, electrostatics plays a vital role in understanding the behavior of particles at the atomic and molecular level.

Electrostatics explains several phenomena such as attraction between charged bodies, chemical bonding, ionic interactions, polarization, and potential energy. The chapter provides fundamental laws and concepts that are also applied in electric circuits, capacitors, electrochemistry, and atomic structure.

Electrostatics Class 12 Chemistry Notes | Complete Guide with MCQs

Electric Charge

Definition

An electric charge is a fundamental property of matter due to which it experiences a force when placed in an electric field.

Properties of Charge

1. Two types of charges – Positive (+) and Negative (−).

2. Like charges repel, unlike charges attract.

3. Quantization of charge – Charge exists in discrete packets (q = n × e, where e = 1.6 × 10⁻¹⁹ C).

4. Conservation of charge – Charge can neither be created nor destroyed, only transferred.

5. Additivity of charge – Total charge on a system is the algebraic sum of charges.

6. Charge is invariant – It does not depend on the frame of reference.

Coulomb’s Law

Statement

Coulomb’s law states that the electrostatic force between two point charges is directly proportional to the product of charges and inversely proportional to the square of the distance between them.

Mathematical Expression

F=14πϵ0⋅q1q2r2F = \dfrac{1}{4\pi \epsilon_0} \cdot \dfrac{q_1 q_2}{r^2}F=4πϵ0​1​⋅r2q1​q2​​

Where:

• $F$ = Force between charges

• q1,q2q_1, q_2q1​,q2​ = Charges

• $r$ = Distance between charges

• ϵ0\epsilon_0ϵ0​ = Permittivity of free space

Nature of Force

• Attractive for unlike charges.

• Repulsive for like charges.

• Acts along the line joining the charges.

Electric Field

Definition

The electric field is the region around a charged body where its influence can be felt by another charge.

Electric Field Intensity (E)

It is the force experienced by a unit positive charge placed in the field.

E=FqE = \dfrac{F}{q}E=qF​

Properties of Electric Field

1. Vector quantity.

2. Direction: Away from positive charge, towards negative charge.

3. Superposition principle applies.

Electric Field Lines

Properties

1. They start from positive and end at negative charges.

2. Never intersect each other.

3. The density of lines indicates field strength.

4. Perpendicular to the surface of a charged conductor.

Electric Dipole

Definition

A pair of equal and opposite charges separated by a small distance forms an electric dipole.

Dipole Moment (p)

$$p=q\times 2a$$

Where $q$ = charge, $2a$ = separation.

Properties

• Vector quantity directed from negative to positive charge.

• Used to explain polarity in molecules like HCl, H₂O, NH₃.

Electric Flux

Definition

The total number of electric field lines passing through a surface is called electric flux.

Formula

ϕ=E⃗⋅A⃗=EAcos⁡θ\phi = \vec{E} \cdot \vec{A} = EA \cos\thetaϕ=E⋅A=EAcosθ

Where θ is the angle between electric field and normal to surface.

Gauss’s Law

Statement

The total electric flux through a closed surface is equal to 1ϵ0\dfrac{1}{\epsilon_0}ϵ0​1​ times the total charge enclosed by the surface.

ϕ=∮E⃗⋅dA⃗=qϵ0\phi = \oint \vec{E} \cdot d\vec{A} = \dfrac{q}{\epsilon_0}ϕ=∮E⋅dA=ϵ0​q​

Applications

1. Electric field due to a point charge.

2. Electric field due to an infinite line charge.

3. Electric field due to a uniformly charged spherical shell.

Potential Energy in Electrostatics

Electrostatic Potential (V)

Work done in bringing a unit positive charge from infinity to a point in an electric field.

V=WqV = \dfrac{W}{q}V=qW​

Potential Difference

Difference in potential energy per unit charge between two points.

Relation between Field and Potential

E=−dVdrE = -\dfrac{dV}{dr}E=−drdV​

Equipotential Surfaces

Properties

1. Potential is constant at all points on the surface.

2. Work done in moving charge on it = zero.

3. Always perpendicular to electric field lines.

Capacitance and Capacitors

Definition

The ability of a conductor to store charge is called capacitance (C).

C=QVC = \dfrac{Q}{V}C=VQ​

Unit

Farad (F).

Parallel Plate Capacitor

Capacitance of a parallel plate capacitor:

C=ϵ0AdC = \dfrac{\epsilon_0 A}{d}C=dϵ0​A​

Where A = area of plates, d = distance between them.

Factors Affecting Capacitance

1. Area of plates (directly proportional).

2. Distance between plates (inversely proportional).

3. Nature of dielectric material.

Energy Stored in Capacitor

U=12CV2U = \dfrac{1}{2} C V^2U=21​CV2

Combination of Capacitors

Series Combination

1Ceq=1C1+1C2+…\dfrac{1}{C_{eq}} = \dfrac{1}{C_1} + \dfrac{1}{C_2} + \dotsCeq​1​=C1​1​+C2​1​+…

Parallel Combination

Ceq=C1+C2+…C_{eq} = C_1 + C_2 + \dotsCeq​=C1​+C2​+…

Conductors and Insulators

• Conductors: Allow free movement of charges (e.g., metals).

