Electric capacitor is a passive device that is able to accumulate and store electrical energy. It consists of two conductive plates separated by a dielectric material. The application of electrical potentials of different signs to conductive plates leads to the acquisition of a charge by them, which is positive on one plate and negative on the other. In this case, the total charge is zero.
This article discusses the issues of history and the definition of the capacitance of a capacitor.
Invention Story
In October 1745, the German scientist Ewald Georg von Kleist noticed that an electric charge could be stored if an electrostatic generator and a certain amount of water in a glass vessel were connected with a cable. In this experiment, von Kleist's hand and water were conductors, and the glass vessel was an electrical insulator. After the scientist touched the metal wire with his hand, a powerful discharge occurred, which wasmuch stronger than the discharge of an electrostatic generator. As a result, von Kleist concluded that there was stored electrical energy.
In 1746, the Dutch physicist Pieter van Muschenbroek invented a capacitor, which he called the Leiden bottle in honor of the Leiden University where the scientist worked. Daniel Gralat then increased the capacitance of the capacitor by connecting several Leiden bottles.
In 1749, Benjamin Franklin investigated the Leyden capacitor and came to the conclusion that the electric charge is stored not in water, as was believed before, but at the border of water and glass. Thanks to Franklin's discovery, Leyden bottles were made by covering the inside and outside of glass vessels with metal plates.
Industry Development
The term "capacitor" was coined by Alessandro Volta in 1782. Initially, materials such as glass, porcelain, mica and plain paper were used to make electrical capacitor insulators. So, the radio engineer Guglielmo Marconi used porcelain capacitors for his transmitters, and for receivers - small capacitors with a mica insulator, which were invented in 1909 - before World War II, they were the most common in the USA.
The first electrolytic capacitor was invented in 1896 and was an electrolyte with aluminum electrodes. The rapid development of electronics began only after the invention in 1950 of a miniature tantalum capacitor withsolid electrolyte.
During the Second World War, as a result of the development of plastic chemistry, capacitors began to appear, in which the role of an insulator was assigned to thin polymer films.
Finally, in the 50-60s, the industry of supercapacitors develops, which have several working conductive surfaces, due to which the electrical capacity of capacitors increases by 3 orders of magnitude compared to its value for conventional capacitors.
The concept of the capacitance of a capacitor
The electric charge stored in the capacitor plate is proportional to the voltage of the electric field that exists between the plates of the device. In this case, the coefficient of proportionality is called the electric capacitance of a flat capacitor. In SI (International System of Units), electrical capacity, as a physical quantity, is measured in farads. One farad is the electrical capacitance of a capacitor whose plate voltage is 1 volt with a stored charge of 1 coulomb.
Electrical capacitance of 1 farad is huge, and in practice in electrical engineering and electronics, capacitors with capacitances of the order of picofarad, nanofarad and microfarad are commonly used. The only exceptions are supercapacitors, which consist of activated carbon, which increases the working area of the device. They can reach thousands of farads and are used to power prototype electric vehicles.
Thus, the capacitance of the capacitor is: C=Q1/(V1-V2). Here C-electrical capacity, Q1 - electric charge stored in one plate of the capacitor, V1-V2 - the difference between the electric potentials of the plates.
The formula for the capacitance of a flat capacitor is: C=e0eS/d. Here e0and e is the universal dielectric constant and the dielectric constant of the insulator material S is the area of the plates, d is the distance between the plates. This formula allows you to understand how the capacitance of a capacitor will change if you change the material of the insulator, the distance between the plates or their area.
Types of used dielectrics
For the manufacture of capacitors, various types of dielectrics are used. The most popular are the following:
- Air. These capacitors are two plates of conductive material, which are separated by a layer of air and placed in a glass case. The electrical capacity of air capacitors is small. They are usually used in radio engineering.
- Mica. The properties of mica (the ability to separate into thin sheets and withstand high temperatures) are suitable for its use as insulators in capacitors.
- Paper. Waxed or varnished paper is used to protect against getting wet.
Stored Energy
As the potential difference between the plates of the capacitor increases, the device stores electrical energy due tothe presence of an electric field inside it. If the potential difference between the plates decreases, then the capacitor is discharged, giving energy to the electrical circuit.
Mathematically, the electrical energy that is stored in an arbitrary type of capacitor can be expressed by the following formula: E=½C(V2-V1)2, where V2 and V1 are the final and initial stress between the plates.
Charge and discharge
If a capacitor is connected to an electrical circuit with a resistor and some source of electric current, then current will flow through the circuit and the capacitor will begin to charge. As soon as it is fully charged, the electric current in the circuit will stop.
If a charged capacitor is connected in parallel with a resistor, then a current will flow from one plate to another through the resistor, which will continue until the device is completely discharged. In this case, the direction of the discharge current will be opposite to the direction of electric current flow when the device was being charged.
Charging and discharging a capacitor follows an exponential time dependence. For example, the voltage between the plates of a capacitor during its discharge changes according to the following formula: V(t)=Vie-t/(RC), where V i - initial voltage on the capacitor, R - electrical resistance in the circuit, t - discharge time.
Combining in an electrical circuit
To determine the capacitance of the capacitors that are available inelectrical circuit, it should be remembered that they can be combined in two different ways:
- Serial connection: 1/Cs =1/C1+1/C2+ …+1/C.
- Parallel connection: Cs =C1+C2+…+C.
Cs - total capacitance of n capacitors. The total electrical capacitance of capacitors is determined by formulas similar to mathematical expressions for the total electrical resistance, only the formula for series connection of devices is valid for parallel connection of resistors and vice versa.
