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Class 12  »  Physics  »  Electric Charges and Fields

Coulombs Law - UNSOLVED PRACTICE SET

Class 12

Chapter: Electric Charges and Fields | Topic: Coulombs Law

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Class 12

COULOMBS LAW - UNSOLVED PRACTICE SET

Topic: Coulombs Law

Time: 40 mins | Marks: 30 | Difficulty: Medium

Multiple Choice Questions

Q1. Coulomb's Law states that the electrostatic force between two point charges is:

  1. Directly proportional to the product of charges and inversely proportional to the square of the distance between them
  2. Inversely proportional to the product of charges and directly proportional to the square of the distance
  3. Independent of the distance between the charges
  4. Directly proportional to the sum of charges

Q2. The SI unit of the constant k in Coulomb's Law is:

  1. N m² C⁻²
  2. N m C⁻²
  3. N⁻¹ m² C²
  4. N m⁻² C²

Q3. Two charges q₁ and q₂ are placed in a medium of dielectric constant K. The Coulomb force between them in the medium compared to vacuum is:

  1. K times greater
  2. K times smaller
  3. Unchanged
  4. K² times smaller

Q4. The electrostatic force between two charges is 36 N when they are 3 m apart. If the distance is increased to 6 m, the new force will be:

  1. 9 N
  2. 18 N
  3. 72 N
  4. 144 N

Q5. Coulomb's Law is valid for:

  1. Only moving charges
  2. Only point charges at rest
  3. All types of charges in any state
  4. Only charges in conductors

Q6. The ratio of electrostatic force to gravitational force between two electrons is approximately:

  1. 10³⁶
  2. 10⁻³⁶
  3. 10¹⁸
  4. 10⁻¹⁸

Short Answer Questions

Q7. State Coulomb's Law in vector form. What does the negative sign in the expression indicate?

Q8. Two identical charges repel each other with a force F. If the distance between them is halved, what will be the new force? Explain.

Q9. How does the presence of a dielectric medium affect the electrostatic force between two charges? Explain with the concept of dielectric constant.

Q10. Why is Coulomb's Law called an inverse square law? Compare it with Newton's Law of Gravitation.

Q11. Two point charges +q and −q are placed at a distance d apart. What is the nature of the force between them? Write the expression for this force.

Q12. In an experiment, two charged pith balls are suspended by silk threads. When brought near each other, they repel. What can you conclude about the charges on the balls?

Long Answer Questions

Q13. Derive Coulomb's Law from experimental observations. Explain how the force depends on the magnitude of charges and the distance between them. Draw a diagram showing two point charges and the force between them.

Q14. Compare and contrast Coulomb's Law with Newton's Law of Gravitation. List at least three similarities and three differences between the two laws.

Q15. Two point charges q₁ = +4 μC and q₂ = −6 μC are placed in vacuum at a distance of 30 cm from each other.

(a) Calculate the magnitude and direction of the electrostatic force on q₁.

(b) What would be the force if the charges are placed in water (dielectric constant K = 80)?

(c) If a third charge q₃ = +2 μC is placed midway between q₁ and q₂, what is the net force on q₃?

[Given: k = 9 × 10⁹ N m² C⁻²]

Numerical / Application-Based Problems

Q16. Two small identical conducting spheres A and B are placed in air with charges +12 μC and −4 μC respectively. The distance between their centers is 40 cm.

(a) Calculate the initial force of attraction between them.

(b) The spheres are brought into contact and then separated back to the same distance. Calculate the new force between them. Is it attractive or repulsive?

(c) If the spheres are now placed in a medium with dielectric constant K = 4, what will be the force between them?

[Given: k = 9 × 10⁹ N m² C⁻²]

Q17. Two point charges q and 4q are placed at a distance r apart in vacuum.

(a) At what point on the line joining the two charges is the electric field zero?

(b) If a test charge q₀ is placed at that point, what is the net force on it?

(c) If the distance between the charges is doubled and the charge q is also doubled, by what factor does the force between them change?

Q18. In a physics lab experiment, three point charges are placed at the vertices of an equilateral triangle of side 20 cm. The charges are q₁ = +2 μC, q₂ = +2 μC, and q₃ = −2 μC.

(a) Calculate the force on q₁ due to q₂.

(b) Calculate the force on q₁ due to q₃.

(c) Find the magnitude and direction of the net force on q₁.

[Given: k = 9 × 10⁹ N m² C⁻²]


Total: 30 Marks | Time: 40 mins

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