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Chapter 18. Electric Forces and Electric Fields. 18.1 The Origin of Electricity. The electrical nature of matter is inherent in atomic structure. coulombs. 18.1 The Origin of Electricity. In nature, atoms are normally found with equal numbers of protons - PowerPoint PPT Presentation
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Chapter 18 Electric Forces and Electric Fields
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Page 1: Chapter 18

Chapter 18

Electric Forces and Electric Fields

Page 2: Chapter 18

18.1 The Origin of Electricity

The electrical nature of matter is inherentin atomic structure.

kg10673.1 27pm

kg10675.1 27nm

kg1011.9 31em

C1060.1 19e

coulombs

Page 3: Chapter 18

18.1 The Origin of Electricity

In nature, atoms are normallyfound with equal numbers of protonsand electrons, so they are electricallyneutral.

By adding or removing electronsfrom matter it will acquire a netelectric charge with magnitude equalto e times the number of electronsadded or removed, N.

Neq

Page 4: Chapter 18

18.1 The Origin of Electricity

Example 1 A Lot of Electrons

How many electrons are there in one coulomb of negative charge?

Neq

1819-

1025.6C101.60

C 00.1

e

qN

Page 5: Chapter 18

18.2 Charged Objects and the Electric Force

It is possible to transfer electric charge from one object to another.

The body that loses electrons has an excess of positive charge, whilethe body that gains electrons has an excess of negative charge.

Page 6: Chapter 18

18.2 Charged Objects and the Electric Force

LAW OF CONSERVATION OF ELECTRIC CHARGE

During any process, the net electric charge of an isolated system remainsconstant (is conserved).

Page 7: Chapter 18

18.2 Charged Objects and the Electric Force

Like charges repel and unlike charges attract each other.

Page 8: Chapter 18

18.2 Charged Objects and the Electric Force

Page 9: Chapter 18

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 electricalconductors.

Materials that conduct electric charge poorly are called electricalinsulators.

Page 10: Chapter 18

18.4 Charging by Contact and by Induction

Charging by contact.

Page 11: Chapter 18

18.4 Charging by Contact and by Induction

Charging by induction.

Page 12: Chapter 18

18.4 Charging by Contact and by Induction

The negatively charged rod induces a slight positive surface chargeon the plastic.

Page 13: Chapter 18

18.5 Coulomb’s Law

Page 14: Chapter 18

18.5 Coulomb’s Law

COULOMB’S LAW

The magnitude of the electrostatic force exerted by one point chargeon another point charge is directly proportional to the magnitude of the charges and inversely proportional to the square of the distance betweenthem.

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r

qqkF

229 CmN1099.841 ok

2212 mNC1085.8

Page 15: Chapter 18

18.5 Coulomb’s Law

Example 3 A Model of the Hydrogen Atom

In the Bohr model of the hydrogen atom, the electron is in orbit about thenuclear proton at a radius of 5.29x10-11m. Determine the speed of the electron, assuming the orbit to be circular.

2

21

r

qqkF

Page 16: Chapter 18

18.5 Coulomb’s Law

N1022.8

m1029.5

C1060.1CmN1099.8 8211

219229

2

21

r

qqkF

rmvmaF c2

sm1018.2

kg109.11

m1029.5N1022.8 631-

118

mFrv

Page 17: Chapter 18

18.5 Coulomb’s Law

Example 4 Three Charges on a Line

Determine the magnitude and direction of the net force on q1.

Page 18: Chapter 18

18.5 Coulomb’s Law

N7.2m20.0

C100.4C100.3CmN1099.82

66229

221

12

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qqkF

N4.8m15.0

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66229

231

13

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5.7NN4.8N7.21312 FFF

Page 19: Chapter 18

18.5 Coulomb’s Law

Page 20: Chapter 18

18.6 The Electric 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 charge should have a small magnitude so it doesn’t affect the other charge.

Page 21: Chapter 18

18.6 The Electric 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 coulomb that the test chargeexperiences.

(b) Predict the force that a charge of +12x10-8Cwould experience if it replaced the test charge.

CN0.2C100.3

N100.68

8

oq

F(a)

(b) N1024C100.12CN0.2 88 F

Page 22: Chapter 18

18.6 The Electric Field

DEFINITION OF ELECRIC FIELD

The electric field that exists at a point is the electrostatic force experiencedby 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)

Page 23: Chapter 18

18.6 The Electric Field

It is the surrounding charges that create the electric field at a given point.

Page 24: Chapter 18

18.6 The Electric Field

Example 7 An Electric Field Leads to a Force

The charges on the two metal spheres and the ebonite rod create an electricfield at the spot indicated. The field has a magnitude of 2.0 N/C. Determinethe force on the charges in (a) and (b)

Page 25: Chapter 18

18.6 The Electric Field

N1036C100.18CN0.2 88 EqF o(a)

(b) N1048C100.24CN0.2 88 EqF o

Page 26: Chapter 18

18.6 The Electric Field

Electric fields from different sourcesadd as vectors.

