Electrostatics - Cbse Physics Class 12 By Gurukul24x7

©Copyright Gurukul24x7 Electrostatics, Important facts. 1.Two bodies attract or repel each other because they have different kind of charges. There are two types of charges, negative polarity and positive polarity charges. 2.When unlike charges come in contact with each other they get neutralized if the magnitude is same. 3.A body can have electric charge, which is when it is called electrified or charged and if its not charged it's said to be neutral. 4.Charge can't be created nor it can be destroyed. Thus when a body is charged by direct contact then like rubbing etc, some of the charge gets transferred from one body to another body however the total charge remains constant. 5.Conductors are those materials that can conduct electricity very easily. By very easily it is meant that even the smallest amount of voltage applied can make current to flow within a conductor. 6.Insulators are those materials that can't conduct electricity easily. That is even if a very large amount of voltage is applied, a negligible amount of current,if any, will be noticed in an insulator. 7.Both conductors and insulators are required in everyday life. For example, the household wiring is made from conductors but since its dangerous to have open wires, insulation is provided to wires using plastic materials or other insulating materials. 8.When charge is placed on a conductors it spreads out uniformly over its surface, however with insulators, the charge will remain where it is placed. 9.When a charged body is connected to earth, the excess charge flows to ground, causing a momentay current to flow. This is called as earthing. In many industries like gunpowder and electronic industries, the shoes of workers contain a small clip which helps to make the excess charge flow to ground. Similarly any outsider is first made to step onto a specially made conducting plate connected to ground so that any excess charge a person has is transferred to ground. 10.Charging by Induction implies that the bodies never come in contact of each other. Charge equilibrium takes place before a final charge appears, however this equilibrium takes a very short time to reach and the effect appears to be instantaneous. 11.Charges can be added. Charges should be added keeping the sign of the polarity. 12.Charge is quantized. There's a smallest unit of charge which is the charge of electron −19 e = 1.6×10 C . All charges must be integral multiples of this charge. Thus any charge must be q = n e where n is an integer positive or negative. This fact is an experimental fact. 13.At the microscopic level, quantization of charge is of very importance. By microscopic level it means, when we are talking about a few hundred of thousand charges. While in a macroscopic level, Phone: +91-011-22148115, +91-011-28715025 , +91 9911425050, +91 9911193737

©Copyright Gurukul24x7 since the number of charges can be huge, of the order of 1013 we can ignore quantization and assume that charges are continuous. 14.The unit Coulomb is very large. For example if 109 electrons are transferred per second then it would take about 200 years to transfer 1C charge!. For practical purposes the amount of charge is of the order of  C . 15.Coulomb's Law: The magnitude of the force between two charges separated by a distance r is given k q 1 q2 ∣ = by ∣F . 2 r 16.The direction of force between the charges can be towards each other(attractive) or away from each other(repulsive) depending upon the sign of the charges. The direction is along the line joining the two charges. 17.When writing down the force, always write the magnitude first. For direction, use unit vector of the line joining the two charges. If the force is attractive then reverse the direction of unit vector by placing a minus sign, otherwise leave the unit vector as it is. 18.Shell Theorem: ◦A shell of uniform charge attracts or repels another charged particle outside the shell as if the shell's charge were concentrated at its center. That is consider shell in this case as another charged particle. ◦If a charged particle is located inside a shell of uniform charge, then there's no net electrostatic force on the charged particle inside the shell. This is because each force cancel outs the other irrespective of the polarity of the charge on the shell and of the particle. Spherical Conductors: When Excess charge on spherical conductor exists, then it'll spread out uniformly over the surface of the conductor. This is due to the fact that since charges placed would be like charges, thus they would cause repulsion between each other. Since the surface is that of a conductor, therefore charges can move around easily and would thus spread out uniformly so as to minimize the repulsion between them. In this condition, we can assume that the whole charge is concentrated at the center of the shell.

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