Definition
Capacitance
C = Q/V (charge stored per unit voltage applied). SI unit: farad (F) = C/V. Depends only on geometry and dielectric, NOT on Q or V.
-- NCERT Class 12 Physics, Ch. 2, p. 68Capacitors Capacitance
8 MCQs9-step worked example
Source: NCERT ElectrostaticsOfficial key: NTA-verifiedLast updated: 21 Sep 2026
Capacitors Capacitance, explained for NEET
The trap: reading C = Q/V as "capacitance goes up when you add charge" or "capacitance halves when you double the voltage." NCERT Class 12 Physics Part I, Chapter 2 (Section 2.11, page 67) says the opposite: the field between the conductors is proportional to Q, so V is also proportional to Q, and the ratio Q/V is a constant. That constant does not depend on Q or V.
The definition. A capacitor is a system of two conductors separated by an insulator. In practice the conductors carry +Q and −Q with potential difference V between them, and
C = Q/V (NCERT Chapter 2, Eq. 2.38, page 67)
Q is called "the charge of the capacitor", although it is the charge on one conductor — the total charge of the capacitor is zero (page 67).
What C depends on. Only the geometrical configuration of the two conductors — shape, size, separation (pages 67–68) — and, as NCERT adds, the nature of the insulator (dielectric) between them (page 68). Change Q or V and C stays put; the other one of Q and V adjusts in proportion.
The farad. SI unit: 1 F = 1 C V⁻¹ (page 68). NCERT calls the farad a very big unit; practical values are in μF (10⁻⁶ F), nF (10⁻⁹ F) and pF (10⁻¹² F).
Why large C matters. For a given Q, larger C means smaller V (page 68). A high potential difference means a strong field, which can ionise the surrounding air and let the charge leak away. NCERT gives the dielectric strength of air as about 3 × 10⁶ V m⁻¹, so there is a limit to how much charge a given capacitor can hold without significant leaking.
Bridge to NEET. Direct questions give two of Q, C, V and ask for the third. Distractors come from multiplying where you should divide (Q × V), inverting the ratio (V/Q), and prefix slips (μ read as m).
Watch out: when V is changed on the same capacitor, Q changes in proportion — C does not.
Can you answer these Capacitors Capacitance MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
According to NCERT, the capacitance of a capacitor depends on:
Show answer and why every option is right or wrong
Answer: C. NCERT Class 12 Physics Part I, Chapter 2, Section 2.11 (pages 67–68): C is independent of Q or V; it depends only on the geometrical configuration of the two conductors and on the nature of the dielectric separating them.
Why A is wrong: A is wrong because NCERT states C is independent of Q; adding charge raises V in proportion, so Q/V is unchanged.
Why B is wrong: B is wrong because NCERT states C is independent of V; changing V changes Q in proportion, leaving Q/V constant.
Why D is wrong: D is wrong because C = Q/V defines capacitance as a ratio that stays constant; it is not set by the values of Q and V themselves.
MCQ 2Easy RecallPractice
One farad is equal to:
Show answer and why every option is right or wrong
Answer: D. From C = Q/V, the SI unit is 1 farad = 1 coulomb per volt, 1 F = 1 C V⁻¹ (NCERT Class 12 Physics Part I, Chapter 2, page 68).
Why A is wrong: A is wrong because it multiplies charge by potential difference instead of dividing; C = Q/V, not Q × V.
Why B is wrong: B is wrong because 1 J C⁻¹ is the volt, the unit of potential, not of capacitance.
Why C is wrong: C is wrong because it inverts the ratio; V C⁻¹ is the unit of V/Q, which is 1/C, not capacitance.
MCQ 3Easy RecallPractice
The two conductors of a charged capacitor carry charges +Q and −Q. Which statement matches NCERT's convention?
Show answer and why every option is right or wrong
Answer: B. NCERT Class 12 Physics Part I, Chapter 2, Section 2.11 (page 67): Q is called the charge of the capacitor, though it is the charge on one of the conductors; the total charge of the capacitor is zero.
Why A is wrong: A is wrong because it adds the magnitudes of the two plate charges; the charges are +Q and −Q, so they sum to zero, and the capacitor's charge is quoted as Q.
Why C is wrong: C is wrong because the total adds +Q and −Q, which gives zero, not 2Q.
Why D is wrong: D is wrong because only the total charge is zero; by convention the charge of the capacitor is Q, the charge on one conductor.
