Capacitor and its principle

 Capacitor : 

A capacity of the order of 1 μF is extensively used in the assembly of radio sets. The capacity can be achieved by means of a simple spherical capacitor. The radius of that capacitor can be calculated as follows 

  Since       C = r/9 × 10⁹

              10−6  =  r/9 × 10⁹

               r = 9 × 10³ m       or       r = 9 km.                   

 This value of 'r' is so large that it is impossible to assemble a radio set using capacities from a single conductor. Therefore, some method has to be devised to obtain a large capacity in a smaller space. 

A capacitor or a condenser is an arrangement which provides a large capacity in a smaller space. 

Principle of a capacitor : 

It is based on the principle that when, an earthed conductor is placed in the neighbourhood of a charged conductor, the capacity of the system increases considerably, this can be shown by the following simple experiment. 

Consider a plate P charged positively and connected to the knob of a gold leaf electroscope. Note the divergence of the leaves [Fig.(i)]. The divergence is a measure of potential 'V' of the plate. Now place another plate Q near it [Fig.(ii)]. Negative charge is induced on the inner side of Q. Positive charge is induced on the outer side Q. Induced negative charge on inner side of Q tries to decrease the potential of P while induced positive charge on the outer side of Q tries to increase it. On the whole, there is a net decrease in the potential of P because negative charge is nearer to P than the positive charge. This is indicated by a decrease in the divergence of leaves. 

  Principle of a capacitor     

But            C = Q/V

Therefore, the capacity of the system increases. 

Now connect the plate Q to earth [Fig. (iii)]. The free positive charge on its outer surface disappears, thereby, causing a further reduction in the potential of P. Hence the capacity of the system increases further. 

Types of Capacitors:

1. Paper capacitor. It consists of a pair of longer tin foils having a piece of wax paper in between them [Fig. (i)]. The wax paper acts as a di-electric medium. For the sake of convenience, the foils along with wax paper are rolled into the form of a cylinder [Fig. (ii)].                

Paper capacitor. 

Two leads connected to the foils are taken out from the two sides of the roll. 

2. Mica capacitor. It consists of a number of mica sheets arranged one above the other in such a way that there is a mica sheet in between every two copper plates [in fig.]. Alternate copper plates are connected to terminals A and B. This way different condensers get connected in parallel with each other. Thus net capacity of the capacitor is equal to the sum of the individual capacities.                                                       

Mica capacitor. 

3. Variable condenser. It is a condenser whose capacity can be varied at will. It consists of two sets X and Y of parallel plates. One set of plates, say Y, is fixed while the other set X can be turned with the help of knob k [in fig.]. As the knob turns, the set X either moves into the spacings of set Y Or comes out of them, thereby changing the common area of the plates. Since capacity also undergoes a change. Generally a pointer attached to the knob moves over a graduated scale enabling us to read the capacity directly.                          

Variable condenser. 

4. Electrolytic capacitor. It consists of a pair of aluminium plates A and C dipping in a solution of aluminium borate. A and C are connected to the positive and negative terminals of a source of steady current [in fig.]. Due to electrolysis a very fine lyer (≈ 10^-6 cm) of aluminium oxide is formed on plate A. This layer acts as a di-electric medium while rhe solution along with plate C acts as cathode. Since the thickness of the di-electric is very small, the capacitor has a large capacity.             

Electrolytic capacitor. 

                                                                                      In a wet type electrolytic condenser, the anode is taken in the form of a cylinder immersed in the electrolyte contained in a metal can which works as a cathode. In case of a dry type electrolytic capacitor, two long strips of aluminium (one of which has a thin layer of a aluminium oxide) have cotton or paper gauze soaked in electrolyte in between them.                                                                        

 The oxide layer of aluminium has a low resistance in one direction and a high resistance in the other. So the electrolytic capacitors are used only in cases of sources of unidirectional current in such a way that the oxide plate is always positive with respect to the other plate.