• Insulators: Do not allow movement of charges (e.g., rubber, wood).

Electrostatic Shielding

The phenomenon of nullifying electric field inside a cavity enclosed by a conductor is called electrostatic shielding.

Example: Faraday cage.

Applications of Electrostatics

1. Photocopiers and laser printers.

2. Electrostatic precipitators (removing dust from industries).

3. Capacitors in circuits.

4. Lightning arresters.

5. Painting and spraying techniques.

Summary Notes

• Electric charge: quantized and conserved.

• Coulomb’s law defines force between charges.

• Electric field & potential are fundamental quantities.

• Gauss’s law simplifies field calculations.

• Capacitors store charge and energy.

• Electrostatic shielding protects sensitive instruments.

  Electrostatics – MCQs for Practice

  Q1. Which of the following is a scalar quantity?

  (a) Electric field
  (b) Electric potential
  (c) Force
  (d) Dipole moment

  Answer: Electric potential

  Q2. The SI unit of charge is:

  (a) Volt
  (b) Coulomb
  (c) Farad
  (d) Newton

  Answer: Coulomb

  Q3. Coulomb’s law is similar to which law of gravitation?

  (a) Kepler’s law
  (b) Newton’s law of gravitation
  (c) Hooke’s law
  (d) Ohm’s law

  Answer: Newton’s law of gravitation

  Q4. The value of permittivity of free space (ε₀) is:

  (a) 9 × 10⁹ Nm²C⁻²
  (b) 8.85 × 10⁻¹² C²N⁻¹m⁻²
  (c) 1.6 × 10⁻¹⁹ C
  (d) None of these

  Answer: 8.85 × 10⁻¹² C²N⁻¹m⁻²

  Q5. Which type of field lines never intersect each other?

  (a) Magnetic field lines
  (b) Electric field lines
  (c) Both a and b
  (d) None

  Answer: Electric field lines

  Q6. The SI unit of electric field is:

  (a) C/m²
  (b) N/C
  (c) J/C
  (d) C²/N

  Answer: N/C

  Q7. Which of the following has no charge?

  (a) Proton
  (b) Electron
  (c) Neutron
  (d) Positron

  Answer: Neutron

  Q8. What is the direction of the electric field due to a positive charge?

  (a) Towards the charge
  (b) Away from the charge
  (c) Circular around the charge
  (d) Random

  Answer: Away

  Q9. Electric dipole moment is a:

  (a) Scalar
  (b) Vector
  (c) Tensor
  (d) None

  Answer: Vector

  Q10. The unit of dipole moment is:

  (a) C·m
  (b) C/m
  (c) N·m
  (d) V·m

  Answer: C·m

  Q11. The work done in moving a charge on an equipotential surface is:

  (a) Maximum
  (b) Minimum
  (c) Zero
  (d) Infinite

  Answer: Zero

  Q12. The capacitance of a parallel plate capacitor is directly proportional to:

  (a) Distance between plates
  (b) Area of plates
  (c) Voltage applied
  (d) None

  Answer: Area

  Q13. The energy stored in a capacitor is:

  (a) ½CV²
  (b) ½QV
  (c) Q²/2C
  (d) All of these

  Answer: All

  Q14. In series combination of capacitors, the equivalent capacitance is:

  (a) Greater than each capacitor
  (b) Smaller than each capacitor
  (c) Equal to maximum capacitor
  (d) Infinite

  Answer: Smaller

  Q15. Gauss’s law is based on the concept of:

  (a) Electric flux
  (b) Magnetic flux
  (c) Gravitational force
  (d) Induction

  Answer: Electric flux

  Q16. Which of the following is an example of electrostatic shielding?

  (a) Lightning arrester
  (b) Faraday cage
  (c) Capacitor
  (d) Transformer

  Answer: Faraday cage

  Q17. The SI unit of potential difference is:

  (a) Volt
  (b) Ampere
  (c) Coulomb
  (d) Watt

  Answer: Volt

  Q18. The charge on an electron is:

  (a) +1.6 × 10⁻¹⁹ C
  (b) −1.6 × 10⁻¹⁹ C
  (c) +1.6 × 10⁻¹⁸ C
  (d) −9.1 × 10⁻³¹ C

  Answer: −1.6 × 10⁻¹⁹ C

  Q19. The force between two charges becomes four times if the distance between them is:

  (a) Halved
  (b) Doubled
  (c) Tripled
  (d) Reduced to one-fourth

  Answer: Halved

  Q20. Which of the following devices works on electrostatics?

  (a) Transformer
  (b) Photocopier
  (c) Electric motor
  (d) Generator

  Answer: Photocopier

  Electrostatics – Short Answer Questions

  Q1. Define electric charge and write its properties.

  Answer: Electric charge is a fundamental property of matter that causes particles to experience force in an electric field. Its properties are: charges are of two types (positive and negative), like charges repel while unlike charges attract, charge is quantized, charge is conserved, and charge is invariant.

  Q2. State Coulomb’s law in electrostatics.

