electric forces & electric fields
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Chapter 18
Elec tr ic Forces andElec tr ic Fields
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18.1 The Origin o f Electr ic i ty
The electrical nature of matter is inherent
in atomic structure.
kg10673.1 27pm
kg10675.1 27
nm
kg1011.9 31em
C1060.1 19e
coulombs
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18.1 The Origin o f Electr ic i ty
In nature, atoms are normally
found with equal numbers of protonsand electrons, so they are electrically
neutral.
By adding or removing electrons
from matter it will acquire a net
electric charge with magnitude equalto etimes the number of electrons
added or removed, N.
Neq
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18.1 The Origin o f Electr ic i ty
Example 1 A Lot of Electrons
How many electrons are there in one coulomb of negative charge?
Neq
18
19-
1025.6C101.60
C00.1
e
qN
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18.2 Charged Objects and the Electr ic Force
It is possible to transfer electric charge from one object to another.
The body that loses electrons has an excess of positive charge, while
the body that gains electrons has an excess of negative charge.
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18.2 Charged Objects and the Electr ic Force
LAW OF CONSERVATION OF ELECTRIC CHARGE
During any process, the net electric charge of an isolated system remainsconstant (is conserved).
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18.2 Charged Objects and the Electr ic Force
Like charges repel and unlike
charges attract each other.
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18.3 Conductors and Insulators
Not only can electric charge exist on an object, but it can also movethrough an object.
Substances that readily conduct electric charge are called electr ical
conductors.
Materials that conduct electric charge poorly are called electricalinsulators.
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18.4 Charging b y Contact and by Indu ction
Charging by contact.
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18.4 Charging b y Contact and by Indu ction
Charging by induction.
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18.5 Coulombs Law
COULOMBS LAW
The magnitude of the electrostatic force exerted by one point charge
on another point charge is directly proportional to the magnitude of the
charges and inversely proportional to the square of the distance between
them.
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21
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qqkF
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2212 mNC1085.8
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18.5 Coulombs Law
Example 3 A Model of the Hydrogen Atom
In the Bohr model of the hydrogen atom, the electron is in orbit about the
nuclear proton at a radius of 5.29x10-11m. Determine the speed of the
electron, assuming the orbit to be circular.
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r
qqkF
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C l b L
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18.5 Coulombs Law
N1022.8m1029.5
C1060.1CmN1099.8 8211
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rqqkF
rmvmaF c2
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118
mFrv
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18 C l b L
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18.5 Coulombs Law
Example 4 Three Charges on a Line
Determine the magnitude and direction of the net force on q1.
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18 5 C l b L
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18.5 Coulombs Law
N7.2m20.0
C100.4C100.3CmN1099.82
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12
r
qqkF
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qq
kF
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18.5 Coulombs Law
Figure (a) shows three point charges that lie in the x, y plane in a vacuum.
Find the magnitude and direction of the net electrostatic force on q1.
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18 6 Th El t i Fi ld
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18.6 The Electr ic Field
The positive charge experiences a force which is the vector sum of the
forces exerted by the charges on the rod and the two spheres.
This test chargeshould have a small magnitude so it doesnt affectthe other charge.
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18 6 The Electr ic Field
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18.6 The Electr ic Field
Example 6 A Test Charge
The positive test charge has a magnitude of
3.0x10-8C and experiences a force of 6.0x10-8N.
(a) Find the force per coulombthat the test charge
experiences.
(b) Predict the force that a charge of +12x10-8C
would experience if it replaced the test charge.
CN0.2C100.3
N100.6
8
8
oq
F
(a)
(b) N1024C100.12CN0.2 88 F
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18 6 The Electr ic Field
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18.6 The Electr ic Field
DEFINITION OF ELECRIC FIELD
The electric field that exists at a point is the electrostatic force experienced
by a small test charge placed at that point divided by the charge itself:
oq
FE
SI Units of Electric Field: newton per coulomb (N/C)
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18 6 The Electr ic Field
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18.6 The Electr ic Field
It is the surrounding charges that create the electric field at a given point.
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18.6 The Electr ic Field
Example 7 An Electric Field Leads to a Force
The charges on the two metal spheres and the ebonite rod create an electric
field at the spot indicated. The field has a magnitude of 2.0 N/C. Determine
the force on the charges in (a) and (b)
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18.6 The Electr ic Field
N1036C100.18CN0.2 88 EqF o(a)
(b) N1048C100.24CN0.2 88 EqF o
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18 6 The Electr ic Field
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18.6 The Electr ic Field
Electric fields from different sourcesadd as vectors.
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18.6 The Electr ic Field
Example 10 The Electric Field of a Point Charge
The isolated point charge of q=+15C is
in a vacuum. The test charge is 0.20mto the right and has a charge qo=+0.80C.
Determine the electric field at point P.
oq
FE
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21
r
qqkF
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18 6 The Electr ic Field
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18.6 The Electr ic Field
N7.2m20.0 C1015C1080.0CmN1099.8 266229
2
r
qqkF o
CN104.3C100.80
N7.2 66-
oq
FE
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18.6 The Electr ic Field
2r
qkE
The electric field does not depend on the test charge.
o
o
o qrqqk
qFE 1
2
Point charge q:
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18.6 The Electr ic Field
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18.6 The Electr ic Field
Example 11 The Electric Fields from Separate Charges May Cancel
Two positive point charges, q1=+16C and q2=+4.0C are separated in a
vacuum by a distance of 3.0m. Find the spot on the line between the chargeswhere the net electric field is zero.
2r
qkE
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18.6 The Electr ic Field
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18.6 The Electr ic Field
2
6
2
6
m0.3
C100.4C1016
dk
dk
21
EE 2r
q
kE
22m0.30.4 dd
m0.2d
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18.6 The Electr ic Field
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8 6 e ect c e d
Conceptual Example 12 Symmetry and the
Electric Field
Point charges are fixed to the corners of a rectangle in twodifferent ways. The charges have the same magnitudes
but different signs.
Consider the net electric field at the center C of the rectangle
in each case. Which field is stronger?
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18.6 The Electr ic Field
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THE PARALLEL PLATE CAPACITOR
Parallel plate
capacitorooA
qE
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charge density
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18.7 Electric Field Lines
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Elect r ic field l ines or l ines of forc eprovide a map of the electric field
in the space surrounding electric charges.
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18.7 Electric Field Lines
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Property 2: Electric field lines always begin on a positive charge
and end on a negative charge and do not stop in
Midspace and they are ALWAYS perpendicular to the charges.
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18.7 Electric Field Lines
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Property 3: The number of lines leaving a positive charge or entering a
negative charge is proportional to the magnitude of the charge.
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18.7 Electric Field Lines
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Conceptual Example 13 Drawing Electric
Field Lines
There are three things wrong with part (a) ofthe drawing. What are they?
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18.8 The Electr ic Field Inside a Conducto r: Shielding
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At equilibrium under electrostatic conditions, any
excess charge resides on the surface of a conductor.
At equilibrium under electrostatic conditions, the
electric field is zero at any point within a conductingmaterial.
The conductor shields any charge within it fromelectric fields created outside the conductor.
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18.8 The Electr ic Field Inside a Conducto r: Shielding
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The electric field just outside the surface of a conductor is perpendicular to
the surface at equilibrium under electrostatic conditions.
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