Tiêu đề 01.Electric charge and fields.pdf ✅

https://www.chemcontent.com/ 1 1.ELECTRIC CHARGES AND FIELDS Physics Smart Booklet Theory + NCERT MCQs + Topic Wise Practice MCQs + NEET PYQs
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https://www.chemcontent.com/ 3 Electric Charges and Fields Charge and its properties • Charge, like mass is a basic property of matter. • Charges are of two types − the one on proton, by convention is named positive and the other an electron, by convention is named negative. A neutron has no net charge and is electrically neutral. Fundamental properties of charges (a) Charge is conserved − quantity the net of charge in an isolated system remains constant. (b) Charge is quantized − the charge on any particle is an integral multiple of a fundamental unit of charge e, namely the charge on an electron or proton. Q ne =  : n = 1, 2, 3 ... where 19 e 1.6 10 C − =  . Protons and neutrons are made up of quarks with fractional values of e. However, quarks do not occur in free state. (c) Charge is invariant − unlike mass, length and time, charge on a body is independent of its state of motion (reference frames). (d) Charge is a scalar and is additive. (e) Like charges repel and unlike charges attract. Auxillary properties of electric charge (i) An electric charge in motion produces a magnetic field in addition to the electric field. (ii) An accelerated charge radiates energy in the form of electromagnetic waves. Methods of charging a body Bodies can be charged by • Friction (E.g. rubbing of glass with wool etc.) The two bodies acquire opposite kind of charge, due to one body losing electrons to other body. • Conduction (E.g. bringing an uncharged body in contact with a charged body) During conduction, the body getting charged acquires part of the charge from the source (charging body). • Induction (E.g. bringing a uncharged body near a charged body) During induction, the body getting charged acquires opposite kind of charge on its surface close to the charging body. The net charge on the charged body is still zero. However, by earthing, the charged body can be made to retain one type of charge. The charge on the source body remains intact. This method is applicable to only conductors. • irradiating a conducting body by suitable electromagnetic radiation • heating a body to a high temperature • by applying a large electric field • In all the above methods the body regains electrical neutrality over a period of time. • Whether a body is charged or not and if charged, the type and extent of charge can be detected by a device called electroscope. The amount of charge on a body can be measured by a device called electrometer. Coulomb’s law: The force (F) of attraction or repulsion between two point charges is directly proportional to the product of the charges (Q1 and Q2) and inversely proportional to the square of the distance (r) between them. 1 2 2 Q Q F r   1 2 2 0 1 Q Q F 4 r =  where 9 2 2 0 1 9 10 Nm C 4 − =   0 is called the permittivity of free space and 0 = 8.854  10−12 C 2 N−1 m−2 Dimensional formula of permittivity is 1 3 4 2 M L T A . − −
https://www.chemcontent.com/ 4 r is greater than one for a medium other than vacuum. For air  1. r r is called dielectric constant and is denoted by K. Coulomb’s law is strictly valid for point charges in vacuum. The presence of a medium modifies the force which is approximately given by 1 2 2 0 r 1 Q Q F 4 r =   where r is called the permittivity of the medium. Coulomb’s law in vector form Coulomb’s law of force between two point charges q1 and q2 located at 1 2 r and r is written as 1 2 21 2 21 0 21 1 q q F r 4 r =  where F21 is the force exerted on q2 by q1 and 21 r is the unit vector towards q2 from q1 and 21 2 1 r r r = − Similarly force exerted on q1 by q2 is 1 2 12 2 12 0 12 1 q q F r 4 r =  12 1 2 12 r r r and r = − is the unit vector towards q1 from q2. Since, the above equations are in vector form, q1 and q2 should be used with appropriate signs.  ▪ Coulomb’s law is strictly applicable to point charges. However, in practice, charges are always associated with matter, which occupy finite volume. To simplify the situation, usually a charge is replaced by a charge of equal magnitude imagined to be concentrated at a single point. Such a charge is called a point charge. ▪ The gravitational force is usually neglected while studying the force between electric charges because gravitational force is negligible compared to electrostatic force. The ratio of electrostatic force of repulsion to the gravitational force of attraction between two electrons separated by 1 m is about 1.7  1043 . ▪ The relative permittivity r of a medium is nearly constant. r depends both on the property of the medium and the magnitude of the charges. ▪ r is only a number and has no units. r is also called dielectric constant. ▪ The unit of charge is coulomb (C). ▪ The charge on an electron is equal to −1.6  10−19 C. ▪ One coulomb charge has 6.25  1018 electrons. ▪ A test charge is a hypothetical infinitesimally small charge, kept at a point to evaluate the electric field at that point without disturbing the field. Electric Field The electric field at a point is defined as the force experienced per unit positive test charge (q0 ) placed at that point. i.e., 0 F E q = Electric field is measured in newton per coulomb (NC−1 ). Its direction is always from a positive charge and towards a negative charge. Electric field (E) at distance r from a point charge Q is 2 0 1 Q E 4 r =  E is a vector and the resultant electric field at a point due to several charges is equal to the vector sum of the fields due to the individual charges. Coulomb's law in vector form