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Transcript
Chapter 18 - Electric Forces and Electric Fields • Atomic nature of matter: matter consists of atoms. Atoms consist of a central part - the nucleus with neutrons and protons and electrons circling the nucleus. • Electric charge is an intrinsic property of matter - two types, positive and negative. • Protons have a positive charge and electrons have an equal and opposite amount of negative charge. Neutrons have no charge. • Unit of charge is the Coulomb • Electron has a charge of −1.6 × 10−19 C, proton has a charge of +1.6 × 10−19 C. • Charge is quantized - a multiple of 1.6 × 10−19 C. • Usually atoms are electrically neutral: equal amount of positive and negative charge ie. the same number of electrons and protons q = Ne. Here N is an integer. • Ex 1, p. 530 • Like charges repel, unlike charges attract. • This is the electric force or the electrostatic force - force due to electric charges that are stationary. • Coulomb’s Law: |q1 ||q2 | , r2 where the force is measured in Newtons, q1 , q2 in Coulombs and k = 8.99 × 109 N.m2 /C 2 . F =k • Ex. 3, p. 536, Ex 4, p. 537, Ex. 538, p. 538. • The Electric Field, E= F , q0 • The units of electric field are N/C. • Ex. 7, p. 540 • The Coulomb force is a vector force and the Electric field is a vector field. 1 • Ex. 8,9,10 p. 541. • Parallel Plate Capacitor - two parallel plates of area A have an electric field between the plates of q . E= ǫ0 A • Electric field lines represent the direction a positive test charge would move in the presence of an electric field. They run from positive to negative. • Conductors/insulators • At equilbrium underelectrostatic conditions, any excess charge resides on the surface of a conductor. • At equilbrium, under electrostatic conditions, the electric field is zero at any point within a conducting material. • The conductor shields any charge within it from electric fields created outside the conductor. The electric field just outside the surface of a conductor is perpendicular to the surface at equilbrium under electrostatic conditions. • Ex. 14, p. 549 • Flux: ΦA = EA. • Gauss’ Law: The electric flux, ΦE through a Gaussian surface is equal to the net charge, Q, enclosed by the surface divided by ǫ0 , the permittivity of free space. • Ex. 15,16 p. 551 • At equilbrium, under electrostatic conditions, the electric field is zero at any point within a conducting material. 2