Two small identical metallic balls having charges q and -2q are kept far at a separation r. They are brought in contact and then separated at distance r/2. Compared to the initial force F, they will now : (A) attract with a force F/2 (B) repel with a force F/2 (C) repel with a force F (D) attract with a force F

PhysicsCBSE Class 12 · Board exam 2026, set 55/5/1 · 1 mark · MCQ

CBSE Class 12 Physics 2026: question 1

Set in the CBSE Class 12 board exam of 2026, paper 55/5/1, section A, question 1, for 1 mark. Try it first; the hints and our step-by-step solution are below.

Q1 · 1 mark · MCQElectric Charges and Fields

Two small identical metallic balls having charges q and $- 2 q$ are kept far at a separation r. They are brought in contact and then separated at distance $\frac{r}{2}$ . Compared to the initial force F, they will now :

(A)attract with a force $\frac{F}{2}$
(B)repel with a force $\frac{F}{2}$
(C)repel with a force F
(D)attract with a force F

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 55/5/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Find the charge on each ball after contact.

Hint 2

Like charges repel. Use Coulomb's law with separation r/2.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

Initially the force is $F = \frac{k ( q ) ( 2 q )}{r ^{2}} = \frac{2 k q ^{2}}{r ^{2}}$ , and it is attractive.
On contact the total charge $- q$ is shared equally, so each ball has $- q /2$ . The new force is $F^{'} = \frac{k ( q /2 ) ^{2}}{( r /2 ) ^{2}} = \frac{k q ^{2}}{r ^{2}} = \frac{F}{2}$ . Both charges are now negative, so the balls repel.
1 mark

Answer: Repel with force F/2, option (B)

Where the marks are

Identifies a repulsive force of magnitude F/2 (1 mark)

Revise the chapter

Chapter: Electric Charges and Fields (notes, formulas, where marks are lost and every past question)
Topic: Coulomb's Law and Superposition

Practise this chapterBoard-style questions from Electric Charges and Fields, one at a time, marked against the answer key and marking points.Back to the full paper2026 board paper, set 55/5/1: every question, by section.

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CBSE Class 12 Physics 2026: question 1

Hints

Step-by-step solution

Revise the chapter

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