Capacitors & Capacitance
Updated: Sep 6, 2020
A #capacitor is a component that has the ability to store #electrical energy in it in the form of electrical charge producing a potential difference across its plates. Capacitors are also called as condenser. It is a passive electronic component (cannot generate electricity but can store it) with two terminals. Individual capacitors do not hold a great deal of energy, it can provide only enough power to operate the electrical during temporary power outages.
The applications of capacitors are #embedded in our day-to-day activities. Almost all the sensors which are currently in use are constructed with the help of capacitors. They are also employed in power conditioning, signal processing.
In this article, we are going to discuss in details about the capacitors, types, and applications of it.
As mentioned above, the capacitors hold an electric charge in it. When a voltage is applied across it, it gives up the stored charge to the required #circuit. The basic construction of a capacitor involves two parallel conductors (metallic plates) separated by a dielectric material.
Materials commonly used as dielectrics are glass, ceramic, paper, mica, plastic firm, air medium, and even oxide layers. The conductors (metallic plates) can be any foil, thin film, electrolyte. The dielectric placed between the plates helps to increase the storage capacity of the capacitor. An ideal capacitor does not dissipate energy, although real-life capacitors do dissipate a small amount.
When a voltage source is connected across a capacitor, the metal plate connected to the positive terminal becomes positively charged and the one connected to the negative terminal becomes negatively charged. Due to the presence of dielectric in between the conductors, no charge can migrate from one plate to another. So, there will be a difference in charging the two plates and results in an electric potential difference across the plates.
The voltage across the capacitor rises exponentially until it becomes equal to the applied voltage. If a time-varying voltage is applied across the leads of the capacitor, the source circuit experiences an ongoing current due to the charging and discharging cycles of the capacitor.
The amount of charge stored in the capacitor for developing a particular voltage across the capacitor is termed as the change holding capacity of the capacitors. This capacity of the capacitor is measured in a unit called the capacitance. The term capacitance can be defined as the quantity of charge stored in the capacitor for developing 1-volt potential difference across it. The charge accumulated in the capacitor is directly proportional to the voltage developed across the capacitor will be
Then Q=CV, where Q is the amount of charge, V is the applied voltage, and C is the capacitance. The value of #capacitance depends on the active area of the capacitor conductors (metal plates), the distance between the plates, and the permittivity of the dielectric medium will be
where, ε is the permittivity of the dielectric medium, A is the active area of the plates, and d is the perpendicular distance between plates.
To increase the amount of charge stored and voltage in the capacitor, certain work must be done by an external power source to move the charges from the negatively charged plate to the positively charged plate against the opposing force exhibited by the electric field.
The generated energy is stored between the plates in the increased electric fields. The total energy stored in a capacitor is equal to the total work done (W) in establishing the electric field from an uncharged state will be stated as
The potential energy will remain in the capacitor until the charge is removed.
Relation with Coulomb's law
The principle of coulomb's law is applied in the construction of capacitors. The law states that "The electrostatic force is directly proportional to the product of the magnitude of two point charges and inversely proportional to the square of the distance between the two-point charges."
According to this law, in capacitors, a charge on one conductor will exert a force on the charge carriers within the other conductor. Therefore it attracts opposite polarity charges.
The conductors hold equal and opposite charges on their facing surfaces and dielectric developed an electric field. Capacitance (C) is measured in farads in the SI system of units.
One farad of capacitance can be defined as one coulomb of charge on each conductor causes a voltage of one volt across the device. A large number of charges can be stored in the capacitor when two plates of it are close together.
In practical capacitors, building up of charges may affect the capacitor mechanically. In such cases, the capacitance can be defined as,
Types of capacitors
The capacitors are generally classified based on the dielectric material used between the plates. Like resistors, the capacitance of the capacitors also varies with respect to the voltage and frequency involved in them.
Small capacitors are constructed using ceramic materials and then dipped into epoxy resin to seal them. Whereas, the commercial capacitors are made by using metallic foil interlaced with thin sheets of polyester as the dielectric material.
Dielectric capacitors possess a continuous variation of capacitance value which is used for tuning receivers, transmitters, and transistor radios. These capacitors have multi-plates.
A set of fixed plates and a set of movable plates in between them. The position of moving plates with respect to the fixed plates determines the capacitance value.
Film capacitors consist of a relatively large family of capacitors with the difference being in their dielectric properties. The dielectrics used here include polystyrene, polyester, polycarbonate, polypropylene, and Teflon. The capacitance value of these capacitors varies from 5pF to 100uF.
Based on the size and shape, film capacitors are divided into three types. They are wrap & fill the epoxy case, and metal hermetically sealed capacitors. The above case styles are available in both axial and radial leads.
Film capacitors made using Teflon, polystyrene, or polycarbonate are known as "Plastic capacitors" and they have a long service life, high reliability, and also can be operated at high temperatures.
Ceramic or disc capacitors are made by coating two sides of porcelain or ceramic disc with silver and are then stacked together. Ceramic disc of about 3-6mm is used for capacitors with low capacitance values.
These are used as de-coupling or by-pass capacitors because they show non-linear changes with respect to temperature. ceramic capacitors generally have 3-digit code printed on them to identify their capacitance value in pico-farads.
The first two digits indicate the capacitor's value and the last digit indicates the number of zero's to be added. For example, the marking of 303 indicates a capacitance value equivalent to 30,000 pF.
Electrolytic capacitors are used when a very large value of capacitance is required. Here one of the thin-film electrodes is replaced by a semi-liquid electrolyte solution. The dielectric of these capacitors is a very thin layer of oxide in order to acquire a high capacitance value. The majority of electrolytic capacitors are polarised (i.e) DC voltage should be applied to the respective terminals of the capacitor.
They are majorly employed in DC power supply circuits to reduce the ripple voltage and also for coupling, decoupling applications. Based on the film of the electrode, the electrolytic capacitors are classified as Aluminium electrolytic capacitors and tantalum electrolytic capacitors.
Applications of capacitors:
In all our houses, we have Uninterrupted Power Supply (UPS). They can be equipped with maintenance-free capacitors in order to extend their service life.
The car audio systems employ capacitors to store energy for the amplifier to use on-demand.
Low-inductance high-voltage capacitors are employed in pulse forming networks, Marx generators, electromagnetic forming, and in particle accelerators.
Capacitors are also employed in noise filters and motor starters.
They are also employed in smooth distribution of power to both industries as well as domestic houses.
Capacitors play a vital role in the functioning of all kinds of #sensors.