Post on 02-Apr-2015
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Physics for Scientistsand Engineers
Chapter 21:The Electric Field I:
Discrete Charge Distributions
Copyright © 2004 by W. H. Freeman & Company
Paul A. Tipler • Gene Mosca
Fifth Edition
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21-1Electric Charge
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Electric Charge
Q = ± Ne, (N = integer)
Charge of Matter
qproton = +e qneutron = 0 qelectron = -e
Examples:
He = (2p, 2n, 2e-) charge = 0
He+ = (2p, 2n, 1e-) = +1e
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Charge Interaction
Like charges repelUnlike charges attract
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Relative affinity of materials
in attracting electrons.
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21-2Conductors and Insulators
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Conductors
• “free” electrons • electron motion = “conduction”
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Evidence of electron conduction away from central metallic element (gold leaf repulsion)
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Charging by Induction (1 of 3)
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Charging by Induction (2 of 3)
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Charging by Induction (3 of 3)
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attraction of an uncharged conductor
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“Ground” = source or sink of charge
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Charge re-distributes quickly after rod moves far away
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21-3Coulomb’s Law
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Example in 2D
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Electric Field
force per charge-size [N/C]
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Direction of E
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Electric Field of a Point Charge
Electric Field due to Multiple Point Charges
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Example: Approximating E above Uniform Line of Charge
Given: 6 m long uniform strip of charge Q lies along x axis 0, 6 m. Estimate E at P = (6, 2)m by breaking strip into 3 discrete point charges q = Q/3 located at 1, 3, and 5 m along the x-axis.
r1/r1 = {(6, 2) – (1, 0)}/(29). = (5,2)/ (29)
r2/r2 = (3, 2)/ (13)
r3/r3 = (1, 2)/ (5)
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)01281.0,03202.0()29(
)2,5(33
1
1
1
12
11 kq
kq
r
rkq
r
r
r
kqE
)04267.0,06400.0()13(
)2,3(233
2
2
2
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22 kq
kq
r
rkq
r
r
r
kqE
)1789.0,08944.0()5(
)2,1(233
3
3
3
32
33 kq
kq
r
rkq
r
r
r
kqE
Components of Total Electric Field
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The net electric field at (6,2) is E = E1 + E2 + E3.
E = kq(0.1855, 0.2344)
angle of E = tan-1(.2344/.1855) = 51.6.
magnitude of E = kq(0.18552 + 0.23442) = 0.2989kq= 0.2989kQ/3 = 0.09963kQ
The exact value of E = kQ/12 = 0.08333kQ
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Dipole – two equal size opposite sign charges at small separation distance
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Electric Field around a Dipole
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21-5Electric Field Lines
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Effect of point charge on oil soaked threads
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Electric Field Lines (EFLs) inferred from experiment
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Effect of two like point charges on oil soaked threads
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Electric Field Lines (EFLs) inferred from experiment
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Effect of two unlike point charges on oil soaked threads
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Electric Field Lines (EFLs) inferred from experiment
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EFL Conventions
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EFLs lines for +q as distance from charges increas
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Relative charge = #lines leaving – #lines entering
Example:
Each sphere has count of +8
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21-6Motion of Point Charges in
Electric Fields
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Force on – charge is opposite to E.
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21-7Electric Dipoles in
Electric Fields
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Problems
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