Join Electric Charge and Its Properties
Short Questions & Answers (2 Marks Each)
Here are important 2-mark short questions with precise answers and explanations:
1. Define electric charge. What are its two types?
Answer: Electric charge is a fundamental property of matter that causes it to experience a force when placed in an electromagnetic field. It is a scalar quantity and is measured in Coulombs (C). There are two types of electric charges: Positive charge and Negative charge.
Explanation: A body having an excess of electrons is negatively charged, while a body with a deficiency of electrons is positively charged. Like charges repel each other and unlike charges attract each other.
2. State the law of conservation of charge.
Answer: The law of conservation of charge states that the total electric charge in an isolated system remains constant. Charge can neither be created nor destroyed; it can only be transferred from one body to another.
Explanation: For example, when a glass rod is rubbed with silk, the glass rod becomes positively charged and silk becomes negatively charged. The total charge of the system (glass + silk) remains zero.
3. What do you mean by quantization of electric charge?
Answer: Quantization of electric charge means that the charge on any body is always an integral multiple of the elementary charge (e). That is, Q = ±ne, where n is an integer and e = 1.6 × 10⁻¹⁹ C.
Explanation: A body cannot have a fractional charge like 0.5e or 1.7e. This property was discovered by Millikan through his oil drop experiment.
4. State Coulomb’s law in electrostatics.
Answer: Coulomb’s law states that the electrostatic force of attraction or repulsion between two point charges is directly proportional to the product of the magnitude of the charges and inversely proportional to the square of the distance between them. F = k |q₁q₂| / r², where k = 9 × 10⁹ Nm²/C² (in vacuum).
Explanation: This is the basic law governing the force between charges and is valid for point charges.
5. Mention any two properties of electric charge.
Answer:
- Additivity: Total charge of a system is the algebraic sum of all individual charges.
- Charge is conserved: Total charge remains constant in an isolated system.
Explanation: Electric charge is a scalar quantity. It is independent of velocity and does not depend on the medium (though force does).
6. Differentiate between conductors and insulators.
Answer:
| Property | Conductors | Insulators |
|---|---|---|
| Free electrons | Large number | Very few |
| Charge movement | Can move freely | Cannot move freely |
| Examples | Metals, human body | Glass, rubber, plastic |
Explanation: In conductors, charges can redistribute easily. In insulators, charges stay at the place where they are transferred.
7. What is the principle of superposition for electrostatic forces?
Answer: The principle of superposition states that the total force acting on a charge due to a number of other charges is equal to the vector sum of all the individual forces acting on it.
Explanation: Forces are calculated pairwise and then added vectorially. This principle is used to find net force on a charge due to multiple charges.
8. What is an electric dipole? Give its SI unit.
Answer: An electric dipole consists of two equal and opposite charges separated by a small fixed distance. The electric dipole moment (p) is a vector quantity given by p = q × d, where q is the magnitude of each charge and d is the separation vector. Its SI unit is Coulomb-metre (Cm).
9. What is charging by friction?
Answer: Charging by friction is the process in which two neutral bodies are rubbed against each other, and electrons are transferred from one body to another due to friction. One body becomes positively charged and the other negatively charged.
Explanation: This occurs because electrons are loosely bound in some materials. For example, when a glass rod is rubbed with silk, glass loses electrons and becomes positive while silk gains electrons and becomes negative.
10. Explain charging by conduction.
Answer: Charging by conduction is the process of charging a neutral body by bringing it in direct contact with a charged body. The neutral body acquires the same type of charge as the charging body.
Explanation: Charge is shared between the two bodies until they have the same potential. The total charge is conserved.
11. What is charging by induction?
Answer: Charging by induction is the process of charging a neutral conductor without actual contact with a charged body. It involves redistribution of charges due to the electric field of the charged body.
Explanation: The neutral body develops opposite charge on the near side and same charge on the far side. When earthed and then disconnected, it acquires opposite charge.
12. Define electrostatic induction.
Answer: Electrostatic induction is the temporary redistribution of charges in a neutral conductor when it is brought near a charged body, without any transfer of charge.
Explanation: It occurs due to the force of attraction or repulsion between charges. This is the basic principle behind charging by induction.
13. What is the triboelectric series?
Answer: The triboelectric series is a list of materials arranged in the order of their tendency to gain or lose electrons when rubbed against each other.
