NEET Physics Notes Electrostatics-Conductors and Insulators
Conductors and Insulators
Conductors and Insulators
Conductors are those materials through which electricity can pass through easily, because they contain a large number of free electrons, e.g. metals like copper, silver, iron etc. Insulators are those materials through which electricity cannot pass through, because they do not contain free electrons, e.g. rubber, ebonite, mica etc.
Dielectrics and Polarisation
Dielectrics are insulating materials which transmit electric effect without actually conducting electricity.
e.g. mica, glass, water etc. .
When a dielectric is placed in an external electric field, the centres of positive and negative charges gets separated in non-polar dielectrics and get farther away in polar dielectrics, so the molecules of dielectric gain a permanent electric dipole moment. This process is called polarisation.
Electrical Capacitance
Capacitance of a conductor is the amount of charge needed in order to raise the potential of the conductor by unity.
Mathematically, Capacitance 
Electrical capacitance is a scalar. SI unit of capacitance is 1 farad (IF), where
Its dimensional formula is 
The electrostatic potential energy of a charged conductor having capacitance C, charge Q and a potential V, is given by
Sharing of Charges
Let us have two charged conductors having charges Q1 and Q2 (or potentials V1,V2 and capacitances C1,C2 respectively). If these are joined together by means of a connecting wire, charge begins to flow from the higher potential to the lower potential side, till their potential is the same, which is called the common potential. In such a cases
During sharing of charges, there is some loss of electrostatic energy, which in turn reappears as heat or light. The loss of electrostatic energy
NOTE
- When charges are shared between any two bodies, their potential become equal. The charges acquired are in the ratio of their capacities.
- No charge is really lost, but same loss of energy does occur.
Capacitor
A capacitor is a device which stores electrostatic energy. It consists of conductors of any shape and size carrying charges of equal magnitudes and opposite signs and separated by an insulating medium.
Net charge on a capacitor is zero. However, ordinarily we talk in terms of charge on either plate of a capacitor and that is finite and non zero.
We conclude that the capacitance of an insulated conductance is increased considerably by bringing near it an uncharged each conductor.
Combination of Capacitors
There are two common methods of grouping of capacitors
Series Grouping
In a series arrangement, the charge on each plate of each capacitor has the same magnitude, equal to the charge supplied by the battery.
The potential difference is distributed inversely in the ratio of capacitors,
Parallel Grouping
In a parallel arrangement, the potential across each of the capacitor is exactly same.
Charges on different capacitors are different. In fact, the charge is distributed in the ratio of capacitance,
If n identical conductor plates are arranged such that alternate plates are joined together, then the combination is equivalent to (n – 1) capacitors all joined in parallel.
Capacitance of a Parallel Plate Capacitor
The parallel plate capacitor consists of two metal plates parallel to each other and separated by a distance that is very small as compared to the dimensions of the plates.
Capacitor without Dielectric Medium between the Plates
If the magnitude of charge on each plate of a parallel plate capacitor be Q and the overlapping area of plates be A, then
Electric field between the plates 
Potential difference between the plates 
, where d = separation between the two plates
Capacitance 
Capacitor with Dielectric Medium between the Plates
(i) If a dielectric medium of dielectric constant K is completely filled between the plates of a capacitor, then its capacitance becomes,
Energy Stored in a Capacitor
If a capacitor of capacity C is charged to a potential V, the electrostatic energy stored in it is,
Energy Loss During Parallel Combination
When two capacitor of C1 capacitance charge to potential V1 whereas another of C2 charge to potential of V2, then after parallel combination.
Loss in energy = Vi – Vf
Van de Graaff Generator
Van de Graaff in 1931, devised an electrostatic generator in order to produce very high potential of the order of 106 V. It is used for accelerating electrons and other charged particles required to study nucleus reactions. It makes use of electrostatic phenomena while functioning. If a charged conducting object has sharp points on its surface, then the charge density is so high at these points that the surrounding air becomes highly conducting and produce corona discharge.
If a charged conductor is brought into internal contact with a second hollow conductor, all of its charge is transferred to the hollow conductor, no matter how high the potential of the latter may be. Thus,the charge and hence the potential of the hollow conductor can be raised to a high value by successively adding charges to it by internal contact. The limit is set only by insulation difficulties.
