Capacitors generally is made of two conducting surfaces (say metal plates) which are separated by an insulating material (e.g., air, mica, paper, etc.). The dielectric media has the property to store electrical energy in the form of electrostatic charge.
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Different media hold different charges on the application the same potential. This ability of the body to hold the charge is called capacitance or capacity of the body. In order to store sufficient charge, a device called a capacitor is purposely constructed.
It can be connected to a circuit so that this stored energy in the capacitor can be made to flow in a desired part of the circuit to perform an intended function. Capacitance has essential attributes in DC as well as AC circuits. In many circuits (e.g., radio and television circuits), it is intentionally inserted to introduce the desired capacitance.
Capacitors
Any two conducting surfaces separated by an insulating material are called as capacitors or condensers. The purpose of the capacitor is to store the charge in a small and closed space. The name is derived from the fact that this arrangement has the capacity to store charges. The name condenser is given to the device due to the fact that when a potential difference is applied across it, the electric lines of force are condensed in the small space between the plates. The conducting surfaces are called the plates of the capacitor and the insulating material which seperates the plates is called the dielectric.
A steady current cannot pass through an insulator but an electric field can. For this reason, an insulator is often referred to as a dielectric. Air, mica, waxed paper, ceramics, etc. are among the most used dielectrics in the industry.
The following points may be noted carefully:
- The ability of capacitors to store charge (i.e. its capacitance) depends upon the area of plates, the distance between plates, and the nature of the insulating material (or dielectric).
- A capacitor is generally named after the dielectric used e.g. air capacitors, paper capacitors, mica capacitors, etc.
- The capacitors may be in the form of parallel plates, concentric cylinder,s or other arrangement.
CHARGING OF A CAPACITOR
A capacitor stores charge when connected to a supply. The parallel plate capacitors having plates A and B is connected across a battery of V volts as shown in Figure.
(i). When the switch S is open as shown, the capacitor plates are neutral i.e. there is no charge on the plates. When the switch is closed as shown (ii), the electrons from plate A will be attracted by the +ve terminal of the battery and these electrons start accumulating on plate B.
The result is that plate A attains more positive charge and correspondingly plate B gets more negative charge. This action is referred to as charging a capacitor because the capacitor plates are becoming charged. This process of flow of electrons also called as charging (i.e. detaching electrons from plate A and accumulating on B) continues till potential difference across capacitor plates becomes equal to external supply voltage V. When the capacitor is charged to supply voltage V, the current flow ceases as shown in (iii)
If now the switch is opened as shown in Figure (iv), the capacitor plates will retain the charges. Thus, the plates of the capacitor which were electrically neutral to start with now posses the charges on them. This shows that a capacitor stores charge.
Capacitance
The ability of a capacitor to store charge in it’s dielectric is known as its capacitance. It has been found experimentally that charge Q stored in a capacitor is directly proportional to the potential difference V across it i.e.
Q ∝ V or, Q / V = Constant = C
The constant C is called the capacitance of the capacitor. Hence capacitance of a capacitor can be defined as the ratio of charge on capacitor plates to the potential difference across the plates.
Unit of capacitance
We know that C = Q/V
The SI unit of charge is Coulomb and that of voltage is Volt. Therefore, the SI unit of capacitance is coulomb/volt which is also called farad (Symbol F) in honor of Michael Faraday.
1 farad = 1 coulomb/ 1 volt
A capacitor has a capacitance of 1 farad if a charge of 1 coulomb accumulates on each plate of the capacitor, when the potential difference of 1 volt is applied across the plates. Thus, if a charge of 0·1C accumulates on each plate of a capacitor when a potential difference of 10V is applied across its plates, then the capacitance of the capacitor = 0·1/10 = 0·01 F.
The farad is a large unit of capacitance. However, most practical application of capacitors have capacitances of the order of microfarad (μF) and micro-microfarad (μμF) or picofarad (pF).
1 μF = 10−6F; 1μμF (or 1 pF) = 10−12 F
Factors Affecting Capacitance
The ability of a capacitor to store the charge in the dielectric field (i.e. the capacitance) depends upon the following factors:
(i) Area of plate:
The greater the area of capacitor plates, the larger will be the capacitance of the capacitor and vice-versa. It is because the larger the plates, the greater the charge they can hold for a given p.d. and hence greater the capacitance.
(ii) Thickness of dielectric:
The capacitance of a capacitor is inversely proportional to the thickness (i.e. distance between plates) of the dielectric. The smaller the thickness of the dielectric, the greater the capacitance and vice-versa. When the plates of the capacitor are brought closer, the electrostatic field between them is intensified along with the charge density, and hence capacitance increases.
(iii) Relative permittivity of dielectric:
The greater the relative permittivity of the insulating material (i.e., dielectric), lead to greater capacitance of the capacitor and vice-versa. This is because the nature of dielectric affects the electrostatic field between the plates of the capacitor and hence impacts the charge that accumulates on the plates of the capacitor.
Dielectric Constant or Relative Permittivity
The insulating material which seperates the plates of a capacitor is called dielectric. When the capacitor is charged, the electrostatic field extends across the dielectric media. The presence of dielectric increases the concentration of electric lines of force between the plates and hence the charge on each plate. The degree of concentration of electric lines of force between the plates depends upon the nature of the dielectric.
The ability of a dielectric material to concentrate electric lines of force between the plates of a capacitor is called the dielectric constant or relative permittivity of the material.
Normally, the electrons in the atoms of the dielectric revolves around the nuclei in the regular orbits. When the capacitor is charged, the electrostatic field causes distortion of the orbits of the electrons of the dielectric. This distortion of orbits causes an additional electrostatic field within the dielectric which causes more electrons to be transferred from one plate to the other. Hence, the presence of dielectric increases the charge on the capacitor plates and hence the capacitance.
Air has been assigned a reference value of dielectric constant (or relative permittivity) as 1. The dielectric constant of all other insulating materials is greater than unity. The dielectric constants of materials commonly used in capacitors range from 1 to 10. For example, the dielectric constant of mica is 6. It means that if mica is used as a dielectric between the plates of a capacitor, the charge on each plate will be 6 times the value when air is used, with other things remaining equal.
Hence, the dielectric constant (or relative permittivity) of a dielectric material can also be called the ratio of the capacitance of a capacitor with that material as a dielectric to the capacitance of the same capacitor with air as a dielectric.
Voltage rating of a capacitor
It is the voltage that the dielectric medium of the capacitor can withstand without breaking down. On the occasion of a dielectric medium breakdown, it will trigger a short circuit, which is hazardous. In the capacitors workbook, the voltage rating is mentioned as working voltage. It is to be noted that, a capacitor that is capable of handling x volts DC must not be connected to x volts AC. This is because the rms value of AC is x volts but its peak value will be √2 times x volts, which is enough to cause a short circuit.
For this reason, it is recommended that the AC voltage should be at least 40% lower than the working voltage of the capacitor.
Applications
- Capacitors are used in most DC control circuits to smooth out the fluctuations of current.
- Capacitors can be used as temporary batteries as they can store energy after disconnection from a charging circuit.
- Capacitors are used to counterbalance the inductive loading of induction motors and electric motors etc in a distribution circuit and make the load resistive. Thus it helps in improving the power factor in power transmission and distribution.
- Capacitors can be used to suppress undesired frequencies by blocking them. As capacitors pass the AC but block DC signals, they are also used to filter or separate the two components known as AC coupling or capacitive coupling.
