The discussion of electrostatics is thoroughly covered in two chapters of Physics (Part I) of the NCERT Class 12 textbook. The definition of the electrostatic phenomenon, creation of electrostatic fields, charge distribution, and electrostatic related laws are discussed in the first chapter, entitled Electric Charges, and Fields. A brief definition headlining the basics of electrostatic is given below:
- Electrostatics is the branch of physics that deals with the study of the electromagnetic phenomenon when electric charges do not move or change with time.
- The creation of electrostatic phenomenon in a system is due to the electrification of the objects in the system. For example, on rubbing a glass rod with a wool or silk cloth, we electrify the rod such that after rubbing, the rod will attract another wool or silk cloth. However, a charged wool or silk cloth will repel another piece of cloth. Within this system of glass rods and cloth pieces, the objects are electrified by rubbing making each body possess a type of entity called the electric charge. Initially, the rod and cloth are stable, and on rubbing against each other charges are transferred from the rod to the cloth. The property that differentiates the two kinds of charges is called the polarity of charge. There are two types of electrification, (i) like charges repel and (ii) unlike charges attract each other. Notably, the electrified glass rod will no longer attract the piece of cloth with which it was rubbed. This shows that charges acquired after rubbing are lost when charged bodies are contacting each other, neutralizing each other effects.
- Naming the charges positive and negative, such that when they are added up, the same magnitude of positive and negative charges nullifies the net charge. By convention, the charge on rubbed glass rod is called positive while on the wool or silk cloth is termed negative. When no charge is available then it is said to be neutral.
- Some materials readily allow the passage of electricity or transfer of charge through them while others do not. Those substances that allow electricity to pass through them easily are called conductors and those that don’t are insulators.
- Not only by rubbing the two objects that electricity is created but a body at rest can also be charged by inducing charges by contacting with a charged body. The induced charged body will induce similar effects on the surrounding objects.
- If the sizes of charged bodies are very small as compared to the distance between them, the bodies will be treated as point charges. The properties of electric charge are:
- The total charge of a system containing n number of point charges will be the sum of all charges in the system. The total charge of the system can be positive or negative.
- The total charge of an isolated system is always conserved.
- Quantization of charge on a body, q is a multiple, n of the basic unit of charge, e, given by q=ne. n is an integer, either positive or negative and the basic unit of charge is the charge that a proton or electron carries, its value equal to 1.602192 × 10–19
Important Question:Threepoint charges of +2muC, -3muC, and -3muC are kept at the vertices A, B, and C, respectively of an equilateral triangle of side 20 cm. what should be the sign and magnitude of the charge (q) to be placed at the midpoint (M) of side BC so that the charge at A remains in equilibrium?
- Coulomb’s law states that the force (F) between two point charges varies inversely as the square of the distance between them (r) and is directly proportional to the product of the magnitude of the two charges (q1 and q2), and acting along the line joining the two charges.
- By Coulomb’s law, the force exerted by a point charge Q located at the origin, O, on another charge, q placed at r distance from the location of Q, the electric field, E produced by the charge Q at point r from the origin, is given as
ε0 is called the permittivity of free space. The value of ε0 in SI units is 8.854 × 10–12 C2 N–1m–2
, is a unit vector from the origin to the point r. The force F exerted by the charge Q on the charge q is
- A system of charges q1, q2, …, qn with position vector r1, r2, …, rn relative to an origin. At point P, Electric field E1 due to q1 at position vector r1 is given by
Where is the unit vector in the direction from q1 to P and is the distance between q1 and P. The electric field due to the superposition of all the charges is
- The charge, ΔQ distributedalong the line (Δl), on conductor surface (ΔS), and within volume (ΔV)elements are given as
- The flux through an area element ΔS is . Due to a single charge, q, the electric field E on the surface of the sphere, the magnitude of is 1. ΔS and have the same direction.