Capacitance

What is Capacitance

Capacitance is a measure of an object’s ability to store electrical charge when a potential difference (voltage) is applied across it. It is defined as the ratio between the amount of energy stored in an object and the amount of charge applied to it. The greater the capacitance, the more electrical charge an object can accumulate for a given voltage.

The amount of capacitance is determined by several factors, including the geometry of the conductive plates, the distance between them, and the material properties of the dielectric medium that separates them. The larger the area of the plates, the closer they are to each other, and the higher the permittivity (ability to store electric fields) of the dielectric material, the greater the capacitance.

Capacitance is an essential concept in the design and operation of many electronic devices, such as capacitors, transmission lines, and integrated circuits.

Understanding Capacitors

Diagram of how a capacitor works in an electric circuit

Capacitors are electronic components designed to store electrical energy in an electric field. They consist of two conductive plates that are separated by a dielectric material, allowing the flow of electric charges between them while preventing direct contact. When a voltage is applied to a capacitor, it creates an electric field between the two conductive plates, which stores electrical energy in the capacitor.

The amount of electrical energy that a capacitor can store depends on its capacitance, which is determined by the physical properties of the capacitor, such as the distance between the plates and the type of dielectric material used. Capacitors with a larger surface area and a smaller distance between the plates have a higher capacitance and can store more electrical energy.

Formula & Units

The capacitance of a component can be found as:

C = \frac{Q}{V}

Where:

$C$ is the capacitance in farads (F)
$Q$ is the electric charge in coulombs (C) stored on the plates of the capacitor
$V$ is the potential difference or voltage in volts (V) between the plates of the capacitor

The SI unit of capacitance is Farad (F).

Capacitance Example

A capacitor has a charge of 6×10^-4C when the potential difference across its plates is 240V. Find its capacitance.

Solution

Step 1) We write down the given parameters:

Q = 6×10^-4C V = 240V

Step 2) We can find the capacitance by simply substituting the values in the formula as:

C = \frac{Q}{V} = \frac{6 \times 10^{- 4}}{240} = 2.5 µ F

Factors Affecting Capacitance

The capacitance of a material can be affected by several factors, including:

Distance between the plates: Capacitance is inversely proportional to the distance between the plates of a capacitor. The closer the plates, the greater the capacitance.
Surface area of the plates: Capacitance is directly proportional to the surface area of the plates. The larger the surface area, the greater the capacitance.
Dielectric constant: The dielectric constant of the material between the capacitor plates plays a major role in determining capacitance. Higher dielectric constants correspond to higher capacitance values.
Temperature: The capacitance of a material can be affected by temperature changes. An increase in temperature can cause an increase in capacitance due to changes in the material’s dielectric constant.
Frequency: Capacitance can also be affected by the frequency of the signal passing through the capacitor. At high frequencies, the capacitance of a capacitor can change due to the effects of parasitic capacitance and other factors.

Applications of Capacitors

Capacitors have a wide range of applications in various fields. Some of the most common applications of Capacitors are:

Energy storage: Capacitors are commonly utilized as energy storage devices in electronic circuits, enabling them to rapidly discharge stored energy to provide a power surge to electronic devices.
Filtering: Capacitors are used with resistors to develop filters that eliminate undesired signals or noise from electronic circuits.
Sensing: Capacitors can be used as sensors to measure the level of fluids in tanks, the position of moving objects, and the humidity in the air.
Power factor correction: Capacitors are used in power factor correction circuits to increase electrical system efficiency by minimizing the amount of reactive power generated.
Coupling: Capacitors are used in electronic circuits to couple or connect signals between different circuits while blocking direct current (DC) flow.

Capacitance Summary
Definition	A measure of an object’s ability to store electrical charge.
Symbol	$C$
Formula	$C = \frac{Q}{V}$
Units	Farad (F)

What is the unit of capacitance?: The unit of capacitance is the farad (F), named after Michael Faraday. However, it is a very large unit of measurement, and it’s more common to use smaller units like microfarads (μF), nanofarads (nF), or picofarads (pF).

What is the difference between capacitance and resistance?: Capacitance refers to an object’s capacity to hold an electric charge, whereas resistance relates to a material’s resistance to the flow of electric current.

What is the purpose of using capacitors in power supply circuits?: Capacitors are used in power supply circuits to reduce AC ripple voltage in the output, resulting in a stable DC voltage.

How do you calculate the capacitance of a capacitor?: The capacitance of a capacitor can be calculated by dividing the amount of electric charge stored on the plates of the capacitor by the voltage applied across them. The formula for capacitance is $C = \frac{Q}{V}$ , where C is capacitance in farads, Q is charge in coulombs, and V is voltage in volts.

Can the capacitance of a capacitor be changed?: Yes, the capacitance of a capacitor can be changed by adjusting its physical dimensions, such as plate distance, surface area, or the dielectric material used between the plates.