Introduction to Capacitors, Capacitance and Charge & Characteristics

What is a Capacitor?

Capacitors are also known as Electric-condensers. A capacitor is a two-terminal electric component.(+) & (-)Terminal, It has the ability or capacity to store energy in the form of electric charge (Dielectric Medium in Center Path). Capacitors are usually designed to enhance and increase the effect of capacitance.  The storing capacity of capacitance may vary from small storage to high storage.

Capacitance

Capacitance is nothing but the ability of a capacitor to store the energy in form of electric charge. In other words, the capacitance is the storing ability of a capacitor. It is measured in

Unit: Farads

Construction of Capacitor

Most capacitors usually contain two electrical conductors. These conductors are separated by metallic plates. Conductors may be in form of electrolyte, thin film, a sintered bead of metal etc.

The parallel plate capacitor is the simplest form of capacitor. It can be constructed using two metal or metallised foil plates at a distance parallel to each other, with its capacitance value in Farads, being fixed by the surface area of the conductive plates and the distance of separation between them. Altering any two of these values alters the the value of its capacitance and this forms the basis of operation of the variable capacitors.

Also, because capacitors store the energy of the electrons in the form of an electrical charge on the plates the larger the plates and/or smaller their separation the greater will be the charge that the capacitor holds for any given voltage across its plates. In other words, larger plates, smaller distance, more capacitance.

By applying a voltage to a capacitor and measuring the charge on the plates, the ratio of the charge Q to the voltage V will give the capacitance value of the capacitor and is therefore given as: C = Q/V this equation can also be re-arranged to give the familiar formula for the quantity of charge on the plates as: Q = C x V

Although we have said that the charge is stored on the plates of a capacitor, it is more exact to say that the energy within the charge is stored in an “electrostatic field” between the two plates. When an electric current flows into the capacitor, it charges up, so the electrostatic field becomes much stronger as it stores more energy between the plates.

Likewise, as the current flowing out of the capacitor, discharging it, the potential difference between the two plates decreases and the electrostatic field decreases as the energy moves out of the plates.

The property of a capacitor to store charge on its plates in the form of an electrostatic field is called the Capacitance of the capacitor. Not only that, but capacitance is also the property of a capacitor which resists the change of voltage across it.

Standard Units of Capacitance

  • Microfarad  (μF)   1μF = 1/1,000,000 = 0.000001 = 10-6 F
  • Nanofarad  (nF)   1nF = 1/1,000,000,000 = 0.000000001 = 10-9 F
  • Picofarad  (pF)   1pF = 1/1,000,000,000,000 = 0.000000000001 = 10-12 F

Then using the information above we can construct a simple table to help us convert between pico-Farad (pF), to nano-Farad (nF), to micro-Farad (μF) and to Farads (F) as shown.

Capacitance of a Parallel Plate Capacitor

The capacitance of a parallel plate capacitor is proportional to the area, A in metres2 of the smallest of the two plates and inversely proportional to the distance or separation, d (i.e. the dielectric thickness) given in metres between these two conductive plates.

The generalised equation for the capacitance of a parallel plate capacitor is given as: C = ε(A/d) where ε represents the absolute permittivity of the dielectric material being used. The permittivity of a vacuum, εo also known as the “permittivity of free space” has the value of the constant 8.84 x 10-12 Farads per metre.

To make the maths a little easier, this dielectric constant of free space, εo, which can be written as: 1/(4π x 9×109), may also have the units of picofarads (pF) per metre as the constant giving: 8.84 for the value of free space. Note though that the resulting capacitance value will be in picofarads and not in farads.

Generally, the conductive plates of a capacitor are separated by some kind of insulating material or gel rather than a perfect vacuum. When calculating the capacitance of a capacitor, we can consider the permittivity of air, and especially of dry air, as being the same value as a vacuum as they are very close.

Capacitor Rating

The capacitance value of two different capacitors may exactly be the same and the voltage rating of the two capacitors are different. Let us take two capacitors, one which has a small voltage rating and other with high voltage rating. If we substitute a smaller rated voltage capacitor in place of a higher rated voltage capacitor, the smaller capacitor.

This can happen because of the unexpected increases in voltage. The common working DC voltage of capacitors are usually 10V, 16V, 25V, 35V, 50V, 63V, 100V, 160V, 250V, 400V and 1000V.

Multi-plate Capacitor

Now we have five plates connected to one lead (A) and four plates to the other lead (B). Then BOTH sides of the four plates connected to lead B are in contact with the dielectric, whereas only one side of each of the outer plates connected to A is in contact with the dielectric. Then as above, the useful surface area of each set of plates is only eight and its capacitance is therefore given as:

Modern capacitors can be classified according to the characteristics and properties of their insulating dielectric:

  • Low Loss, High Stability such as Mica, Low-K Ceramic, Polystyrene.
  • Medium Loss, Medium Stability such as Paper, Plastic Film, High-K Ceramic.
  • Polarized Capacitors such as Electrolytic’s, Tantalum’s.

Characteristics of Capacitors

The properties or characteristics of capacitors may differ from one another. Few characteristics of capacitors are:

Capacitance (C)

Capacitance is the basic and important characteristic of a capacitor. We measure it in pico-Farads (pF), nano-Farads (nF) or micro-Farads (µF). Usually, we can find this value printed on the capacitor body in form of a number or text. Hence, you can get this value easily. You can see capacitance in the Solved example below.

Working Voltage

The total amount of direct current (DC) or alternating current (AC) is applied to a capacitor without any failure in the capacitor’s whole lifetime. Working Voltage defines this statement.

Tolerance

Just like the voltage rating, capacitors also have a tolerance rating. They vary from plus to minus value.

Leakage Current

The capacitors Leakage Current is the small DC current flow in the region of Nano-Amps (nA). Leakage current is a result of electrons physically making their way through the dielectric medium. It could also be the movement around its edges or across its leads. Therefore, these electrons can fully discharge the capacitor over time if you remove the supply voltage.

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