Explanation: Materials higher in the series lose electrons (become positive) and materials lower in the series gain electrons (become negative). Example: Glass > Silk > Rubber.
14. State the principle of conservation of charge with one example.
Answer: The total charge of an isolated system remains constant. Charge can only be transferred from one body to another. Example: In any charging process (friction, conduction, or induction), the algebraic sum of charges before and after remains the same.
Explanation: This is a fundamental law of nature, similar to conservation of mass and energy.
15. What do you mean by additivity of charge?
Answer: Additivity of charge means that the total charge of a system is equal to the algebraic sum of all the individual charges present in the system.
Explanation: Charge is a scalar quantity. For example, if charges +2q, –3q, and +5q are present, total charge = +4q.
16. Mention two basic properties of electric charge.
Answer:
- Electric charge is quantized (exists only in integral multiples of e).
- Like charges repel and unlike charges attract each other.
Explanation: These properties distinguish electric charge from other physical quantities and govern all electrostatic interactions.
17. Differentiate between positive and negative charge.
Answer:
| Positive Charge | Negative Charge |
|---|---|
| Deficiency of electrons | Excess of electrons |
| Produced by loss of electrons | Produced by gain of electrons |
| Attracted towards negative charge | Attracted towards positive charge |
Explanation: The nature of charge depends on the imbalance between protons and electrons in an atom.
18. What is an electric field?
Answer: An electric field is the space around a charged particle or body where another charged particle experiences an electrostatic force.
Explanation: It is a vector quantity. The direction is the direction of force on a positive test charge.
19. Define electric field intensity.
Answer: Electric field intensity (E) at a point is the force experienced by a unit positive test charge placed at that point. E = F / q₀ (SI unit: N/C or V/m).
Explanation: It gives the strength and direction of the electric field.
20. What is a test charge? Why is it taken very small?
Answer: A test charge is a small positive charge used to measure the strength of an electric field. It is taken very small so that it does not disturb the original charge distribution.
Explanation: A large test charge would induce charge redistribution, giving incorrect field measurement.
21. What is the SI unit of electric charge?
Answer: The SI unit of electric charge is Coulomb (C). 1 Coulomb = Charge equal to 6.25 × 10¹⁸ electrons.
Explanation: Elementary charge e = 1.6 × 10⁻¹⁹ C.
22. Define electric dipole moment.
Answer: Electric dipole moment (p) is a vector quantity whose magnitude is equal to the product of the magnitude of either charge and the separation distance between them. p = q × d (SI unit: C m). Direction: From negative to positive charge.
Explanation: It measures the strength of the dipole.
23. What is an electroscope? State its use.
Answer: An electroscope is a device used to detect the presence and nature (positive or negative) of electric charge on a body.
Explanation: It works on the principle of repulsion of like charges. Gold leaf electroscope is the most common type.
24. Explain the concept of earthing (grounding).
Answer: Earthing is the process of connecting a conductor to the Earth, allowing excess charge to flow to the ground, making the conductor neutral.
Explanation: Earth acts as a large reservoir of charge. It is commonly used in charging by induction.
25. State two differences between conductors and insulators.
Answer:
| Conductors | Insulators |
|---|---|
| Allow free movement of charges | Do not allow free movement of charges |
| Have large number of free electrons | Have very few free electrons |
Explanation: Conductors can be charged only by induction or conduction when isolated, while insulators are charged mainly by friction.
These questions cover the complete chapter as per CBSE and most state board syllabi. They are precise and ideal for scoring full 2 marks.
If you need numerical-based short questions, diagrams, or previous year board questions, please let me know.
Electric Charge and Its Properties Short Questions & Answers (2 Marks Each) – Questions 26 to 50
26. What are electric field lines?
Answer: Electric field lines are imaginary continuous lines drawn in the electric field such that the tangent at any point gives the direction of the electric field at that point.
Explanation: They represent the electric field visually. The density of lines indicates the strength of the field.
27. State any two properties of electric field lines.
Answer:
- Electric field lines start from a positive charge and end on a negative charge or go to infinity.
- Electric field lines never intersect each other.
Explanation: If two field lines intersect, it would mean two different directions of electric field at the same point, which is not possible.
28. Define electric flux.
Answer: Electric flux (Φ) through a surface is the total number of electric field lines passing through that surface. Φ = E · A = EA cosθ (SI unit: Nm²/C).
Explanation: It is a scalar quantity and measures the flow of electric field through a given area.
