Read the following passage and answer the questions.
Coulomb used a torsion balance to study the force between two small charged metal spheres. A torsion balance is a very sensitive device for measuring small forces. The spheres were small compared with the distance between them, so they could be treated as point charges. At the start, Coulomb did not know the charge on either sphere.
To change a charge in a known way, he touched a charged sphere with an identical uncharged sphere. Then he took the second sphere away. He repeated this step to get smaller and smaller charges. In one set of readings, he kept the charges fixed and changed the distance between the spheres. In another set, he kept the distance fixed and changed the charges in pairs. By comparing all the readings, he arrived at a simple law for the force between two point charges.
Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): The force exerted by the first sphere on the second is equal in magnitude and opposite in direction to the force exerted by the second sphere on the first.
Reason (R): The electrostatic force between two point charges acts at right angles to the line joining them.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A) is correct but (R) is not correct.
Assertion (A) The force exerted by the first sphere on the second is equal in magnitude and opposite in direction to the force exerted by the second sphere on the first. — True. From the vector form of Coulomb’s law, \( \vec{F}_{12}=-\vec{F}_{21} \). So the two forces form an action-reaction pair, as in Newton’s third law.
Reason (R) The electrostatic force between two point charges acts at right angles to the line joining them. — False. The force between two point charges acts along the line joining them, as a push for like charges and a pull for unlike charges.
NCERT (2026-27):
Coulomb’s Law – “Thus, Coulomb’s law agrees with the Newton’s third law.”
Coulomb’s Law – “Coulomb measured the force between two point charges and found that it … acted along the line joining the two charges.”