Capacitance
Capacitance is the ability of a capacitor to store electrical charge when a voltage is applied across its terminals. It is one of the most important properties of a capacitor and determines how much electrical energy the component can store. The greater the capacitance, the more electrical charge and energy the capacitor can store at a given voltage.
What Is Capacitance?
Capacitance is defined as the amount of electrical charge stored for each volt applied across a capacitor.
C = Q / V
| Symbol | Meaning |
|---|---|
| C | Capacitance (farads) |
| Q | Electric charge (coulombs) |
| V | Voltage (volts) |
For example, if a capacitor stores one coulomb of charge when one volt is applied across it, its capacitance is one farad.
Unit of Capacitance
Capacitance is measured in farads (F), named after the English scientist Michael Faraday.
One farad is a very large value, so most electronic circuits use much smaller units.
| Unit | Value |
|---|---|
| 1 F | 1 Farad |
| 1 mF | 0.001 F |
| 1 µF | 0.000001 F |
| 1 nF | 0.000000001 F |
| 1 pF | 0.000000000001 F |
What Determines Capacitance?
The capacitance of a capacitor depends on its physical construction.
| Factor | Effect |
|---|---|
| Larger plate area | Higher capacitance. |
| Smaller plate spacing | Higher capacitance. |
| Higher dielectric constant | Higher capacitance. |
| Better dielectric material | Improved stability and lower leakage. |
Parallel Plate Capacitor Equation
The capacitance of a simple parallel-plate capacitor is given by:
C = εA / d
| Symbol | Meaning |
|---|---|
| C | Capacitance |
| ε | Permittivity of the dielectric |
| A | Area of one plate |
| d | Distance between the plates |
This equation shows that increasing the plate area or using a dielectric with a higher permittivity increases the capacitance, while increasing the distance between the plates reduces it.
Energy Stored in a Capacitor
A charged capacitor stores electrical energy according to the following equation:
E = ½CV²
| Symbol | Meaning |
|---|---|
| E | Stored energy (joules) |
| C | Capacitance (farads) |
| V | Voltage (volts) |
This equation shows that stored energy increases directly with capacitance and with the square of the applied voltage.
Typical Capacitance Values
| Capacitor Type | Typical Capacitance |
|---|---|
| Silver Mica | 1 pF – 10 nF |
| Ceramic | 1 pF – 100 µF |
| Film | 100 pF – 100 µF |
| Electrolytic | 0.47 µF – 100,000 µF |
| Supercapacitor | 1 F – 5,000 F+ |
Applications of Different Capacitance Values
| Capacitance Range | Typical Application |
|---|---|
| 1–100 pF | RF tuning and oscillators. |
| 100 pF–100 nF | Signal coupling and bypassing. |
| 100 nF | Power supply decoupling. |
| 1–100 µF | Audio coupling and timing circuits. |
| 100–10,000 µF | Power supply filtering. |
| 1–5,000 F | Energy storage and backup power. |
Measuring Capacitance
Capacitance can be measured using several instruments.
- Digital multimeter with capacitance function.
- Capacitance meter.
- LCR meter.
- Component tester.
- Oscilloscope with RC time-constant measurements.
Factors That Affect Capacitance
- Temperature.
- Applied voltage.
- Ageing.
- Humidity.
- Dielectric material.
- Mechanical stress (especially ceramic capacitors).
- Frequency.
Common Mistakes
| Mistake | Explanation |
|---|---|
| Confusing capacitance with voltage rating. | Capacitance and maximum working voltage are different specifications. |
| Using incorrect units. | µF, nF and pF differ by factors of 1,000. |
| Ignoring tolerance. | Actual capacitance may differ from the marked value. |
| Ignoring temperature effects. | Some capacitor types change value significantly with temperature. |
Real-World Examples
| Equipment | Typical Capacitor |
|---|---|
| Arduino | 100 nF decoupling capacitor. |
| Audio Amplifier | 4,700 µF power supply filter capacitor. |
| Radio Receiver | 5–365 pF tuning capacitor. |
| Camera Flash | Large electrolytic capacitor. |
| RTC Backup | 0.47 F supercapacitor. |
Key Points
- Capacitance is the ability to store electrical charge.
- It is measured in farads (F).
- Higher capacitance allows more charge and energy to be stored.
- Plate area, plate spacing and dielectric material determine capacitance.
- Capacitance is one of the most important specifications of a capacitor.