29. State Gauss’s theorem.
Answer: Gauss’s theorem states that the total electric flux through a closed surface is equal to 1/ε₀ times the net charge enclosed by the surface. Φ = q_enclosed / ε₀.
Explanation: This theorem is used to calculate electric field due to symmetric charge distributions.
30. What is a Gaussian surface?
Answer: A Gaussian surface is an imaginary closed surface used for applying Gauss’s theorem to calculate the electric field.
Explanation: It is chosen according to the symmetry of the charge distribution (e.g., sphere, cylinder, plane).
31. Give the expression for electric field due to an infinite line charge.
Answer: Electric field due to an infinite line charge having linear charge density λ is E = λ / (2πε₀r), directed radially outward (for positive charge).
Explanation: This is derived using Gauss’s theorem with a cylindrical Gaussian surface.
32. Give the expression for electric field due to an infinite plane sheet of charge.
Answer: Electric field due to an infinite plane sheet with surface charge density σ is E = σ / (2ε₀), directed perpendicular to the sheet.
Explanation: The field is uniform and independent of distance from the sheet.
33. What is the electric field inside a charged conductor?
Answer: The electric field inside a charged conductor is zero.
Explanation: Free charges reside only on the surface. If there were a field inside, charges would move until the field becomes zero.
34. State the properties of conductors in electrostatic equilibrium.
Answer:
- Electric field inside the conductor is zero.
- Excess charge resides only on the outer surface.
Explanation: Any net charge on a conductor distributes itself on the surface to make the internal field zero.
35. Define electric potential.
Answer: Electric potential at a point is the work done per unit positive test charge in bringing it from infinity to that point. V = W / q₀ (SI unit: Volt).
Explanation: It is a scalar quantity and represents the potential energy per unit charge.
36. What is the potential difference between two points?
Answer: The potential difference between two points is the work done per unit charge in moving a positive test charge from one point to the other. ΔV = W / q₀.
Explanation: It is measured in volts and is the cause of current flow in conductors.
37. What is an equipotential surface?
Answer: An equipotential surface is a surface on which the electric potential is the same at every point.
Explanation: No work is done in moving a charge along an equipotential surface.
38. Give two properties of equipotential surfaces.
Answer:
- Equipotential surfaces are always perpendicular to electric field lines.
- Equipotential surfaces never intersect each other.
Explanation: Intersection would imply two different potentials at the same point, which is impossible.
39. What is an electric dipole?
Answer: An electric dipole is a pair of equal and opposite charges separated by a small fixed distance.
Explanation: It has a non-zero dipole moment and produces a characteristic field pattern.
40. Derive the expression for torque on an electric dipole in a uniform electric field.
Answer: Torque τ = p × E = pE sinθ (where p is dipole moment, E is electric field, θ is angle between them).
Explanation: The torque tends to align the dipole along the direction of the electric field.
41. What is the net force on an electric dipole in a uniform electric field?
Answer: The net force on an electric dipole placed in a uniform electric field is zero.
Explanation: The forces on +q and –q are equal in magnitude and opposite in direction.
42. Define electric potential energy of a dipole.
Answer: The electric potential energy of a dipole in a uniform electric field is U = –p · E = –pE cosθ.
Explanation: It is minimum when the dipole is aligned parallel to the field.
43. What is the electric field due to a dipole at axial point?
Answer: Electric field at a point on the axial line of a dipole is E_axial = (2p) / (4πε₀r³) (for r >> d).
Explanation: It is directed along the dipole moment.
44. What is the electric field due to a dipole at equatorial point?
Answer: Electric field at a point on the equatorial line of a dipole is E_equatorial = –p / (4πε₀r³) (for r >> d).
Explanation: It is directed opposite to the direction of dipole moment.
45. State the principle of superposition of electric fields.
Answer: The net electric field at any point due to a system of charges is equal to the vector sum of the electric fields due to individual charges.
Explanation: E_net = E₁ + E₂ + E₃ + … (vector addition).
46. What is polarization of a dielectric?
Answer: Polarization is the process in which a dielectric develops induced dipole moments when placed in an external electric field.
Explanation: It reduces the effective electric field inside the dielectric.
47. Define dielectric constant.
Answer: Dielectric constant (K or ε_r) of a material is the ratio of the permittivity of the medium to the permittivity of free space. K = ε / ε₀.
Explanation: It indicates the ability of the material to reduce the electric field.
