CAPACITOR:-
Capacitor is a passive element that stores electric charge statistically and temporarily as an static electric field. It is composed of two parallel conducting plates separated by non-conducting region that is called dielectric, such as vacuum, ceramic, air, aluminum, etc. The capacitance formula of the capacitor is represented by,
C is the capacitance that is proportional to the area of the two conducting plates (A) and proportional with the permittivity ε of the dielectric medium. The capacitance decreases with the distance between plates (d). We get the greatest capacitance with a large area of plates separated by a small distance and located in a high permittivity material. The standard unit of capacitance is Farad, most commonly it can be found in micro-farads, pico-farads and nano-farads.
Capacitor is a passive element that stores electric charge statistically and temporarily as an static electric field. It is composed of two parallel conducting plates separated by non-conducting region that is called dielectric, such as vacuum, ceramic, air, aluminum, etc. The capacitance formula of the capacitor is represented by,
C is the capacitance that is proportional to the area of the two conducting plates (A) and proportional with the permittivity ε of the dielectric medium. The capacitance decreases with the distance between plates (d). We get the greatest capacitance with a large area of plates separated by a small distance and located in a high permittivity material. The standard unit of capacitance is Farad, most commonly it can be found in micro-farads, pico-farads and nano-farads. Types of Capacitors
The various types of capacitors have been developed to overcome these problems in a number of ways.Paper Capacitor
It is one of the simple forms of capacitors. Here, a waxed paper is sandwiched between two aluminium foils. Process of making this capacitor is quite simple. Take place of aluminium foil. Cover this foil with a waxed paper. Now, cover this waxed paper with another aluminium foil. Then roll up this whole thing as a cylinder. Put two metal caps at both ends of roll. This whole assembly is then encapsulated in a case. By rolling up, we make quite a large cross-sectional area of capacitor assembled in a reasonably smaller space.Air Capacitor
There are two sets of parallel plates. One set of plates is fixed and another set of plates is movable. When the knob connected with the capacitor is rotated, the movable set of plates rotates and overlapping area as between fixed and movable plates vary. This causes variation in effective cross-sectional areas of the capacitor. Consequently, the capacitance varies when one rotates the knob attached to the air capacitor. This type of capacitor is generally used to tune the bandwidth of a radio receiver.Plastic Capacitor
When various plastic materials are used as dielectric material, the capacitors are said to be plastic capacitors. The plastic material may be of polyester, polystyrene, polycarbonate or poly propylene. Each of these materials has slightly different electrical characteristics, which can be used to advantage, depending upon the proposed application.
This type of capacitors is constructional, more or less same as paper capacitor. That means, a thin sheet one of the earlier mentioned plastic dielectrics, is kept between two aluminium foils. That means, here the flexible thin plastic sheet is used as dielectric instead of waxed paper. Here, the plastic sheet covered by aluminium foil from two sides, is first rolled up, then fitted with metal end caps, and then the whole assembly is encapsulated in a case.
Plastic Film Capacitor
Plastic capacitor can be made also in form of film capacitor. Here, thin strips or films of plastic are kept inside metallic strips. Each metallic strip is connected to side metallic contact layer alternatively; as shown in the figure below. That means, if one metallic strip is connected to left side contact layer, then the very next is connected to right side contact layer. And there are plastic films in between these metallic strips. The terminals of this type of capacitors are also connected to side contact layer and whole assembly is covered with insulated non metallic cover as shown.Silvered Mica Capacitor
A silvered mica capacitor is very accurate and reliable capacitor. This type of capacitors has very low tolerance. But on the other hand, cost of this capacitor is quite higher compared to other available capacitors in the market. But this high cost capacitor can easily be compensated by its high quality and performance. A small ceramic disc or cylinder is coated by silver compound. Here, electrical terminal is affixed on the silver coating and the whole assembly is encapsulated in a casing.Ceramic Capacitor
Construction of ceramic capacitor is quite simple. Here, one thin ceramic disc is placed between two metal discs and terminals are soldered to the metal discs. Whole assembly is coated with insulated protection coating as shown in the figure below.Principle of Capacitor
As a capacitor is passive component, it does not generate energy. But it is able to store energy from an energy source like a battery or another charged capacitor. When a battery (DC Source) is connected across a capacitor, one surface, named plate I gets positive end of the battery and another surface, named plate II gets negative end of the battery. When battery is connected, the full voltage of that battery is applied across that capacitor. At that situation, plate I is in positive potency with respect to the plate II. Current from the battery tries to flow through this capacitor from its positive plate (plate I) to negative plate (plate II) but cannot flow at max value due to separation of these plates with an insulating material. Rather a very small current will flow through this insulating material (dielectric) from Positive to Negative plate depending upon the value of strength of this dielectric.
