Electric capacitance is one of the basic concepts of electrostatics. This term refers to the ability to accumulate an electric charge. You can talk about the capacity of a separate conductor, you can talk about the capacity of a system of two or more conductors. The physical processes are similar.
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Basic concepts related to electrical capacity
If the conductor has received a charge q, a potential φ arises on it. This potential depends on the geometry and environment - for different conductors and conditions, the same charge will cause a different potential. But φ is always proportional to q:
φ=Cq
The coefficient C is called the electrical capacitance.If we are talking about a system of several conductors (usually two), then when a charge is imparted to one conductor (plate), a potential difference or voltage U occurs:
U=Cq, hence С=U/q
Capacitance can be defined as the ratio of the potential difference to the charge that caused it. The SI unit for capacitance is the farad (they used to say farad). 1 F \u003d 1 V / 1 C. In other words, a system has a capacity of 1 farad, in which, when a charge of 1 coulomb is imparted, a potential difference of 1 volt arises. 1 Farad is a very large value. In practice, fractional values \u200b\u200bare most often used - picofarad, nanofarad, microfarad.
In practice, such a connection makes it possible to obtain a battery that can withstand a greater breakdown voltage of the dielectric than that of a single cell.
Calculation of the capacitance of capacitors
In practice, as elements with a normalized electric capacitance, most often used capacitors, consisting of two flat conductors (plates), separated by a dielectric. The formula for calculating the electric capacitance of such a capacitor looks like this:
C=(S/d)*ε*ε0
where:
- C - capacity, F;
- S is the area of the facings, sq.m;
- d is the distance between the plates, m;
- ε0 - electrical constant, constant, 8.854 * 10−12 f/m;
- ε is the electrical permittivity of the dielectric, a dimensionless quantity.
From this it is easy to understand that the capacitance is directly proportional to the area of the plates and inversely proportional to the distance between the conductors. Also, the capacity is affected by the material that separates the plates.
To understand how the quantities that determine capacitance affect the ability of a capacitor to store charge, you can do a thought experiment to create a capacitor with the largest possible capacitance.
- You can try to increase the area of the plates. This will lead to a sharp increase in the dimensions and weight of the device. To reduce the size of the lining with a dielectric separating them, they are rolled up (into a tube, flat briquette, etc.).
- Another way is to reduce the distance between the plates. It is not always possible to place the conductors very close, since the dielectric layer must withstand a certain potential difference between the plates. The smaller the thickness, the lower the dielectric strength of the insulating gap. If you take this path, there will come a time when the practical use of such a capacitor will become meaningless - it can only work at extremely low voltages.
- Increasing the electrical permeability of the dielectric. This path depends on the development of production technologies that exist at the moment. The insulating material must not only have a high permeability value, but also good dielectric properties, and also maintain its parameters in the required frequency range (with an increase in the frequency at which the capacitor operates, the characteristics of the dielectric decrease).
Some specialized or research installations may use spherical or cylindrical capacitors.
The capacitance of a spherical capacitor can be calculated by the formula
C=4*π*ε*ε0 *R1R2/(R2-R1)
where R are the radii of the spheres, and π=3.14.
For a cylindrical capacitor, the capacitance is calculated as:
C=2*π*ε*ε0 *l/ln(R2/R1)
l is the height of the cylinders, and R1 and R2 are their radii.
Fundamentally, both formulas do not differ from the formula for a flat capacitor. The capacitance is always determined by the linear dimensions of the plates, the distance between them and the properties of the dielectric.
Series and parallel connection of capacitors
Capacitors can be connected in series or in parallel, obtaining a set with new characteristics.
Parallel connection
If you connect the capacitors in parallel, then the total capacity of the resulting battery is equal to the sum of all the capacities of its components. If the battery consists of capacitors of the same design, this can be considered as the addition of the area of \u200b\u200ball the plates. In this case, the voltage on each cell of the battery will be the same, and the charges will add up. For three capacitors connected in parallel:
- U=U1=U2=U3;
- q=q1+q2+q3;
- C=C1+C2+C3.
serial connection
When connected in series, the charges of each capacitance will be the same:
q1=q2=q3=q
The total voltage is distributed proportionally capacitances of capacitors:
- U1=q/C1;
- U2=q/C2;
- U3= q/C3.
If all capacitors are the same, then equal voltage drops across each. The total capacity is found as:
С=q/( U1+U2+U3), hence 1/С=( U1+U2+U3)/q=1/C1+1/S2+1/S3.
The use of capacitors in technology
It is logical to use capacitors as electrical energy storage devices. In this capacity, they cannot compete with electrochemical sources (galvanic batteries, capacitors) due to the small stored energy and rather fast self-discharge due to charge leakage through the dielectric.But their ability to accumulate energy for a long period, and then almost instantly give it away is widely used. This property is used in flash lamps for photography or lamps for excitation of lasers.
Capacitors are widely used in radio engineering and electronics. Capacitances are used as part of resonant circuits as one of the frequency-setting elements of the circuits (the other element is inductance). It also uses the ability of capacitors not to pass direct current without delaying the variable component. Such an application is common for separating amplifying stages in order to exclude the influence of the DC modes of one stage on another. Large capacitors are used as smoothing filters in power supplies. There are also a huge number of other applications of capacitors where their properties are useful.
Some practical capacitor designs
In practice, various designs of flat capacitors are used. The design of the device determines its characteristics and scope.
variable capacitor
A common type of variable capacitor (VPC) consists of a block of movable and fixed plates separated by air or a solid insulator. The movable plates rotate around the axis, increasing or decreasing the overlap area. When the moving block is removed, the interelectrode gap remains unchanged, but the average distance between the plates also increases. The dielectric constant of the insulator also remains unchanged. The capacity is regulated by changing the area of the plates and the average distance between them.
oxide capacitor
Previously, such a capacitor was called electrolytic. It consists of two strips of foil separated by a paper dielectric impregnated with an electrolyte. The first strip serves as one plate, the second plate serves as an electrolyte. The dielectric is a thin layer of oxide on one of the metal strips, and the second strip serves as a current collector.
Due to the fact that the oxide layer is very thin, and the electrolyte closely adjoins it, it became possible to obtain sufficiently large capacities with moderate sizes. The price for this was a low operating voltage - the oxide layer does not have high electrical strength. With an increase in the operating voltage, it is necessary to significantly increase the dimensions of the capacitor.
Another problem is that the oxide has one-sided conductivity, so such containers are used only in DC circuits with polarity.
Ionistor
As shown above, the traditional methods of increasing Capacitors have natural limitations. Therefore, the real breakthrough was the creation of ionistors.
Although this device is considered an intermediate link between a capacitor and a battery, in essence it is still a capacitor.
The distance between the plates is drastically reduced thanks to the use of a double electrical layer. The plates are layers of ions with opposite charges. It became possible to sharply increase the area of the plates due to foamed porous materials. As a result, it is possible to obtain supercapacitors with a capacity of up to hundreds of farads.A congenital disease of such devices is low operating voltage (usually within 10 volts).
The development of technology does not stand still - lamps from many areas have been replaced by bipolar transistors, which, in turn, are replaced by unipolar triodes. When designing circuits, they try to get rid of inductances wherever possible. And capacitors have not lost their positions for the second century, their design has not fundamentally changed since the invention of the Leyden jar, and there are no prospects for ending their career.
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