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A cell of emf E and internal… (Class 12 Physics 2025, Q17)

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PhysicsCBSE Class 12 · Board exam 2025, set 55/7/1 · 2 marks · Very short answer

CBSE Class 12 Physics 2025: question 17

Set in the CBSE Class 12 board exam of 2025, paper 55/7/1, section B, question 17, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q17 · 2 marks · Very short answerCurrent Electricity

A cell of emf E and internal resistance r is connected across a resistor of variable resistance R. Show graphically the variation of
(a) the terminal voltage across the cell,
(b) the current supplied by the cell,
with R as it is increased from 0 to the maximum value.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 55/7/1.

Hints

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

Hint 1

Write the current using Ohm's law for the complete circuit, including r.

Hint 2

Use V = E − Ir and check what happens at R = 0 and when R becomes very large.

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.

  1. Current in the circuit: . Terminal voltage: .

  2. (a) V versus R graph: at , . As increases, rises quickly at first and then more slowly. The curve is concave downward and approaches asymptotically as . It never crosses the line , and when .

    1 mark

  3. (b) I versus R graph: at , (the maximum, short-circuit current). As increases, falls along a hyperbola-like curve, concave upward, and tends to zero as .

    1 mark

Answer: (a) V rises from 0 and tends asymptotically to E; (b) I falls from E/r and tends to 0.

Where the marks are

  • Correct V–R graph: starting at 0 and saturating at E (1 mark)
  • Correct I–R graph: starting at E/r and decreasing towards zero (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Current Electricity, one at a time, marked against the answer key and marking points.Back to the full paper2025 board paper, set 55/7/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.