48. What is electrostatic shielding?
Answer: Electrostatic shielding is the process of protecting a region from external electric fields by surrounding it with a conducting surface.
Explanation: Electric field inside a charged conductor or cavity is zero. This principle is used in Faraday cages.
49. Why do we use lightning conductors?
Answer: Lightning conductors protect buildings from lightning by providing a low-resistance path for the charge to flow to the earth, thus preventing damage.
Explanation: They are pointed at the top to facilitate corona discharge and safely conduct charge to ground.
50. What is corona discharge?
Answer: Corona discharge is the phenomenon of gradual leakage of charge from a pointed conductor into the surrounding air due to high electric field near sharp points.
Explanation: It is responsible for the glowing effect around high-voltage transmission lines and is used in lightning conductors.
Electric Charge and Its Properties Short Questions & Answers (2 Marks Each) – Questions 51 to 80
51. What is the relation between electric field and electric potential?
Answer: The electric field intensity (E) is equal to the negative gradient of electric potential (V). E = –dV/dr (in magnitude, E = –ΔV/Δr).
Explanation: The electric field points in the direction of steepest decrease in potential. The negative sign indicates that E is directed from high to low potential.
52. Define potential due to a point charge.
Answer: Electric potential due to a point charge q at a distance r is V = (1/(4πε₀)) × (q/r).
Explanation: It is a scalar quantity. For a positive charge, potential is positive; for a negative charge, it is negative.
53. Write the expression for electric potential due to an electric dipole.
Answer: Potential due to a dipole at a point is V = (1/(4πε₀)) × (p cosθ / r²) (for r >> d).
Explanation: On the axial line (θ = 0°), potential is maximum; on the equatorial line (θ = 90°), potential is zero.
54. What is the potential energy of a system of two point charges?
Answer: The electrostatic potential energy of two point charges q₁ and q₂ separated by distance r is U = (1/(4πε₀)) × (q₁q₂ / r).
Explanation: It represents the work done in assembling the charges from infinity.
55. Give the expression for potential energy of an electric dipole in a uniform electric field.
Answer: U = –p · E = –pE cosθ.
Explanation: Potential energy is minimum when the dipole is aligned parallel to the field (θ = 0°) and maximum when anti-parallel (θ = 180°).
56. State two differences between electric potential and electric potential energy.
Answer:
| Electric Potential (V) | Electric Potential Energy (U) |
|---|---|
| Work done per unit charge | Total work done for the charge |
| Scalar, unit: Volt | Scalar, unit: Joule |
Explanation: Potential is independent of the test charge, while potential energy depends on the magnitude of the charge.
57. Why is the electric field inside a hollow charged conductor zero?
Answer: The electric field inside a hollow charged conductor is zero because any excess charge resides only on the outer surface, and charges rearrange themselves so that the net field inside becomes zero.
Explanation: This follows from Gauss’s theorem — flux through a Gaussian surface inside the conductor is zero, hence enclosed charge is zero.
58. What is the electric field inside a uniformly charged spherical shell?
Answer: The electric field inside a uniformly charged thin spherical shell is zero.
Explanation: By applying Gauss’s theorem, the flux through a Gaussian surface inside the shell is zero, so E = 0.
59. Give the expression for electric field due to a uniformly charged solid sphere.
Answer: Inside the sphere (r < R): E = (1/(4πε₀)) × (ρr / 3ε₀) Outside the sphere (r > R): E = (1/(4πε₀)) × (Q / r²)
Explanation: Inside, field increases linearly with r; outside, it behaves like a point charge.
60. What is a Van de Graaff generator?
Answer: A Van de Graaff generator is a machine that produces very high electrostatic potential difference (millions of volts) by continuously transferring charge to a large hollow spherical conductor.
Explanation: It is used to accelerate charged particles for nuclear physics experiments.
61. State the importance of Gauss’s theorem.
Answer: Gauss’s theorem helps in calculating the electric field due to symmetric charge distributions (plane sheet, line charge, spherical shell) easily, without integration.
Explanation: It relates electric flux to the enclosed charge: Φ = q_enclosed / ε₀.
62. Define linear charge density (λ).
Answer: Linear charge density (λ) is the charge per unit length. λ = dq/dl. Its SI unit is C/m.
Explanation: It is used for line charges such as infinite straight wires.
