A-Level Physics: Capacitors – How They Work and What They Are Used For

What is a Capacitor?

A capacitor is a device that stores electrical energy by separating charge. It consists of two conducting plates separated by an insulator, called the dielectric.

When connected to a power supply:

• One plate becomes positively charged

• The other becomes negatively charged

• Energy is stored in the electric field between them

Unlike a battery, a capacitor does not produce energy – it stores energy that was supplied to it.

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How a Capacitor Works (GCSE → A-Level Bridge)

When a capacitor is connected to a DC supply:

1. Electrons flow onto one plate and are removed from the other

2. The potential difference (p.d.) across the capacitor rises

3. The charging current decreases over time

4. Eventually, the capacitor is fully charged and current falls to zero

At this point:

$$Q=CV$$

Where:

• $Q$ = charge (C)

• $C$ = capacitance (F)

• $V$ = potential difference (V)

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What Affects Capacitance?

For a parallel-plate capacitor:

$$C=\frac{\epsilon A}{d}$$

Capacitance increases if:

• Plate area $A$ increases

• Plate separation $d$ decreases

• A dielectric with higher permittivity is used

This is why real capacitors often use thin insulating layers and materials such as ceramics or plastics.

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Charging and Discharging Capacitors (A-Level Core)

In an RC circuit:

• Voltage across the capacitor rises exponentially during charging

• Voltage falls exponentially during discharging

The time constant:

$$\tau =RC$$

After one time constant:

• Charging capacitor reaches 63% of its final voltage

• Discharging capacitor falls to 37% of its initial voltage

This behaviour is essential for:

• Timing circuits

• Signal smoothing

• Sensor data logging

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Energy Stored in a Capacitor

The energy stored is:

$$E=\frac{\mathrm{1/}}{2}C{V}^{\mathrm{^2}}$$

Key A-Level insight:

• Energy is stored in the electric field, not “in the charges”

• Increasing voltage dramatically increases stored energy (square law)

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What Are Capacitors Used For?

1. Camera Flashes
A capacitor charges slowly and discharges rapidly to produce a bright flash
(see any compact camera or studio strobe)

2. Defibrillators
Large capacitors store energy and release it in a controlled, life-saving pulse

3. Power Supplies
Capacitors smooth rectified DC by reducing voltage ripple

4. Timing Circuits
RC circuits control delays in alarms, indicators, and microcontrollers

5. Signal Processing & Audio
Used in filters and crossovers to block or pass certain frequencies

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Why Capacitors Matter at A-Level

Capacitors bring together:

• Electric fields

• Exponential mathematics

• Practical electronics

• Graph interpretation

• Energy storage

They are a perfect exam topic because questions often mix:

• Calculations

• Graphs

• Explanations

• Real-world applications

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Exam Tip

If a question mentions:

• “exponential”

• “time constant”

• “RC circuit”

• “charging or discharging”

👉 Sketch the graph first – it often unlocks the marks.