PhysicsCBSE Class 12 · Board exam 2026, set 55/5/1 · 3 marks · Short answer

CBSE Class 12 Physics 2026: question 22

Set in the CBSE Class 12 board exam of 2026, paper 55/5/1, section C, question 22, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q22 · 3 marks · Short answerElectrostatic Potential and Capacitance

Two parallel plate capacitors X and Y are connected in series to a 6 V battery. They have the same plate area and same plate separation but capacitor X has air between its plates, whereas capacitor Y contains a material of dielectric constant 4.
(a) Calculate the capacitances of X and Y, if the equivalent capacitance of the combination of X and Y is 4 $μ$ F.
(b) Calculate the potential difference across the plates of X and Y.

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

A dielectric of constant K multiplies the capacitance by K.

Hint 2

In series, the charge on both capacitors is the same; V = Q/C for each.

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.

Same A and d, so $C_{X} = \frac{ε _{0} A}{d} = C$ and $C_{Y} = \frac{K ε _{0} A}{d} = 4 C$ .
½ mark
Series: $C_{e q} = \frac{C \cdot 4 C}{C + 4 C} = \frac{4 C}{5} = 4 μ F \Rightarrow C = 5 μ F$ .
So $C_{X} = 5 μ$ F, $C_{Y} = 20 μ$ F.
1 mark
(b) Charge on each (series): $Q = C_{e q} V = 4 μ F \times 6 V = 24 μ$ C
½ mark
$V_{X} = \frac{Q}{C _{X}} = \frac{24}{5} = 4.8$ V; $V_{Y} = \frac{Q}{C _{Y}} = \frac{24}{20} = 1.2$ V (check: 4.8 + 1.2 = 6 V)
1 mark

Answer: (a) C_X = 5 μF, C_Y = 20 μF; (b) V_X = 4.8 V, V_Y = 1.2 V

Where the marks are

Recognises $C_{Y} = 4 C_{X}$ (½ mark)
Uses series formula to get $C_{X} = 5$ μF, $C_{Y} = 20$ μF (1 mark)
Finds common charge 24 μC (½ mark)
Finds $V_{X} = 4.8$ V and $V_{Y} = 1.2$ V (1 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 paper2026 board paper, set 55/5/1: every question, by section.

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CBSE Class 12 Physics 2026: question 22

Hints

Step-by-step solution

Revise the chapter

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