An electric field will form inside the capacitor dielectric from positive to negative plate. As time goes on, positive plate (plate I) will accumulate positive charge from the battery and negative plate (plate II) will accumulate negative charge from negative end of the battery. After a certain time, the capacitor holds maximum amount of charge as per its capacitance with respect to this voltage. This time span is called charging time of this capacitor.
Now, after removing this battery from this capacitor, these two plates will hold positive and negative charges with respect to a certain voltage level for long time. Thus this capacitor acts as energy source.
If two ends (plate I and plate II) get shorted through a load, a current will flow through this load from plate I to plate II up to all charges get vanished from both plates. This time span is known as discharging time of the capacitor.
Now, after removing this battery from this capacitor, these two plates will hold positive and negative charges with respect to a certain voltage level for long time. Thus this capacitor acts as energy source.If two ends (plate I and plate II) get shorted through a load, a current will flow through this load from plate I to plate II up to all charges get vanished from both plates. This time span is known as discharging time of the capacitor.Energy Stored in Capacitor
While capacitor is connected across a battery, charges come from the battery and get stored in the capacitor plates. But this process of energy storing is step by step only.At the very beginning, capacitor does not have any charge or potential. i.e. V = 0 volts and q = 0 C.
Now at the time of switching, full battery voltage will fall across the capacitor. A positive charge (q) will come to the positive plate of the capacitor, but there is no work done for this first charge (q) to come to the positive plate of the capacitor from the battery. It is because of the capacitor does not have own voltage across its plates, rather the initial voltage is due to the battery.
Now at the time of switching, full battery voltage will fall across the capacitor. A positive charge (q) will come to the positive plate of the capacitor, but there is no work done for this first charge (q) to come to the positive plate of the capacitor from the battery. It is because of the capacitor does not have own voltage across its plates, rather the initial voltage is due to the battery.
First charge grows little amount of voltage across the capacitor plates, and then second positive charge will come to the positive plate of the capacitor, but gets repealed by the first charge. As the battery voltage is more than the capacitor voltage then this second charge will be stored in the positive plate. At that condition a little amount of work is to be done to store second charge in the capacitor. Again for the third charge, same phenomenon will appear. Gradually charges will come to be stored in the capacitor against pre-stored charges and their little amount of work done grows up.
It can’t be said that the capacitor voltage is fixed. It is because of the capacitor voltage is not fixed from the very beginning. It will be at its maximum limit when potency of capacitor will be equal to that of the battery. As storage of charges increases, the voltage of the capacitor increases and also energy of the capacitor increases. So at that point of discussion the energy equation for the capacitor can’t be written as energy (E) = V.q As the voltage increases the electric field (E) inside the capacitor dielectric increases gradually but in opposite direction i.e. from positive plate to negative plate.
Here dx is the distance between two plates of the capacitor.
Charge will flow from battery to the capacitor plate until the capacitor gains as same potency as the battery. So, we have to calculate the energy of the capacitor from the very begging to the last moment of charge getting full.
It can’t be said that the capacitor voltage is fixed. It is because of the capacitor voltage is not fixed from the very beginning. It will be at its maximum limit when potency of capacitor will be equal to that of the battery. As storage of charges increases, the voltage of the capacitor increases and also energy of the capacitor increases. So at that point of discussion the energy equation for the capacitor can’t be written as energy (E) = V.q As the voltage increases the electric field (E) inside the capacitor dielectric increases gradually but in opposite direction i.e. from positive plate to negative plate.Here dx is the distance between two plates of the capacitor.Charge will flow from battery to the capacitor plate until the capacitor gains as same potency as the battery. So, we have to calculate the energy of the capacitor from the very begging to the last moment of charge getting full.
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