  Answer: Coulomb’s law states that the electrostatic force between two point charges is directly proportional to the product of charges and inversely proportional to the square of the distance between them.

  Q3. What is meant by quantization of charge?

  Answer: Quantization of charge means that charge exists in discrete packets and cannot be divided further. It is always an integral multiple of the elementary charge, i.e., $q=n\times e$.

  Q4. Define electric field intensity.

  Answer: Electric field intensity at a point is the force experienced by a unit positive charge placed at that point. It is a vector quantity.

  Q5. Write the SI unit of electric potential.

  Answer: The SI unit of electric potential is volt (V), which is equal to one joule per coulomb.

  Q6. What is an equipotential surface?

  Answer: An equipotential surface is one on which the potential is the same at every point. Work done in moving a charge along such a surface is zero.

  Q7. Define dipole moment.

  Answer: Dipole moment is the product of the magnitude of one charge and the distance between the charges in a dipole. It is a vector quantity directed from negative to positive charge.

  Q8. State Gauss’s law in electrostatics.

  Answer: Gauss’s law states that the total electric flux through a closed surface is equal to the total charge enclosed divided by the permittivity of free space.

  Q9. What is electrostatic shielding?

  Answer: Electrostatic shielding is the phenomenon of eliminating the effect of external electric fields inside a cavity enclosed by a conductor.

  Q10. Mention two applications of capacitors.

  Answer: Capacitors are used for storing charge and energy, in electronic circuits for tuning radios, in smoothing rectified current, and in flash cameras.

  Electrostatics – Long Answer Questions

  Q1. Derive an expression for the force between two point charges using Coulomb’s law.

  Answer: According to Coulomb’s law, the force between two point charges is directly proportional to the product of charges and inversely proportional to the square of the distance between them. Mathematically,

  F∝q1q2r2F \propto \dfrac{q_1 q_2}{r^2}F∝r2q1​q2​​ F=kq1q2r2F = k \dfrac{q_1 q_2}{r^2}F=kr2q1​q2​​

  Where k=14πϵ0k = \dfrac{1}{4\pi \epsilon_0}k=4πϵ0​1​. Thus, the force is given by:

  F=14πϵ0⋅q1q2r2F = \dfrac{1}{4\pi \epsilon_0} \cdot \dfrac{q_1 q_2}{r^2}F=4πϵ0​1​⋅r2q1​q2​​

  This expression shows the dependence of force on charges, distance, and medium.

  Q2. Explain the properties of electric field lines with a diagram.

  Answer: Electric field lines start from positive charges and end on negative charges, they never intersect, they are perpendicular to the surface of a conductor, and the density of field lines represents the strength of the field. A diagram shows that closer lines mean a stronger field and widely spaced lines indicate a weaker field.

  Q3. Derive the expression for the electric field due to a point charge.

  Answer: Consider a point charge $q$ placed in free space. The force on a test charge q0q_0q0​ at a distance $r$ is:

  F=14πϵ0⋅qq0r2F = \dfrac{1}{4\pi \epsilon_0} \cdot \dfrac{q q_0}{r^2}F=4πϵ0​1​⋅r2qq0​​

  The electric field at that point is:

  E=Fq0=14πϵ0⋅qr2E = \dfrac{F}{q_0} = \dfrac{1}{4\pi \epsilon_0} \cdot \dfrac{q}{r^2}E=q0​F​=4πϵ0​1​⋅r2q​

  This gives the field due to a point charge.

  Q4. Explain the principle and working of a capacitor.

  Answer: A capacitor works on the principle that when two conducting plates are separated by an insulating material and connected to a battery, one plate accumulates positive charge and the other negative charge. The capacitance is the ratio of charge stored to potential difference. Capacitors store electrical energy which can be released when required.

  Q5. Derive the expression for the energy stored in a charged capacitor.

  Answer: Work done in charging a capacitor is stored as potential energy.

  dW=Vdq=qCdqdW = V dq = \dfrac{q}{C} dqdW=Vdq=Cq​dq

  On integrating from 0 to Q,

  W=Q22CW = \dfrac{Q^2}{2C}W=2CQ2​

  Since $V=Q/C$, energy stored is:

  U=12CV2U = \dfrac{1}{2} CV^2U=21​CV2

  Conclusion

  

  Electrostatics is one of the most fundamental and important concepts in Class 12 Chemistry and Physics. It explains the behavior of charges at rest and provides the foundation for understanding electric forces, electric fields, potentials, capacitance, and energy storage. From Coulomb’s law to Gauss’s law, and from electric dipoles to capacitors, every concept connects theoretical knowledge with practical applications in daily life and technology.

  Mastering electrostatics not only helps in scoring well in board exams but also builds a strong base for competitive exams like JEE, NEET, and other entrance tests. Applications such as photocopy machines, electrostatic precipitators, and capacitors in circuits highlight its real-world significance.

  Thus, a clear understanding of electrostatics is essential for every science student as it lays the groundwork for advanced topics in electrodynamics, electrochemistry, and modern electronics.

• Follow My Insta I’d 
• Chemical Kinetics Class 12 Notes | Rate Law, Order, Arrhenius Equation, MCQs