chapter 19 electrostatics electrostatics a bit of history ancient greeks ancient greeks observed...
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Chapter 19Chapter 19
ElectrostaticsElectrostatics
A Bit of HistoryA Bit of History
Ancient GreeksAncient Greeks• Observed electric and magnetic Observed electric and magnetic
phenomena as early as 700 BCphenomena as early as 700 BC Found that amber, when rubbed, became Found that amber, when rubbed, became
electrified and attracted pieces of straw or electrified and attracted pieces of straw or feathersfeathers
Magnetic forces were discovered by Magnetic forces were discovered by observing magnetite attracting ironobserving magnetite attracting iron
A Bit More HistoryA Bit More History
William GilbertWilliam Gilbert• 16001600• Found that electrification was not Found that electrification was not
limited to amberlimited to amber Charles CoulombCharles Coulomb
• 17851785• Confirmed the inverse square Confirmed the inverse square
relationship of electrical forcesrelationship of electrical forces
More Properties of ChargeMore Properties of Charge
Nature’s basic carrier of negative Nature’s basic carrier of negative charge is the electroncharge is the electron• Gaining or losing electrons is how an Gaining or losing electrons is how an
object becomes chargedobject becomes charged Electric charge is always conservedElectric charge is always conserved
• Charge is not created, only exchangedCharge is not created, only exchanged• Objects become charged because Objects become charged because
negative charge is transferred from one negative charge is transferred from one object to anotherobject to another
Properties of Charge, finalProperties of Charge, final
Charge is quantizedCharge is quantized• All charge is a multiple of a fundamental All charge is a multiple of a fundamental
unit of charge, symbolized by eunit of charge, symbolized by e Quarks are the exceptionQuarks are the exception
• Electrons have a charge of –eElectrons have a charge of –e• Protons have a charge of +eProtons have a charge of +e• The SI unit of charge is the Coulomb (C)The SI unit of charge is the Coulomb (C)
e = 1.6 x 10e = 1.6 x 10-19-19 C C
ConductorsConductors
Conductors are materials in which Conductors are materials in which the electric charges move freelythe electric charges move freely• Copper, aluminum and silver are good Copper, aluminum and silver are good
conductorsconductors• When a conductor is charged in a small When a conductor is charged in a small
region, the charge readily distributes region, the charge readily distributes itself over the entire surface of the itself over the entire surface of the materialmaterial
InsulatorsInsulators
Insulators are materials in which Insulators are materials in which electric charges do not move freelyelectric charges do not move freely• Glass and rubber are examples of Glass and rubber are examples of
insulatorsinsulators• When insulators are charged by rubbing, When insulators are charged by rubbing,
only the rubbed area becomes chargedonly the rubbed area becomes charged There is no tendency for the charge to move There is no tendency for the charge to move
into other regions of the materialinto other regions of the material
SemiconductorsSemiconductors
The characteristics of The characteristics of semiconductors are between those of semiconductors are between those of insulators and conductorsinsulators and conductors
Silicon and germanium are examples Silicon and germanium are examples of semiconductorsof semiconductors
Charging by ConductionCharging by Conduction A charged object (the rod) A charged object (the rod)
is placed in contact with is placed in contact with another object (the another object (the sphere)sphere)
Some electrons on the rod Some electrons on the rod can move to the spherecan move to the sphere
When the rod is removed, When the rod is removed, the sphere is left with a the sphere is left with a chargecharge
The object being charged The object being charged is always left with a charge is always left with a charge having the same sign as having the same sign as the object doing the the object doing the chargingcharging
Charging by InductionCharging by Induction When an object is When an object is
connected to a conducting connected to a conducting wire or pipe buried in the wire or pipe buried in the earth, it is said to be earth, it is said to be groundedgrounded
A negatively charged A negatively charged rubber rod is brought near rubber rod is brought near an uncharged spherean uncharged sphere
The charges in the sphere The charges in the sphere are redistributedare redistributed• Some of the electrons in Some of the electrons in
the sphere are repelled the sphere are repelled from the electrons in the from the electrons in the rodrod
Charging by Induction, contCharging by Induction, cont
The region of the sphere nearest the The region of the sphere nearest the negatively charged rod has an negatively charged rod has an excess of positive charge because of excess of positive charge because of the migration of electrons away from the migration of electrons away from this locationthis location
A grounded conducting wire is A grounded conducting wire is connected to the sphereconnected to the sphere• Allows some of the electrons to move Allows some of the electrons to move
from the sphere to the groundfrom the sphere to the ground
Charging by Induction, finalCharging by Induction, final
The wire to ground is removed, the The wire to ground is removed, the sphere is left with an excess of induced sphere is left with an excess of induced positive chargepositive charge
The positive charge on the sphere is The positive charge on the sphere is evenly distributed due to the repulsion evenly distributed due to the repulsion between the positive chargesbetween the positive charges
Charging by induction requires no Charging by induction requires no contact with the object inducing the contact with the object inducing the chargecharge
QUICK QUIZ 19.1
If a suspended object A is attracted to object B, which is charged, we can conclude
that (a) object A is uncharged, (b) object A is
charged, (c) object B is positively charged, or (d)
object A may be either charged or uncharged.
QUICK QUIZ 19.1 ANSWER
(d). Object A could possess a net charge whose sign is opposite that of the excess charge on B. If object A is neutral, B would also attract it by creating an induced charge on
the surface of A.
Coulomb’s LawCoulomb’s Law
Coulomb shows that an electrical force Coulomb shows that an electrical force has the following properties:has the following properties:• It is inversely proportional to the square of It is inversely proportional to the square of
the separation between the two particles the separation between the two particles and is along the line joining themand is along the line joining them
• It is proportional to the product of the It is proportional to the product of the magnitudes of the charges qmagnitudes of the charges q11 and q and q22 on the on the two particlestwo particles
• It is attractive if the charges are of opposite It is attractive if the charges are of opposite signs and repulsive if the charges have the signs and repulsive if the charges have the same signssame signs
Coulomb’s Law, cont.Coulomb’s Law, cont.