MCQ 4Direct ApplicationPractice
A capacitor holds a charge of 6.0 × 10⁻⁴ C when the potential difference between its conductors is 12 V. Its capacitance is:
Show answer and why every option is right or wrong
Answer: C. C = Q/V = 6.0 × 10⁻⁴ C / 12 V = 5.0 × 10⁻⁵ F, i.e. 5.0 × 10¹ μF (NCERT Class 12 Physics Part I, Chapter 2, Eq. 2.38, page 67).
Why A is wrong: A is wrong because 2.0 × 10⁴ comes from V/Q = 12 / 6.0 × 10⁻⁴, inverting the ratio.
Why B is wrong: B is wrong because 7.2 × 10⁻³ comes from Q × V = 6.0 × 10⁻⁴ × 12, multiplying where Q must be divided by V.
Why D is wrong: D is wrong because 5.0 × 10⁻² F is the correct value 5.0 × 10¹ read in millifarads instead of microfarads — a μ-for-m prefix slip.
MCQ 5Direct ApplicationPractice
A capacitor of capacitance 2.0 × 10⁻⁵ F is connected across a potential difference of 15 V. The charge on the capacitor is:
Show answer and why every option is right or wrong
Answer: B. Q = CV = 2.0 × 10⁻⁵ F × 15 V = 3.0 × 10⁻⁴ C (rearranging NCERT Class 12 Physics Part I, Chapter 2, Eq. 2.38, page 67).
Why A is wrong: A is wrong because 1.3 × 10⁻⁶ comes from C/V = 2.0 × 10⁻⁵ / 15, dividing where C must be multiplied by V.
Why C is wrong: C is wrong because 7.5 × 10⁵ comes from V/C = 15 / 2.0 × 10⁻⁵, which is not a charge at all.
Why D is wrong: D is wrong because 3.0 × 10² comes from treating the capacitance as 20 F, i.e. dropping the 10⁻⁶ of the microfarad (20 μF): 20 × 15 = 300.
MCQ 6Direct ApplicationPractice
A 0.50 μF capacitor carries a charge of 2.5 × 10⁻⁵ C. The potential difference between its conductors is:
Show answer and why every option is right or wrong
Answer: D. V = Q/C = 2.5 × 10⁻⁵ C / 5.0 × 10⁻⁷ F = 5.0 × 10¹ V = 50. V (from C = Q/V, NCERT Class 12 Physics Part I, Chapter 2, Eq. 2.38, page 67).
Why A is wrong: A is wrong because 0.050 V comes from reading 0.50 μF as 0.50 mF (5.0 × 10⁻⁴ F): 2.5 × 10⁻⁵ / 5.0 × 10⁻⁴ = 0.050.
Why B is wrong: B is wrong because 1.3 × 10⁻¹¹ comes from Q × C = 2.5 × 10⁻⁵ × 5.0 × 10⁻⁷ = 1.25 × 10⁻¹¹, multiplying where Q must be divided by C.
Why C is wrong: C is wrong because 0.020 comes from C/Q = 5.0 × 10⁻⁷ / 2.5 × 10⁻⁵, inverting the ratio.
MCQ 7CalculationPractice
Capacitor X holds 3.0 μC when the potential difference across it is 1.5 V. Capacitor Y holds 8.0 μC when the potential difference across it is 2.0 V. The ratio of capacitances C_X : C_Y is:
Show answer and why every option is right or wrong
Answer: A. C_X = 3.0 μC / 1.5 V = 2.0 μF and C_Y = 8.0 μC / 2.0 V = 4.0 μF, so C_X : C_Y = 2.0 : 4.0 = 1 : 2 (C = Q/V, NCERT Class 12 Physics Part I, Chapter 2, Eq. 2.38, page 67).
Why B is wrong: B is wrong because 2 : 1 comes from V/Q: 1.5/3.0 = 0.50 and 2.0/8.0 = 0.25, giving 0.50 : 0.25 — the inverted ratio.
Why C is wrong: C is wrong because 3 : 8 is the ratio of the charges alone; capacitance is Q/V, so each charge must be divided by its own potential difference.
Why D is wrong: D is wrong because 3 : 4 is the ratio of the potential differences alone (1.5 : 2.0), ignoring the charges.
