PhysicsCBSE Class 12 · Sample paper, session 2024-25 · 1 mark · MCQ

CBSE Class 12 Physics sample paper 2024-25: question 2

From the official CBSE sample question paper for 2024-25, section A, question 2, for 1 mark. Try it first; the hints and our step-by-step solution are below.

Q2 · 1 mark · MCQElectrostatic Potential and Capacitance

A conducting wire connects two charged conducting spheres of radii $r_{1}$ and $r_{2}$ such that they attain equilibrium with respect to each other. The distance of separation between the two spheres is very large as compared to either of their radii. The ratio of the magnitudes of the electric fields at the surfaces of the spheres of radii $r_{1}$ and $r_{2}$ is

(A) $\frac{r _{1}}{r _{2}}$
(B) $\frac{r _{2}}{r _{1}}$
(C) $\frac{r _{2}^{2}}{r _{1}^{2}}$
(D) $\frac{r _{1}^{2}}{r _{2}^{2}}$

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2024-25.

Hints

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

Hint 1

What quantity becomes equal when two conductors are joined by a wire?

Hint 2

Use $E = k q / r^{2}$ together with the charge ratio.

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.

The connected spheres reach equal potential: $\frac{k q _{1}}{r _{1}} = \frac{k q _{2}}{r _{2}}$ , so $\frac{q _{1}}{q _{2}} = \frac{r _{1}}{r _{2}}$ .
½ mark
The surface field is $E = \frac{k q}{r ^{2}}$ , so $\frac{E _{1}}{E _{2}} = \frac{q _{1}}{q _{2}} \cdot \frac{r _{2}^{2}}{r _{1}^{2}} = \frac{r _{2}}{r _{1}}$ . The answer is (B).
½ mark

Answer: $r_{2} / r_{1}$ , option (B)

Where the marks are

Equal potentials give $q_{1} / q_{2} = r_{1} / r_{2}$ (0.5 mark)
$E_{1} / E_{2} = r_{2} / r_{1}$ , option (B) (0.5 mark)

Revise the chapter

Chapter: Electrostatic Potential and Capacitance (notes, formulas, where marks are lost and every past question)

Practise this chapterBoard-style questions from Electrostatic Potential and Capacitance, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2024-25: every question, by section.

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CBSE Class 12 Physics sample paper 2024-25: question 2

Hints

Step-by-step solution

Revise the chapter

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