Physics · Class 12 · Electrostatics

Capacitors for JEE

Everything you need for this topic on one page: the ideas, the formulas, the traps, and free lectures that explain it well.

Last updated: 26 October 2026

Direct answer

Capacitance, parallel-plate capacitors, dielectrics, series and parallel combinations, energy stored and charge sharing.

Key ideas

Capacitors: what you must understand

C = Q/V depends only on geometry and medium
A dielectric increases C by the factor K
In series the charge is the same; in parallel the voltage is the same
Charge is conserved when capacitors are reconnected; energy is not

Formula sheet

Must-know formulas for Capacitors

C = Kε₀A/d
Energy U = ½CV² = Q²/2C
Series: 1/C = Σ1/Cᵢ; parallel: C = ΣCᵢ
Partial slab: C = ε₀A / (d − t + t/K)

One-page formula sheet

Printable Capacitors formula sheet: formulas, key ideas and traps on one page. Save it as a PDF for offline revision.

Avoid

Common mistakes students make

Assuming V stays constant after the battery is removed
Forgetting energy lost as heat in charge sharing
Wrong combination formula (it is opposite to resistors)

For the exam

How this topic is asked in JEE

Appears in almost every JEE Main shift. Battery connected vs disconnected is the key question in most dielectric problems.

Prerequisites: Electrostatics (Electric Charges and Fields, Potential).

NCERT map

Where Capacitors is in NCERT

Chapter names follow the current rationalised NCERT textbooks. Read these sections before solving PYQs.

Class	Book	Chapter	Sections JEE asks from
Class 12	Physics Part I	Electrostatic Potential and Capacitance	Capacitors and Capacitance; The Parallel Plate Capacitor; Effect of Dielectric on Capacitance; Combination of Capacitors; Energy Stored in a Capacitor

JEE Advanced level

A JEE Advanced-level example on Capacitors

Question. A parallel-plate capacitor of capacitance 4 μF is charged to 100 V and then disconnected from the battery. A dielectric slab of dielectric constant K = 3 and thickness equal to half the plate separation is inserted between the plates. Find (a) the new capacitance, (b) the new voltage across the capacitor, and (c) the energy lost in the process.

Show solution

Initial charge Q₀ = C₀V₀ = 4 μF × 100 V = 400 μC. Since the battery is disconnected, this charge stays constant throughout.
With dielectric filling half the gap, treat it as two capacitors in series: air part has C₁ = 2C₀ = 8 μF, dielectric part has C₂ = 2KC₀ = 24 μF.
New capacitance: C = C₁C₂/(C₁ + C₂) = (8 × 24)/(8 + 24) = 192/32 = 6 μF.
New voltage: V = Q/C = 400 μC / 6 μF = 200/3 V ≈ 66.7 V.
Initial energy U₀ = ½C₀V₀² = ½ × 4 × 10⁻⁶ × 10000 = 20 mJ. Final energy U = Q²/2C = (400 × 10⁻⁶)²/(2 × 6 × 10⁻⁶) = 40/3 mJ ≈ 13.3 mJ.
Energy lost = 20 − 40/3 = 20/3 mJ ≈ 6.67 mJ. This energy goes into pulling the dielectric slab into the capacitor.

Answer: (a) 6 μF, (b) 200/3 V ≈ 66.7 V, (c) 20/3 mJ ≈ 6.67 mJ

This example was solved twice independently and both solutions agree.

Free lectures

Best free lectures for Capacitors

Full-chapter free lectures picked from YouTube search results by views, from established JEE channels (mostly in Hinglish). Use the subtopic list to jump straight to the part you need (timestamps come from each video's own chapter list). Watch one, then solve PYQs the same day.

CAPACITOR in One Shot: All Concepts & PYQs Covered |JEE Main & Advanced — JEE Wallah, 7:18:52
Jump to a subtopic (15)
- 00:00 Introduction
- 09:09 Capacitor
- 20:00 Law of capacitance
- 35:32 Calculation of capacitance
- 51:25 Combination of Capacitors
- 1:29:40 Wheatstone bridge problem
- 1:53:39 Infinite ladder problem
- 2:00:39 Recap & Topics to be covered
- 4:17:09 Unbalanced Wheatstone bridge
- 4:26:08 Circuit analysis problem
- 4:40:10 Slab problem
- 6:13:02 Battery connected and disconnected
- 6:58:00 Kirchhoff’s voltage law
- 7:02:19 RC circuit
- 7:18:42 Thankyou bachhon
CAPACITOR in 1 Shot - All Concepts Covered || JEE Main & Advanced || Class 12 — Lakshya JEE, 2:09:23
Jump to a subtopic (11)
- 00:00 Introduction
- 02:43 Capacitor
- 40:43 Switching problem
- 50:07 Series and parallel
- 1:00:50 Questions based on connection between capacitor
- 1:11:22 Medium in capacitor
- 1:22:49 Battery connected and Disconnected
- 1:33:08 Plate type Capacitors
- 1:41:01 RC circuit
- 1:53:40 Discharging of capacitor
- 2:08:03 Thank You Bacchon

Practise

Where to practise for free

8 JEE Main 2026 PYQs on Capacitors, with solutions.
Read the chapter in NCERT textbooks (official, free), then solve its NCERT Exemplar (official, free) questions.
Chapter-wise past questions: JEE Main and Advanced PYQs and question bank.
Timed practice tests: NTA Abhyas practice tests (official, free) and our free mock tests.
See it move: interactive simulation.

FAQs

Frequently asked questions

Is Capacitors important for JEE Main 2027?

Appears in almost every JEE Main shift. It is part of the official 'Electrostatics' unit in the JEE Main 2027 syllabus.

What are the most important formulas in Capacitors?

C = Kε₀A/d; Energy U = ½CV² = Q²/2C; Series: 1/C = Σ1/Cᵢ; parallel: C = ΣCᵢ.

How should I study Capacitors for JEE?

Understand the key ideas from NCERT or one full lecture, learn the formula sheet, then solve 25–30 past JEE questions on the topic. Battery connected vs disconnected is the key question in most dielectric problems.