MCQ 8CalculationPractice
Suppose two air-filled capacitors, C₁ = 2.0 × 10⁻¹² F and C₂ = 5.0 × 10⁻¹¹ F, each start leaking charge if the potential difference across them exceeds 3.0 × 10⁴ V. Which statement about the maximum charge each can hold without leaking is correct?
Show answer and why every option is right or wrong
Answer: A. Q_max = C V_max. Q₁ = 2.0 × 10⁻¹² × 3.0 × 10⁴ = 6.0 × 10⁻⁸ C; Q₂ = 5.0 × 10⁻¹¹ × 3.0 × 10⁴ = 1.5 × 10⁻⁶ C; ratio 25. NCERT Class 12 Physics Part I, Chapter 2, page 68: a capacitor with large capacitance can hold a large charge at a relatively small potential difference.
Why B is wrong: B is wrong because 6.0 × 10⁻⁸ C is only C₁'s limit; with the same V limit, Q = CV grows with C, so C₂'s limit is 1.5 × 10⁻⁶ C.
Why C is wrong: C is wrong because 6.7 × 10⁻¹⁷ and 1.7 × 10⁻¹⁵ come from C/V (2.0 × 10⁻¹² / 3.0 × 10⁴ and 5.0 × 10⁻¹¹ / 3.0 × 10⁴), dividing where C must be multiplied by V.
Why D is wrong: D is wrong because it swaps the two correct charges (6.0 × 10⁻⁸ C and 1.5 × 10⁻⁶ C) between the capacitors; for the same V, the larger capacitance holds the larger charge.
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How do you solve a Capacitors Capacitance question? A worked example
- 1
Given
A capacitor holds a charge of 4.8 × 10⁻⁵ C when the potential difference between its conductors is 16 V. Nothing about the capacitor (its conductors, their separation, or the insulator between them) is changed, and the potential difference is then raised to 25 V.
- 2
Required
(a) The capacitance C, in μF. (b) The new charge Q′ at 25 V.
- 3
Concept
C = Q/V is a constant for a given capacitor: it depends on the geometry of the conductors and the insulator, not on Q or V (NCERT Chapter 2, pages 67–68). Find C from the first reading, then use the same C at the new potential difference.
- 4
Formula
C = Q/V, so Q′ = C V′.
- 5
Substitution
(a) C = 4.8 × 10⁻⁵ C / 16 V
(b) Q′ = C × 25 V - 6
Calculation
(a) C = 4.8 × 10⁻⁵ / 16 = 3.0 × 10⁻⁶ F = 3.0 μF
(b) Q′ = 3.0 × 10⁻⁶ × 25 = 7.5 × 10⁻⁵ C
The only exact number here is the prefix conversion 1 μF = 10⁻⁶ F, which is a definition and does not contribute to the significant-figure count. All measured values carry two significant figures, so both answers are given to two significant figures. - 7
Final answer
C = 3.0 μF; charge at 25 V = 7.5 × 10⁻⁵ C
- 8
Common trap
Keeping the charge at 4.8 × 10⁻⁵ C and concluding that the capacitance has fallen to 4.8 × 10⁻⁵ / 25 = 1.9 × 10⁻⁶ F. The capacitor was not altered, so C stays 3.0 μF and Q rises in proportion to V: 4.8 × 10⁻⁵ × (25/16) = 7.5 × 10⁻⁵ C, which agrees with the answer above.
- 9
Similar NEET-style question
"A capacitor holds 9.0 μC when the potential difference across it is 3.0 V. What potential difference is needed for the same capacitor to hold 24 μC?"
Strategy: C = 9.0/3.0 = 3.0 μF, unchanged; V = Q/C = 24/3.0 = 8.0 V. Do not keep V at 3.0 V, and do not multiply Q by C (24 × 3.0 = 72).
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What to remember before solving Capacitors Capacitance questions
Which Capacitors Capacitance formulas do you need for NEET?
1 formula — click to collapse
Parallel plate capacitor
Capacitance of parallel-plate capacitor. eps_r = 1 for vacuum/air.
| Symbol | Quantity | SI Unit |
|---|---|---|
| C | capacitance | F |
| A | plate area | m^2 |
| d | plate separation | m |
| eps_r | relative permittivity | - |
Valid when
- Plate separation << plate dimensions (uniform field)
- Uniform dielectric
More in Electrostatics: 3 exam traps and mistakes · 10 formulas · 6 question patterns from its other lessons.
Capacitors Capacitance questions from past NEET papers
No question in our NEET 2020–2025 set targets this topic directly.
Sources
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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