63. Define surface charge density (σ).
Answer: Surface charge density (σ) is the charge per unit area. σ = dq/dA. Its SI unit is C/m².
Explanation: It is used for plane sheets and spherical surfaces.
64. Define volume charge density (ρ).
Answer: Volume charge density (ρ) is the charge per unit volume. ρ = dq/dV. Its SI unit is C/m³.
Explanation: It is used for uniformly charged solid spheres or cubes.
65. Why do two electric field lines never intersect?
Answer: Two electric field lines never intersect because if they did, it would mean two different directions of electric field at the same point, which is physically impossible.
Explanation: The direction of the field is unique at every point.
66. What is the work done in moving a charge on an equipotential surface?
Answer: The work done in moving a charge along an equipotential surface is zero.
Explanation: Since potential difference (ΔV) is zero, W = qΔV = 0.
67. Compare electrostatic force with gravitational force.
Answer: Both forces follow inverse square law and are central forces. However, electrostatic force is much stronger (≈ 10³⁶ times) than gravitational force and can be attractive or repulsive.
Explanation: Gravitational force is always attractive.
68. Why does charge reside only on the surface of a conductor?
Answer: In electrostatic equilibrium, free electrons rearrange themselves so that the electric field inside the conductor becomes zero. Hence, excess charge resides only on the surface.
Explanation: This minimizes the repulsive forces between like charges.
69. What is the SI unit of electric flux?
Answer: The SI unit of electric flux is Newton metre² per Coulomb (Nm²/C) or Volt metre (Vm).
Explanation: It is also equal to Joule per Coulomb-metre.
70. What happens when a dielectric is placed in an electric field?
Answer: When a dielectric is placed in an external electric field, it gets polarized — induced dipoles are formed, which reduce the effective electric field inside the dielectric.
Explanation: The dielectric constant K > 1.
71. Define permittivity of free space (ε₀).
Answer: Permittivity of free space (ε₀) is a constant that measures the ability of free space to permit electric field lines. Its value is 8.85 × 10⁻¹² C²/Nm².
Explanation: It appears in all electrostatic formulas involving vacuum.
72. What is the physical significance of ε₀?
Answer: ε₀ determines the strength of electric force between charges in vacuum. Higher ε₀ would mean weaker electrostatic forces.
Explanation: It is one of the fundamental constants in physics.
73. Why is the electric field perpendicular to the surface of a conductor?
Answer: If the electric field had a tangential component, charges would move along the surface until the tangential field becomes zero. Hence, E is perpendicular to the conductor surface.
Explanation: This is a condition for electrostatic equilibrium.
74. What is the electric potential at the centre of a charged spherical shell?
Answer: The electric potential at the centre (and anywhere inside) a charged spherical shell is the same as on its surface: V = (1/(4πε₀)) × (Q/R).
Explanation: Potential inside is constant and equal to surface potential.
75. State the superposition principle for electric potential.
Answer: The total electric potential at a point due to a system of charges is the algebraic sum of potentials due to individual charges. V_net = V₁ + V₂ + V₃ + …
Explanation: Unlike field, potential is scalar, so simple addition is used.
76. What is the torque on a dipole when it is perpendicular to the electric field?
Answer: When θ = 90°, torque τ = pE (maximum).
Explanation: The dipole experiences maximum turning effect in this position.
77. Why does a charged balloon stick to a neutral wall?
Answer: The charged balloon induces opposite charge on the near surface of the wall, causing attraction due to induction.
Explanation: This is an example of charging by induction.
78. What is the direction of electric field due to a negative point charge?
Answer: The electric field due to a negative point charge is directed towards the charge.
Explanation: A positive test charge is attracted towards the negative charge.
79. Define one Volt potential difference.
Answer: One Volt is the potential difference when 1 Joule of work is done in moving 1 Coulomb of positive charge between two points.
Explanation: 1 V = 1 J/C.
80. Why is electrostatic shielding important?
Answer: Electrostatic shielding protects sensitive instruments and electronic devices from external electric fields by surrounding them with a conducting surface (Faraday cage).
Explanation: Electric field inside a conductor cavity is zero in electrostatic equilibrium.
Electric Charge and Its Properties Short Questions & Answers (2 Marks Each) – Questions 81 to 100
81. State Coulomb’s law in vector form.
Answer: The force on charge q₁ due to charge q₂ is F₁₂ = (1/(4πε₀)) × (q₁q₂ / r²) × r̂₁₂, where r̂₁₂ is the unit vector from q₂ to q₁.
