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NEET Physics · Electrostatics

Energy stored in a capacitor

Taught in NCERT Class 12 Physics, Chapter 2: Electrostatic Potential and Capacitance.

2 verified questions: Easy 0 · Medium 1 · NEET level 1

Sample questions

Sample 1Medium

A parallel plate capacitor is charged by a battery, and its stored energy is U. The battery is disconnected and a slab of dielectric constant K is then inserted to fill the space between the plates completely. The energy stored now is

  1. A

    KU

  2. B

    U/K

  3. C

    U

  4. D

    K²U

Show the answer
The answer is B.

With the battery disconnected the charge Q stays the same while C becomes KC. Energy Q²/2C therefore becomes U/K.

  • A.

    KU is the result if the battery stays connected (V constant), not when the charge is fixed.

  • C.

    U would require the capacitance to stay unchanged, but it rises K times.

  • D.

    K²U has no basis; with charge fixed, energy varies as 1/C.

NCERT: Class 12 Physics, Chapter 2, Energy stored in a capacitor

Sample 2NEET level

Capacitors of 2 µF and 3 µF are connected in series across a 100 V battery. The total energy stored in the combination is

  1. A

    6 mJ

  2. B

    4.2 mJ

  3. C

    12 mJ

  4. D

    25 mJ

Show the answer
The answer is A.

The series capacitance is C = (2 × 3)/(2 + 3) = 1.2 µF, so U = ½CV² = ½ × 1.2 × 10⁻⁶ × 10⁴ = 6 × 10⁻³ J = 6 mJ. In series the equivalent is smaller than either capacitor.

  • B.

    Takes C = 1/2 + 1/3 = 5/6 µF without inverting the sum of reciprocals.

  • C.

    Uses U = CV², dropping the factor ½.

  • D.

    Adds the capacitances (5 µF) as if they were in parallel.

NCERT: Class 12 Physics, Chapter 2 (Electrostatic Potential and Capacitance), Energy stored in a capacitor

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