A capacitor is an electrical device used to store electric charge and electrical energy.
Capacitors are widely used in electronic circuits, power supplies, communication systems, and electrical appliances.
The ability of a capacitor to store charge is known as its capacitance.
What is a Capacitor?
A capacitor consists of two conducting plates separated by an insulating material called a dielectric.
When the plates are connected to a voltage source, one plate acquires positive charge while the other acquires an equal amount of negative charge.
As a result, an electric field is established between the plates.
The capacitor stores energy in this electric field.
Construction of a Capacitor
A basic capacitor contains:
- Two conducting plates
- An insulating medium between the plates
- Connecting terminals
The insulating material may be air, paper, mica, ceramic, plastic, or glass.
What is Capacitance?
Capacitance is defined as the amount of charge stored per unit potential difference across a capacitor.
Mathematically,
Where:
- C = capacitance
- Q = charge stored
- V = potential difference
SI Unit of Capacitance
From the definition:
The SI unit of charge is coulomb (C) and the SI unit of potential difference is volt (V).
Therefore,
The SI unit of capacitance is farad (F).
Meaning of One Farad
A capacitor has a capacitance of one farad if a charge of one coulomb produces a potential difference of one volt across it.
One farad is a very large unit.
Practical capacitors are usually measured in:
- Microfarad (μF)
- Nanofarad (nF)
- Picofarad (pF)
Factors Affecting Capacitance
The capacitance of a capacitor depends on:
- Area of the plates
- Distance between the plates
- Nature of the dielectric medium
Larger plates store more charge and therefore have higher capacitance.
Greater plate separation decreases capacitance.
Charging of a Capacitor
When a capacitor is connected to a battery, electrons move from one plate to the other through the external circuit.
As charge accumulates on the plates, the potential difference across the capacitor increases.
Charging continues until the capacitor voltage becomes equal to the battery voltage.
Discharging of a Capacitor
When the battery is removed and the plates are connected through a conductor, stored charge flows from one plate to the other.
This process is called discharging.
The stored electrical energy is released during discharge.
Energy Stored in a Capacitor
Work must be done to store charge on a capacitor.
This work is stored as electrical potential energy.
The energy stored is:
Alternative forms are:
Capacitors in Everyday Life
Capacitors are used in many electronic and electrical devices.
- Mobile phone chargers
- Power supplies
- Camera flash units
- Radio circuits
- Television sets
- Computers
- Electric motors
Advantages of Capacitors
- Store electrical energy
- Provide quick energy release
- Filter unwanted signals
- Improve power quality
- Support electronic circuits
Solved Example 1
A capacitor stores a charge of 8 μC at a potential difference of 4 V.
Find its capacitance.
Given:
Using:
Substituting values:
Therefore,
Solved Example 2
A capacitor of capacitance 5 μF is connected across a 12 V battery.
Find the charge stored.
Given:
Using:
Substituting values:
Hence,
Important Points to Remember
- A capacitor stores electric charge and energy.
- Capacitance is charge stored per unit potential difference.
- The SI unit of capacitance is farad.
- Capacitance depends on plate area, separation, and dielectric material.
- Energy stored in a capacitor is given by U = ½CV².
- Capacitors charge and discharge through external circuits.
- Capacitors are widely used in electronic devices.
Frequently Asked Questions
What is a capacitor?
A capacitor is a device used to store electric charge and electrical energy.
What is capacitance?
Capacitance is the charge stored per unit potential difference.
What is the SI unit of capacitance?
The SI unit of capacitance is farad (F).
Can a capacitor store energy?
Yes. A capacitor stores energy in the electric field between its plates.
Where are capacitors used?
Capacitors are used in power supplies, communication systems, computers, mobile chargers, and many other electronic devices.
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