Electric Charges And Fields Class 12 Notes PDF

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Electric Charge And Its Characteristic

(1) Electric charges are the fundamental intrinsic property due to which electric force acts

(2) Electric charges are two types. Traditionally charge on the proton is positive and the charge on the electron is negative. The magnitude of the charge on both is the same

(III) Fundamental charge is the charge on the electron or proton denoted by e.

(lv) Quantization of charges: The magnitude of all charges is found to be the Integral multiple of fundamental charge thus if Q is the total charge then Q=ne here n = 1,2,3,

(v) The Law of conservation of electric charge states that “The algebraic sum of electric charge In an electrically isolated system always remains constant Irrespective of any process taking place.

In other words “In an electrically isolated system, only those processes are possible in which charges of equal magnitude and opposite types are either produced or destroyed.

Example Before rubbing a glass rod on silk algebraic sum of charge is zero. On rubbing and separating the rod from silk, we find equal and opposite amounts of charge developed on silk and glass rods.

(vi) Two charges exert equal and opposite force on each other. Like charges repeals unlike attracts

(vii) S.1 Unit of charge is “coulomb’ denoted by C. CGS unit of charge

(vill) The charge cannot exist without mass though mass can exist without charge

(ix) Charge is invariant: This means that charge is independent of the frame of reference. i.e. change of the body does not charge with whatever its speed

Ways of charging the body

(A) Charging by friction:

When two bodies are rubbed together, a transfer of electrons takes place from one body to another.

The body from which electrons have been transferred is left with an excess of positive charge, so gets positively charged.

The body that receives the electrons becomes negatively charged. “The positive charge and negative charges produced by rubbing are always equal magnitudes”

When a glass rod is rubbed on silk, the glass rod loses its electrons and gets positive charges, while silk acquires equal negative charges: An ebonite or plastic rod acquires a negative charge, if it is rubbed with wool. The piece of wool acquires an equal positive charge.

Consequently, the part of the sphere close to the rod becomes positively charged due to a deficiency of electrons in that region.

As shown in Figure B when the sphere is connected to the earth through a conducting wire, some of the electrons of the spheres will flow to the ground.

As shown in figure c, even if the connection with the earth is removed, the sphere retains the positive charge.

When the negatively charged rod is moved away from the sphere, the electrons get redistributed on the sphere such that the same positive charge is spread all over the surface of the sphere as shown in Figure d Important points regarding electrostatic induction

(a) Inducing body neither gains nor loses charges

(b) The nature of Induced charge is always opposite to that of Inducing charge

(c) Induced charge can be lesser or equal to inducing charge but it is never greater than the inducing charge than

(d) Induction takes place only in bodies (either conducting or non-conducting) and not particles

(C) Charging by conduction

Let us consider two conductors, one charged and the other uncharged. We bring the conductors in contact with each other. The charge under its own repulsion will spread over both the conductors. Thus the conductors will be charged with the same sign. If charged

and uncharged conductors are of the same size charge will be equally divided if separated after contact

Solved numerical

Q) A copper penny has a mass of 3.1g. Being electrically neutral, it contains an equal amount of positive and negative charges. What is the magnitude of these? A copper atom has a positive nuclear charge of 4.6X10 C.

The atomic weight of copper is 64 g/mole and Avogadro’s number is 6X10 atoms/mole Solution: 1 mole of copper le 64 g of copper has 6X102 atoms. Therefore, the number of atoms in a copper penny of 3.1 g is 6×1071 64 x 3.1-2.9×10″

One atom of copper has a positive and negative charge of 4.6×10 “C. So each charge on the penny is (4.6×10″) x ( 2.9×10”)-1.3×10 C.

Language English
No. of Pages35
PDF Size15 MB

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