Mathematically,Mathematically,
kkee is called the is called the Coulomb ConstantCoulomb Constant
• kkee = 8.99 x 10 = 8.99 x 1099 N m N m22/C/C22
Typical charges can be in the µC rangeTypical charges can be in the µC range• Remember, Coulombs must be used in the Remember, Coulombs must be used in the
equationequation Remember that force is a Remember that force is a vector vector quantityquantity
2
21e r
qqkF
Coulomb’s LawCoulomb’s Law
Coulomb’s lawCoulomb’s law
2r
QqkFE
q Q
More than two charges? SUPERPOSE them!!!
ExampleExample
Determine the Coulomb force a -3Determine the Coulomb force a -3C C charge exerts on a -6 charge exerts on a -6 C charge C charge when they are 2 cm apart.when they are 2 cm apart.
+ ++3 C +10 µC
Which angle is bigger, or ?Why?
q
-q
Electrical FieldElectrical Field
Maxwell developed an approach to Maxwell developed an approach to discussing fieldsdiscussing fields
An An electric fieldelectric field is said to exist in the is said to exist in the region of space around a charged region of space around a charged objectobject• When another charged object enters this When another charged object enters this
electric field, the field exerts a electric field, the field exerts a forceforce on on the second charged objectthe second charged object
Electric Field, EElectric Field, E
Force on a positive unit charge (N/C).Force on a positive unit charge (N/C).Therefore, E-field superposable!!!Therefore, E-field superposable!!!
If there is E-field, there is force!!!If there is E-field, there is force!!!• ((++)-charge feels force // E-field)-charge feels force // E-field• (-)-charge feels opposite to E-field.(-)-charge feels opposite to E-field.
qE F E
This does not mean the positive (negative) charge will follow the E-field line.
Electric Field of Point ChargeElectric Field of Point Charge
Electric field at distance r from a Electric field at distance r from a point charge Q ispoint charge Q is
Electric field from many charges: Electric field from many charges: superposition (vector sum)superposition (vector sum)
2r
QkE
...321 EEEE
Direction of Electric FieldDirection of Electric Field
The electric field The electric field produced by a produced by a positive charge is positive charge is directed away directed away from the chargefrom the charge• A positive unit A positive unit
charge would be charge would be repelled from the repelled from the positive source positive source chargecharge
Direction of Electric Field, ContDirection of Electric Field, Cont
The electric field The electric field produced by a produced by a negative charge negative charge is directed toward is directed toward the chargethe charge• A positive unit A positive unit
charge would be charge would be attracted to the attracted to the negative source negative source chargecharge
Electric Field LinesElectric Field Lines
A convenient aid for visualizing A convenient aid for visualizing electric field patterns is to draw lines electric field patterns is to draw lines pointing in the direction of the field pointing in the direction of the field vector at any pointvector at any point
These are called These are called electric field lineselectric field lines and were introduced by Michael and were introduced by Michael FaradayFaraday
Electric Field Lines, cont.Electric Field Lines, cont.
The field lines are related to the field The field lines are related to the field byby• The electric field vector, E, is tangent to The electric field vector, E, is tangent to
the electric field lines at each pointthe electric field lines at each point• The number of lines per unit area The number of lines per unit area
through a surface perpendicular to the through a surface perpendicular to the lines is proportional to the strength of lines is proportional to the strength of the electric field in a given regionthe electric field in a given region
Electric Field Line PatternsElectric Field Line Patterns
Point chargePoint charge The lines radiate The lines radiate
equally in all equally in all directionsdirections
For a positive For a positive source charge, the source charge, the lines will radiate lines will radiate outwardoutward
Electric Field Line PatternsElectric Field Line Patterns
For a negative For a negative source charge, the source charge, the lines will point lines will point inwardinward
Electric Field Line PatternsElectric Field Line Patterns
An electric An electric dipoledipole consists of two consists of two equal and opposite equal and opposite chargescharges
The high density of The high density of lines between the lines between the charges indicates charges indicates the strong electric the strong electric field in this regionfield in this region
Electric Field Line PatternsElectric Field Line Patterns Two equal but like point Two equal but like point
charges charges At a great distance from At a great distance from
the charges, the field the charges, the field would be approximately would be approximately that of a single charge of that of a single charge of 2q2q
The bulging out of the field The bulging out of the field lines between the charges lines between the charges indicates the repulsion indicates the repulsion between the chargesbetween the charges
The low field lines between The low field lines between the charges indicates a the charges indicates a weak field in this regionweak field in this region
Electric Field PatternsElectric Field Patterns
Unequal and unlike Unequal and unlike chargescharges
Note that two lines Note that two lines leave the +2q leave the +2q charge for each charge for each line that line that terminates on -qterminates on -q
Conductors in Electrostatic Conductors in Electrostatic ConditionsConditions
When no net motion of charge occurs within a When no net motion of charge occurs within a conductor, the conductor is said to be in conductor, the conductor is said to be in electrostatic conditionselectrostatic conditions
An isolated conductor has the following An isolated conductor has the following properties:properties:• The electric field is zero everywhere inside the The electric field is zero everywhere inside the
conducting materialconducting material• Any excess charge on an isolated conductor resides Any excess charge on an isolated conductor resides
entirely on its surfaceentirely on its surface• The electric field just outside a charged conductor is The electric field just outside a charged conductor is
perpendicular to the conductor’s surfaceperpendicular to the conductor’s surface
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