CUET · PHYSICS · PYQ PAPER 2025

Two identical metal balls, A and B, with similar charges Q are suspended with insulating threads. They repel each other with a force F. Another identical uncharged metal ball C is first touched with metal ball A and then touched with metal ball B and finally removed. The new force of repulsion between A and B will be

A \( F \)
B \( 2F \)
C \(\frac{1}{8} F\)
D \(\frac{3}{8} F\)

Verified Solution

Answer & Solution

Correct Answer

(D) \(\frac{3}{8} F\)

Step-by-step Solution

Detailed explanation

\( Q_A = Q, Q_B = Q \) \( F = k \frac{Q_A Q_B}{r^2} = k \frac{Q^2}{r^2} \) After C touches A: \( Q_A' = Q_C' = \frac{Q+0}{2} = \frac{Q}{2} \) After C touches B: \( Q_B'' = Q_C'' = \frac{Q + Q_C'}{2} = \frac{Q + Q/2}{2} = \frac{3Q/2}{2} = \frac{3Q}{4} \) New force…

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

Same subject

Explore more questions on app

From CUET

Explore more questions on app

List-I	List-II
(A) Resonant frequency	(I) \( \frac{\omega L}{R} \)
(B) Quality factor	(II) \( 2\pi f L \)
(C) \( X_L \)	(III) \( \frac{1}{2\pi\sqrt{LC}} \)
(D) \( X_C \)	(IV) \( 1/2\pi f c \)