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.
Consider the following statements:
(A) Coulomb’s law has also been found to hold down to the subatomic level.
(B) For two protons, the electric force between them is much weaker than the gravitational force.
(C) One equation of Coulomb’s law gives the right direction of the force for both like and unlike charges.
(D) Like the Coulomb force, the gravitational force between two masses can be attractive or repulsive.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A) and (C) only
Coulomb first found his law with large charged spheres. Later tests and comparisons with gravity tell us more about the law.
(A) Coulomb’s law has also been found to hold down to the subatomic level. — True. Coulomb’s own experiments used large bodies. Later tests showed that the law also holds at the subatomic level.
(B) For two protons, the electric force between them is much weaker than the gravitational force. — False. The electric force between two protons is about \( 1.3\times10^{36} \) times the gravitational force. It is far stronger.
(C) One equation of Coulomb’s law gives the right direction of the force for both like and unlike charges. — True. The signs of the charges in the vector form decide the direction. Like charges give repulsion and unlike charges give attraction.
(D) Like the Coulomb force, the gravitational force between two masses can be attractive or repulsive. — False. Gravitational force is always attractive. Only the Coulomb force has both signs.
NCERT (2026-27):
Coulomb’s Law – “While the original experiments established it at a macroscopic scale, it has also been established down to subatomic level (r ~ 10–10 m).”
Example 1.3 – “The (dimensionless) ratio of the two forces shows that electrical forces are enormously stronger than the gravitational forces.”
Coulomb’s Law – “Thus, we do not have to write separate equations for the cases of like and unlike charges.”
Points to Ponder – “2. Coulomb force and gravitational force follow the same inverse-square law. But gravitational force has only one sign (always attractive), while Coulomb force can be of both signs (attractive and repulsive), allowing possibility of cancellation of electric forces.”