Download Chapter 18 - Electric Forces and Electric Fields • Atomic nature of

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.
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• 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.
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