PhysicsCBSE Class 12 · Sample paper, session 2025-26 · 3 marks · Short answer

CBSE Class 12 Physics sample paper 2025-26: question 23(II)

From the official CBSE sample question paper for 2025-26, section C, question 23(II), for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q23(II) · 3 marks · Short answerCurrent Electricity

How the balancing condition gets affected if you are interchanging the galvanometer and the cell in the Wheat stone bridge?

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2025-26.

Hints

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

Hint 1

Write the usual balancing condition first.

Hint 2

After swapping, the galvanometer joins A and C; what must be true of $V_{A}$ and $V_{C}$ ?

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.

Let arms be AB = P, BC = Q, AD = R, DC = S, with cell across A–C and galvanometer across B–D. Balance condition: $\frac{P}{Q} = \frac{R}{S}$ .
1 mark
After interchange, cell is across B–D and galvanometer across A–C. Current from B divides along B→A→D (P, R) and B→C→D (Q, S): $I_{1} = \frac{V}{P + R}$ , $I_{2} = \frac{V}{Q + S}$ . No deflection requires $V_{A} = V_{C}$ , i.e. $I_{1} P = I_{2} Q$ : $\frac{P}{P + R} = \frac{Q}{Q + S}$ .
1 mark
This gives $P Q + P S = QP + QR \Rightarrow P S = QR \Rightarrow \frac{P}{Q} = \frac{R}{S}$ . So the balance condition is unchanged; the bridge remains balanced on interchanging the cell and galvanometer.
1 mark

Answer: No change; the balance condition P/Q = R/S remains the same

Where the marks are

States the balance condition of the bridge (1 mark)
Sets up the condition for null deflection after interchange (1 mark)
Shows it reduces to the same P/Q = R/S, so no change (1 mark)

Revise the chapter

Chapter: Current Electricity (notes, formulas, where marks are lost and every past question)
Topic: Kirchhoff's Rules and Wheatstone Bridge

Practise this chapterBoard-style questions from Current Electricity, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2025-26: every question, by section.

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CBSE Class 12 Physics sample paper 2025-26: question 23(II)

Hints

Step-by-step solution

Revise the chapter

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