What is the formula for calculating the energy stored in a capacitor?

The energy stored in a capacitor can be calculated using the formula: E = 1/2 * C * V^2, where E is the energy in joules, C is the capacitance in farads, and V is the voltage across the capacitor in volts. This formula is crucial for understanding how capacitors store energy in electrical circuits, a key concept in Cambridge International A Levels Physics.

How does the capacitance of a capacitor affect the energy stored?

The capacitance of a capacitor directly affects the energy stored, as seen in the formula E = 1/2 * C * V^2. A higher capacitance means the capacitor can store more energy for a given voltage. This relationship is important for students studying capacitance in the context of Cambridge International A Levels Physics.

Why is energy stored in a capacitor important in electrical circuits?

Energy stored in a capacitor is crucial because it allows capacitors to release energy quickly when needed, making them essential for applications like power supply smoothing, signal processing, and energy storage in electronic devices. Understanding this concept is vital for students learning about electrical circuits in Cambridge International A Levels Physics.

What factors influence the amount of energy a capacitor can store?

The amount of energy a capacitor can store is influenced by its capacitance, the voltage applied across it, and the dielectric material used. The formula E = 1/2 * C * V^2 shows that both capacitance and voltage are directly proportional to the energy stored. These factors are key considerations in the study of capacitance for Cambridge International A Levels Physics students.

How does the dielectric material in a capacitor affect its energy storage capacity?

The dielectric material in a capacitor affects its energy storage capacity by influencing the capacitance. A better dielectric increases the capacitance, allowing more energy to be stored for the same voltage. This concept is important for understanding how capacitors function in various applications, a topic covered in Cambridge International A Levels Physics.

Why is "Using $P = VI$ for capacitor energy" a common mistake in Energy stored in a capacitor?

Why it happens: Confusion. How to avoid it: P = VI is power in resistor. Capacitor stores ENERGY directly. Use 1/2CV^2. Source: 9702 Examiner Reports 2022-2024

Cambridge International A Levels Physics (9702)

Energy stored in a capacitor

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Short Notes - Energy stored in a capacitor

Detailed Study Notes

Detailed notes on Capacitance for Cambridge International A Levels Physics, covering key concepts, explanations, examples, and exam-focused revision points.

Capacitor Energy Study Notes — Cambridge International A Level Physics 9702 (2025-2027 syllabus)

Three forms of energy stored.

At a glance

$E = \frac{1}{2}CV^2$ .
$= \frac{1}{2}QV$ .
$= Q^2/(2C)$ .
Area under $Q$ - $V$ graph.

What you’ll learn

Mapped to the Cambridge International A Level 9702 syllabus (2025-2027).

19.2.1 — Compute energy stored.

Energy stored

Three equivalent forms.

Three equivalent expressions.

$E = \frac{1}{2}CV^2$ .
$E = \frac{1}{2}QV$ .
$E = Q^2/(2C)$ .

All derived from area under $Q$ - $V$ graph (which is a straight line through origin with slope $1/C$ ).

Pick form based on knowns.

Example. $C = 100$ μF, $V = 50$ V → $E = \frac{1}{2}(100\mu)(50^2) = 0.125$ J.

Cambridge tip. Often easier to use $\frac{1}{2}CV^2$ .

Figure 1 — Energy stored = area under the Q-V graph

Energy stored on a capacitor equals the triangular area under the Q–V graph: E = ½QV = ½CV² = Q²/(2C).

Figure 2 — Why the energy is ½QV and not QV

Unlike a battery, a capacitor's p.d. is not constant while it charges. Because it rises linearly from zero, the average p.d. is V/2 — which is exactly where the factor of a half comes from.

Figure 3 — Choosing between the three energy formulas

The three forms are related by C = Q/V, so no new physics is involved — only algebra. Choosing well saves a rearrangement step.

Figure 4 — Worked example: 100 μF charged to 50 V

The whole calculation is one substitution. The only place marks are usually lost is the prefix conversion at the start.

Which energy formula to reach for.

You are given	Use	Why
$C$ and $V$	$E = \frac{1}{2}CV^2$	The two values most often quoted on a capacitor
$Q$ and $V$	$E = \frac{1}{2}QV$	Straight from the area under the $Q$ - $V$ graph
$Q$ and $C$	$E = \dfrac{Q^2}{2C}$	Avoids having to find $V$ first

All three give the same energy, because $C = Q/V$ links them.

Three equivalent forms.
Area under $Q$ - $V$ .

See the full worked example for energy stored in a capacitor →

Quick recap

$E = \frac{1}{2}CV^2$ .
Three forms available.

Memorise this

Verbatim phrases and definitions Cambridge mark schemes credit.

Three energy forms.

How it’s examined

Paper 4 typically 5 marks.

Sources: Cambridge International A Level Physics 9702 syllabus (2025-2027); 9702 Examiner Reports 2022-2024; 9702/42 May/Jun 2024 question paper and mark scheme. Last reviewed 2026-05-11.

Take this whole topic with you

Step-by-step worked examples — Energy stored in a capacitor

Step-by-step solutions to past-paper-style questions on energy stored in a capacitor, written exactly the way a tutor would explain them at the board.

1Energy stored (5 marks)
Extended• Adapted from 9702/42 May/Jun 2024• energy
Question
Find energy stored in a 100 μF capacitor charged to 50 V. (5 marks)
Step-by-step solution
1. Step 1
  $E = \frac{1}{2}CV^2$ .
  $E = \tfrac{1}{2}(100 \times 10^{-6})(50)^2 = 0.125 \text{ J}$
Answer
$E = 0.125$ J.

Key Formulae — Energy stored in a capacitor

The formulae you need to memorise for energy stored in a capacitor on the Cambridge International A Level 9702 paper, with every variable defined in plain English and a note on when to use it.

Energy in capacitor
$E = \tfrac{1}{2}CV^2 = \tfrac{1}{2}QV = \tfrac{Q^2}{2C}$
When to use
Three equivalent forms. Choose based on knowns.

Key Definitions and Keywords — Energy stored in a capacitor

Definitions to memorise and the exact keywords mark schemes credit for energy stored in a capacitor answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9702 sitting.

Energy in capacitor
Examiner keyword
Area under $Q$ - $V$ graph; stored in electric field between plates.

Common Mistakes and Misconceptions — Energy stored in a capacitor

The traps other students keep falling into on energy stored in a capacitor questions — taken from recent Cambridge International A Level 9702 examiner reports and mark schemes — and how to avoid them.

✕Using $P = VI$ for capacitor energy
9702 Examiner Reports 2022-2024
Why it happens
Confusion.
How to avoid it
$P = VI$ is power in resistor. Capacitor stores ENERGY directly. Use $\frac{1}{2}CV^2$ .