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Chapter 1, Electric Charges and Fields, Electrostatics, Electrostatics deals with the study of forces, fields and potentials arising, from static charges., , Frictional electricity, The electricity produced by rubbing suitable bodies is called frictional, electricity., On rubbing electrons are transferred from one body to the other. The body,, which loses electrons, will become positively charged and which gains, electrons becomes negatively charged., , Electric Charge, From simple experiments on frictional electricity, it is inferred that there, are two types of charges in nature-Positive and Negative ., Like charges repel and unlike charges attract., • When a glass rod is rubbed with silk, glass rod becomes positively, charged and silk negaitive., • When a plastic rod is rubbed with fur, plastic rod becomes negatively, charged and fur positive., 1. Good conductors like copper cannot be charged by friction because any, charge produced on it can easily flow through the rod through our body and, to the ground., 2. Insulators like plastic, ebonite, glass etc can be easily charged by friction, because the charges will stay on them., 3. Electrostatic experiments cannot be performed in moist climate because, moist air is slightly conducting. So the static charges will get conducted away, from the charged body., , How is frictional electrification caused?, The number of protons inside the nucleus of an atom is equal to the number, of electrons outside the nucleus. When a body is rubbed with another, due to, friction, some electrons from one body gets transferred to the other body., The body, which loses electrons, will become positively charged and which, gains electrons becomes negatively charged. The two bodies thus acquire, opposite charges and they are equal in magnitude. This is the reason for, frictional electricity.
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Gold Leaf Electroscope, A simple apparatus to detect charge on a body is called a gold-leaf, electroscope., Apparatus It consists of a vertical metal rod placed in a box. Two thin gold, leaves are attached to its bottom end as shown in figure., , Working, When a charged object touches the metal knob at the top of the rod, charge, flows on to the leaves and they diverge. The degree of divergence is an, indicator of the amount of charge., , Conductors and Insulators, Conductors, Conductors are those substances which allow passage of electricity through, them., Eg. Metals, human and animal bodies and earth are conductors., • They have electric charges (electrons) that are comparatively free to, move inside the material., • When some charge is transferred to a conductor, it readily gets, distributed over the entire surface of the conductor., • Metals cannot be charged by friction,because the charges transferred, to the metal leak through our body to the ground as both are, conductors of electricity.
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Insulators, The substances which offer high resistance to the passage of electricity, through them are called Insulators ., Eg. glass, porcelain, plastic, nylon, wood, • If some charge is put on an insulator, it stays at the same place. So, insulators gets electrified on combing dry hair or on rubbing,, , Earthing (or) Grounding, When a charged body is brought in contact with, earth, all the excess charge pass to the earth, through the connecting conductor. This process of, sharing the charges with the earth is called, grounding or earthing. Earthing provides, protection to electrical circuits and appliances., , Methods of Charging, A body can be charged in different ways, 1)Charging by friction, 2)Charging by conduction, 3)Charging by induction, , 1) Charging by friction, When two bodies are rubbed each other, electrons in one body (in which, electrons are held less tightly) transferred to second body (in which, electrons are held more tightly), When a glass rod is rubbed with silk, some of the electrons from the glass, are transferred to silk. Hence glass rod gets +ve charge and silk gets -ve, charge., , 2) Charging by conduction, Charging a body with actual contact of another body is called charging by, conduction. If a neutral conducting body (A) is brought in contact with, positively charged conducting body (B), the neutral body gets positively, charged.
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3)Charging by induction, When a charged body is brought near to an uncharged conductor (without, touching), that end of the uncharged conductor which is near to the charged, body gets oppositely charged and the farther end is charged with the same, type of charge., , Charging a metal sphere positively without touching it
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Properties of electric charges, 1.Electric charges are of two kinds – positive and negative., 2.Like charges repel and unlike charges attract each other., 3. Quantization of charge : According to quantisation of electric charge,, charge of a body is an integral multiple of a basic charge, which is the, electronic charge., Charge on a body, q=± ne ; where, n is an integer n=0,1,2,3........., e is the electronic charge. e=1.602 x 10−19 C, 4. Charge is conserved: It means that total charge of an isolated system, remains constant. It also implies that electric charges can neither be created, nor destroyed. If an object loses some charge, an equal amount of charge, appears somewhere else., 5.Charge is a scalar quantity., 6. Additivity of charge: The total charge on a surface is the algebraic sum of, individual charges present on that surface., If 𝑞1 , 𝑞2 , 𝑞3 ....................., 𝑞𝑛 are the charges on a surface, then total or net, charge,, 𝒒 = 𝒒𝟏 + 𝒒𝟐 + 𝒒𝟑 +.................. + 𝒒𝒏, , Example 1, How many electronic charges form 1 C of charge?, q=±ne,, 𝑞, n=, 𝑒, , n=, , 1, , 1.602 x 10−19, , =6.25 x1018, , Example 2, A comb drawn through person’s hair causes 1022 electrons to leave the, person’s hair and stick to the comb. Calculate the charge carried by the, comb., q= ne,, q = 1022 x 1.602 x 10−19 C, = −1.602 x 103 C, As the comb gains electrons it gets negatively charged.
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Super position principle, Force on a charge due to a number of charges is the vector sum of forces, due to individual charges.For a system of n charges., , Electric Field, Electric field is the region around a charge where its effect can be felt., Intensity of electric field at a point is the force per unit charge., , 𝐄=, , 𝐅, 𝐪, , Electric field due to a point charge, , 𝐄=, , 𝟏, , 𝐪, , 𝟒𝛑𝛆𝟎 𝐫 𝟐
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Electric field due to a system of charges, Electric field at a point due to a system of charges is the vector sum of the, electric fields at the point due to individual charges., , Electric Field Lines, An electric field line is a curve drawn in such a way that the tangent to it at, each point is in the direction of the net field at that point., (i), Electric Field lines tart from positive charge, end at negative charge., (ii) Electric field lines of a positive charge are radially outwards and, that of a negative charge is radially inwards, (iii) Electric field lines do not form closed loops., (iv) In a charge free region field lines are continuous., (v) Two field lines never intersect.( Two directions for electric field is, not possible at a point), (vi) Field lines are parallel ,equidistant and in same direction in, uniform electric field., Positive Charge, , Negative Charge
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Electric Dipole, An electri dipole is a pair of equal and opposite charges separated by a, distance, , The total charge of the system is +q + -q =0, , ⃗), Electric Dipole moment (𝐩
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Dipole in a non uniform electric field, In a non uniform electric field the dipole experiences a net force as well as a, net torque in general ., Case 1 -when p is parallel to E, , when p is parallel to E, the dipole has a net force in the direction of, increasing field., But the net torque will be zero τ =pE sin0 =0, Case 2-When p is antiparallel to E., , When p is antiparallel to E, the net force on the dipole is in the direction of, decreasing field., But the net torque will be zero, τ =pE sin 180 =0
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c) field inside the shell, ϕ = ES, , (1), , Inside the shell q=0, , ES=0, , (S≠ 𝟎)
