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Two identical non-conducting solid spheres of equal mass and charge hang in air from a common point on two non-conducting massless strings of equal length. At equilibrium the strings make an angle alpha between them. The spheres are then submerged in a dielectric liquid of density 800 kg/m³ and dielectric constant 21. If the angle between the strings stays the same after immersion, then:

1. The electric force between the spheres stays unchanged.
2. The electric force between the spheres decreases.
3. The mass density of the spheres is 840 kg/m³.
4. The tension in the strings holding the spheres stays unchanged.

Correct answer: The electric force between the spheres decreases.

Solution

Immersion in the dielectric reduces the electric force by a factor K = 21, so the force decreases (option B correct). Keeping alpha fixed requires Fₑ/W_eff constant; this gives rho/(rho - 800) = 21, yielding rho = 840 kg/m³ (option C also correct). The classic JEE answer accepts both B and C; the single best/most direct statement is that the electric force reduces.

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