Page 27: Chapter 18

18.6 The Electric Field

Example 10 The Electric Field of a Point Charge

The isolated point charge of q=+15μC isin a vacuum. The test charge is 0.20m to the right and has a charge qo=+0.80μC.

Determine the electric field at point P.

oq

FE

2

21

r

qqkF

Page 28: Chapter 18

18.6 The Electric Field

N7.2m20.0

C1015C1080.0CmN1099.82

66229

2

r

qqkF o

CN104.3C100.80

N 7.2 66-

oq

FE

Page 29: Chapter 18

18.6 The Electric Field

2r

qkE

The electric field does not depend on the test charge.

o

o

o qr

qqk

q

FE

12

Point charge q:

Page 30: Chapter 18

18.6 The Electric Field

Example 11 The Electric Fields from Separate Charges May Cancel

Two positive point charges, q1=+16μC and q2=+4.0μC are separated in avacuum by a distance of 3.0m. Find the spot on the line between the chargeswhere the net electric field is zero.

2r

qkE

Page 31: Chapter 18

18.6 The Electric Field

2

6

2

6

m0.3

C100.4C1016

dk

dk

21 EE 2r

qkE

22m0.30.4 dd

m 0.2d

Page 32: Chapter 18

18.6 The Electric Field

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 magnitudesbut different signs.

Consider the net electric field at the center of the rectanglein each case. Which field is stronger?

Page 33: Chapter 18

18.6 The Electric Field

THE PARALLEL PLATE CAPACITOR

Parallel platecapacitor

ooA

qE

2212 mNC1085.8

charge density

Page 34: Chapter 18

18.7 Electric Field Lines

Electric field lines or lines of force provide a map of the electric fieldin the space surrounding electric charges.

Page 35: Chapter 18

18.7 Electric Field Lines

Electric field lines are always directed away from positive charges andtoward negative charges.

Page 36: Chapter 18

18.7 Electric Field Lines

Electric field lines always begin on a positive chargeand end on a negative charge and do not stop in midspace.

Page 37: Chapter 18

18.7 Electric Field Lines

The number of lines leaving a positive charge or entering a negative charge is proportional to the magnitude of the charge.

Page 38: Chapter 18

18.7 Electric Field Lines

Page 39: Chapter 18

18.7 Electric Field Lines

Conceptual Example 13 Drawing ElectricField Lines

There are three things wrong with part (a) of the drawing. What are they?

Page 40: Chapter 18

18.8 The Electric Field Inside a Conductor: Shielding

At equilibrium under electrostatic conditions, any excess charge resides on the surface of a conductor.

At equilibrium under electrostatic conditions, theelectric field is zero at any point within a conductingmaterial.

The conductor shields any charge within it from electric fields created outside the conductor.

Page 41: Chapter 18

18.8 The Electric Field Inside a Conductor: Shielding

The electric field just outside the surface of a conductor is perpendicular to the surface at equilibrium under electrostatic conditions.

Page 42: Chapter 18

18.8 The Electric Field Inside a Conductor: Shielding

Conceptual Example 14 A Conductor in an Electric Field

A charge is suspended at the center ofa hollow, electrically neutral, spherical conductor. Show that this charge induces

(a) a charge of –q on the interior surface and

(b) a charge of +q on the exterior surface of the conductor.

Page 43: Chapter 18

18.9 Gauss’ Law

22 4 rqrkqE o

oAqE

o

qEA

EAE flux, Electric

Page 44: Chapter 18

18.9 Gauss’ Law

AEE cos

Page 45: Chapter 18

18.9 Gauss’ Law

GAUSS’ LAW

The electric flux through a Gaussiansurface is equal to the net charge enclosed in that surface divided bythe permittivity of free space:

o

QAE

cos

SI Units of Electric Flux: N·m2/C

Page 46: Chapter 18

18.9 Gauss’ Law

Example 15 The Electric Field of a Charged Thin Spherical Shell

A positive charge is spread uniformly over the shell. Find the magnitude of the electric field at any point (a) outside the shell and (b) inside theshell.

o

QAE

cos

Page 47: Chapter 18

18.9 Gauss’ Law

24

0coscos

rEAE

AEAEE

o

QrE

24

Page 48: Chapter 18

18.9 Gauss’ Law

o

QrE

24

(a) Outside the shell, the Gaussian surface encloses all of the charge.

or

qE

24

(b) Inside the shell, the Gaussian surface encloses no charge.

0E

Page 49: Chapter 18

18.9 Gauss’ Law

Page 50: Chapter 18

18.10 Copiers and Computer Printers

Page 51: Chapter 18

18.10 Copiers and Computer Printers

Page 52: Chapter 18

18.10 Copiers and Computer Printers


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