Explanation: The vector form gives both magnitude and direction. Like charges repel and unlike charges attract.
82. What is the principle of superposition of forces?
Answer: The net force on a charge due to a number of other charges is the vector sum of all individual forces acting on it. F_net = F₁ + F₂ + F₃ + …
Explanation: This principle allows calculation of resultant force in a system of multiple point charges.
83. Define one Coulomb of charge.
Answer: One Coulomb is the charge that experiences a force of 9 × 10⁹ N when placed at a distance of 1 m from an equal and similar charge in vacuum.
Explanation: It is the SI unit of charge. 1 C ≈ 6.25 × 10¹⁸ electrons.
84. Why is electrostatic force called a conservative force?
Answer: Electrostatic force is conservative because the work done by it is independent of the path and depends only on initial and final positions.
Explanation: This property allows the definition of electric potential energy.
85. What is the electric potential at infinity?
Answer: The electric potential at infinity is taken as zero.
Explanation: This is a reference point used to define potential at any finite point.
86. Write the relation between electric field and potential gradient.
Answer: E = –dV/dr The magnitude of electric field is equal to the negative rate of change of potential with distance.
Explanation: Electric field is strong where potential changes rapidly.
87. What is the electric potential inside a charged spherical conductor?
Answer: The electric potential inside a charged spherical conductor is constant and equal to the potential on its surface. V = (1/(4πε₀)) × (Q/R)
Explanation: No work is done in moving a charge inside the conductor.
88. Give two differences between electric field and electric potential.
Answer:
Explanation: Field has direction; potential has no direction.
89. What is the total flux through a closed surface enclosing no charge?
Answer: The total electric flux through a closed surface enclosing zero net charge is zero (Φ = 0).
Explanation: This is a direct consequence of Gauss’s theorem.
90. State two applications of electrostatics.
Answer:
- Electrostatic precipitators used to remove dust particles from industrial exhausts.
- Inkjet printers and photocopiers.
Explanation: These devices use charging, induction, and electric fields for practical purposes.
91. Why is the surface of a conductor an equipotential surface?
Answer: The surface of a conductor is an equipotential surface because the electric field inside and tangential to the surface is zero in electrostatic equilibrium.
Explanation: Any potential difference would cause charge flow until it becomes zero.
92. What is the direction of dipole moment?
Answer: The direction of electric dipole moment is from the negative charge to the positive charge.
Explanation: It is a vector quantity p = qd.
93. Write the SI unit of linear, surface, and volume charge densities.
Answer: Linear charge density (λ): C/m Surface charge density (σ): C/m² Volume charge density (ρ): C/m³
Explanation: These are used depending on the geometry of charge distribution.
94. What is the force between two parallel infinite line charges?
Answer: The force per unit length between two parallel line charges having densities λ₁ and λ₂ separated by distance r is F/L = (1/(2πε₀)) × (λ₁λ₂ / r).
Explanation: Derived using Gauss’s theorem.
95. Define capacitance of a conductor.
Answer: Capacitance of a conductor is the ratio of charge given to it to the rise in its potential. C = Q/V (SI unit: Farad).
Explanation: It measures the ability of a conductor to store charge.
96. Why does a conductor become charged only on its outer surface?
Answer: Free electrons inside the conductor move until the internal electric field becomes zero. All excess charge therefore resides on the outer surface.
Explanation: This is a condition of electrostatic equilibrium.
97. What is the electric field near a sharp pointed charged conductor?
Answer: The electric field is very high near sharp points of a charged conductor.
Explanation: This leads to corona discharge and is the principle of lightning conductors.
98. State the law of conservation of charge with one practical example.
Answer: Charge can neither be created nor destroyed; it can only be transferred. Example: During rubbing, total charge of the two bodies remains zero.
Explanation: It is a fundamental conservation law.
99. What is the physical meaning of dielectric constant?
Answer: Dielectric constant (K) indicates how many times the capacitance increases or the electric field decreases when the medium is introduced between charges compared to vacuum.
Explanation: K = 1 for vacuum and K > 1 for all materials.
100. Why is the concept of electric field useful?
Answer: The electric field concept helps us understand the force experienced by a charge at any point without the need for another physical charge to be present at that point. It explains action at a distance.
Explanation: It makes calculations easier for complex charge distributions.
