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Exam level3 past questions

Dielectrics and their effect on capacitors

Study how inserting dielectrics affects capacitance, electric field, and potential difference in various capacitor setups.

Why this shows up in the exam

NEET tests your understanding of how dielectrics modify capacitor properties and circuit behavior.

How NEET tests this

Statement analysis · 2 QsDirect recallApplication

Learn the idea

A dielectric increases a capacitor’s ability to store charge (C becomes κ C₀) by allowing the same voltage to produce more charge, and it does this by reducing the electric field inside the capacitor by a factor of κ.

🧠 Memory hook: Dielectric = ‘kappa‑boost’ for capacitance, but a ‘kappa‑cut’ for field – like turning up the volume (C) while turning down the pitch (E).

Get this one clearly and it pays off every single time it shows up in the paper. 🎯

Formulas & facts to keep ready

  • C₀ = ε₀A/d for a parallel‑plate capacitor
  • With a dielectric, C = κC₀ where κ>1
  • Electric field with dielectric E = E₀/κ
  • For an isolated charged capacitor Q is constant, so V drops to V/κ
  • For a capacitor connected to a battery V is constant, so Q rises to κQ

How to approach it

  1. 1Read the question carefully – is the capacitor isolated or connected to a source?
  2. 2Write the original capacitance C₀ = ε₀A/d and then replace it by C = κC₀
  3. 3Use E = V/d and the relation E = σ/ε₀ → with dielectric E = E₀/κ
  4. 4Apply Q = CV or V = Q/C depending on whether Q or V is fixed
  5. 5Check the final quantity asked (C, E, V or Q) against the case you used

Worked example — watch it click

Assertion A: The capacitance of a capacitor is increased when a dielectric is inserted between the plates. Reason R: The dielectric increases the electric field between the plates.

  • A)If both assertion and reason are true, and reason is the correct explanation of assertion.
  • B)If both assertion and reason are true, but reason is not the correct explanation of assertion.
  • ✅If assertion is true but reason is false.
  • D)If both assertion and reason are false.

The concept behind this problem

The example asks you to decide whether the statement that a dielectric raises the field is correct; it tests the core fact that the field is reduced, even though capacitance rises.

Step by step

  1. 1Assertion: When a dielectric is inserted between capacitor plates, C = κC₀ where κ > 1, so capacitance increases.
  2. 2TRUE.
  3. 3Reason: The dielectric reduces (not increases) the electric field between plates by a factor of κ.
  4. 4The field E = E₀/κ where E₀ is the field without dielectric.
  5. 5FALSE.
  6. 6The capacitance increases because the dielectric reduces the field for the same charge, allowing more charge storage at the same voltage.

Watch out

Students often answer that the dielectric increases the electric field, forgetting that the field is divided by κ.

Common slip-ups that cost marks

  • •Thinking the dielectric increases the field – it actually reduces it
  • •Mixing up the constant‑charge and constant‑voltage situations
  • •Assuming κ = 1 when the dielectric only partially fills the gap

🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.

Practise it

These are real questions from past NEET papers that test this exact idea.

Question 1 of 3

Assertion A: The capacitance of a capacitor is increased when a dielectric is inserted between the plates. Reason R: The dielectric increases the electric field between the plates.

Push further

More challenging

4 harder questions built from the past papers above — a step up in difficulty, with distractors designed so you can't get there by elimination. Written and checked by our reviewers, not from a real paper.

Question 1 of 4

A parallel plate capacitor has an electric field E0 between its plates when air is present. If a dielectric material with dielectric constant K is introduced to completely fill the space between the plates, what is the new electric field E between the plates, assuming the charge on the plates remains constant?

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