101. What is the difference between electrostatic force and gravitational force?
Answer: Electrostatic force can be attractive or repulsive, while gravitational force is always attractive. Electrostatic force is much stronger (≈ 10³⁶ times) than gravitational force.
Explanation: Both follow inverse square law, but electrostatic force depends on the nature of charges.
102. Define the electric field due to a point charge.
Answer: The electric field due to a point charge q at a distance r is E = (1/(4πε₀)) × (q/r²), directed away from the charge if q is positive.
Explanation: It represents the force experienced by a unit positive test charge placed at that point.
103. What is the electric field at the centre of a uniformly charged ring?
Answer: The electric field at the centre of a uniformly charged ring is zero.
Explanation: Due to symmetry, electric fields due to opposite elements cancel each other.
104. State the condition for electrostatic equilibrium.
Answer: In electrostatic equilibrium, the net electric field inside a conductor is zero and the net charge resides only on the surface.
Explanation: No charge flows inside the conductor under this condition.
105. What is the torque experienced by a dipole when aligned with the electric field?
Answer: When θ = 0° (aligned parallel), torque τ = 0.
Explanation: The dipole is in stable equilibrium in this position.
106. Define electric potential difference in terms of work.
Answer: Potential difference between two points is the work done per unit positive charge in moving it from one point to the other against the electric field.
Explanation: ΔV = W/q₀. It is the driving force for current in circuits.
107. Why is the potential constant inside a charged conductor?
Answer: The potential is constant inside a charged conductor because the electric field inside is zero.
Explanation: Since E = –dV/dr = 0, there is no change in potential.
108. What is the electric flux through a closed surface if the net enclosed charge is zero?
Answer: The electric flux through the closed surface is zero.
Explanation: This follows directly from Gauss’s theorem: Φ = q_enclosed / ε₀.
109. Give the expression for electric field due to a short dipole on the axial line.
Answer: E_axial = (1/(4πε₀)) × (2p / r³) (for r >> d).
Explanation: The field is along the direction of the dipole moment.
110. Give the expression for electric field due to a short dipole on the equatorial line.
Answer: E_equatorial = (1/(4πε₀)) × (p / r³) (directed opposite to p).
Explanation: The field is weaker than on the axial line at the same distance.
111. What is the work done to bring a charge from infinity to a point in an electric field?
Answer: The work done is equal to the charge multiplied by the potential at that point: W = qV.
Explanation: This work is stored as electrostatic potential energy.
112. Why do lightning conductors have a pointed tip?
Answer: The pointed tip creates a very high electric field near it, causing corona discharge that safely neutralizes the charge from clouds.
Explanation: This prevents lightning from striking the building.
113. Define relative permittivity (dielectric constant).
Answer: Relative permittivity (ε_r or K) is the ratio of the permittivity of the medium to the permittivity of free space: K = ε / ε₀.
Explanation: It measures how effectively a material can reduce the electric field compared to vacuum.
114. What is the effect of inserting a dielectric between the plates of a capacitor?
Answer: The capacitance increases by a factor of K (dielectric constant) and the electric field inside decreases by a factor of K.
Explanation: This is widely used to increase the storage capacity of capacitors.
115. State Faraday’s cage principle.
Answer: A conducting enclosure (Faraday’s cage) protects the interior from external electric fields because the electric field inside a conductor cavity is zero.
Explanation: This principle is used in safety rooms and vehicles during lightning.
116. What is the total charge on an isolated neutral conductor?
Answer: The total charge on an isolated neutral conductor is zero.
Explanation: Equal number of positive and negative charges balance each other.
117. Why does charge density become higher at sharp edges of a conductor?
Answer: Charge density is higher at sharp points because the electric field is stronger there, causing greater repulsion among like charges.
Explanation: This leads to corona discharge from pointed conductors.
118. Write the dimensional formula of electric field.
Answer: The dimensional formula of electric field E is [MLT⁻³A⁻¹].
Explanation: It can also be expressed as [Force / Charge] = N/C.
119. What is the SI unit of electric dipole moment?
Answer: The SI unit of electric dipole moment is Coulomb-metre (C m).
Explanation: It is a vector quantity used to measure the strength and direction of a dipole.
120. State one limitation of Gauss’s theorem.
Answer: Gauss’s theorem is useful only for charge distributions having high symmetry (spherical, cylindrical, or planar).
Explanation: For irregular charge distributions, integration is required instead.
