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Electric Forces and FieldsElectric Forces and Fields
Chapter 20Chapter 20
Charges and ForcesCharges and ForcesExperiment 1Experiment 1
Nothing happensNothing happensThe objects are neutralThe objects are neutral
Charges and ForcesCharges and ForcesExperiment 2Experiment 2
The two rods repel each otherThe two rods repel each otherA rubbed rod is chargedA rubbed rod is chargedLong range repulsive forceLong range repulsive force
Charges and ForcesCharges and ForcesExperiment 3Experiment 3
These rods attract each otherThese rods attract each otherPositive and negative chargesPositive and negative charges
Charges and ForcesCharges and ForcesExperiment 4Experiment 4
When two rods are rubbed more When two rods are rubbed more vigorously the strength of the forces is vigorously the strength of the forces is greatergreater
The strength of the The strength of the
charges decreasescharges decreases
as the distance as the distance
between the rodsbetween the rods
increasesincreases
Charges and ForcesCharges and ForcesExperiment 5Experiment 5
Rub a plastic rod with woolRub a plastic rod with woolThe rod is weakly attracted to the woolThe rod is weakly attracted to the woolThe rod is repelled by a piece of silk that The rod is repelled by a piece of silk that
has been used to has been used to
rub glassrub glass
Charges and ForcesCharges and ForcesExperiment 6Experiment 6
Charged plastic rod held over paperCharged plastic rod held over paperThe neutral paper is attracted to the rodThe neutral paper is attracted to the rodA charged glass rod attracts the paperA charged glass rod attracts the paperA neutral rod has no effect on the paperA neutral rod has no effect on the paper
Charge SeparationCharge Separation
A charged object can induce a charge A charged object can induce a charge separation on an uncharged object that separation on an uncharged object that can lead to an attractive force. can lead to an attractive force. http://phet.colorado.edu/simulations/sims.php?sim=Balloons_and_Static_Electricity
Insulators and ConductorsInsulators and ConductorsHow does charge move on different How does charge move on different
materials?materials?Charge a plastic rod by rubbing it with Charge a plastic rod by rubbing it with
woolwoolTouch a neutral metal sphereTouch a neutral metal sphereThe sphere acquires the charge of the rodThe sphere acquires the charge of the rod
Insulators and ConductorsInsulators and ConductorsHow does charge move on different How does charge move on different
materials?materials?The metal sphere that is touched by the The metal sphere that is touched by the
charged plastic rod will pick up small charged plastic rod will pick up small pieces of paper (it is charged)pieces of paper (it is charged)
The other sphere will not (it remains The other sphere will not (it remains neutral)neutral)
Insulators and ConductorsInsulators and ConductorsHow does charge move on different How does charge move on different
materials?materials?Touch one sphere with a charged plastic Touch one sphere with a charged plastic
rodrodBoth spheres will attract small bits of Both spheres will attract small bits of
paper (they are charged)paper (they are charged)
Transfer of ChargeTransfer of Charge
Charge can be transferred from one object Charge can be transferred from one object to another when the objects touch.to another when the objects touch.
Removing charge from an object is called Removing charge from an object is called dischargingdischarging
Charge is conserved (it can’t be created or Charge is conserved (it can’t be created or destroyeddestroyed
Transfer of chargeTransfer of charge
Conductors – material through or along Conductors – material through or along which charge easily moveswhich charge easily movesMetalMetal
Insulators – materials on or in which Insulators – materials on or in which charges remain immobilecharges remain immobileGlass and plasticGlass and plasticBoth insulators and conductors can be Both insulators and conductors can be
charged. They differ in the mobility of the charged. They differ in the mobility of the chargecharge
Charge diagramCharge diagram
An insulating rod is charged by rubbingAn insulating rod is charged by rubbingCharges on the insulator rod don’t moveCharges on the insulator rod don’t move
Charge diagramCharge diagram
Charges in a conductor are free to moveCharges in a conductor are free to move
Electrostatic EquilibriumElectrostatic Equilibrium
The charges on an isolated conductor are The charges on an isolated conductor are in static equilibrium other than the brief in static equilibrium other than the brief interval when the charges are adjustinginterval when the charges are adjusting
The charges are at restThe charges are at restNo net force on any chargeNo net force on any charge
ElectroscopeElectroscopeCharge polarizationCharge polarization
The charges move around butThe charges move around butCharge is conserved Charge is conserved Slight separation ofSlight separation of
the positive and the positive and
negative charge in anegative charge in a
neutral objectneutral object
Polarization ForcePolarization Force
The polarization force arises because the The polarization force arises because the charges in the metal are separated NOT charges in the metal are separated NOT because the rod and metal are oppositely because the rod and metal are oppositely chargescharges
Fundamental ChargeFundamental Charge
Charge is represented by the symbol qCharge is represented by the symbol qThe SI unit of charge is the coulomb (C)The SI unit of charge is the coulomb (C)Protons and electrons have the same Protons and electrons have the same
amount of charge but opposite signsamount of charge but opposite signsThe fundamental or elementary charge (e) The fundamental or elementary charge (e)
is the magnitude of the charge of a proton is the magnitude of the charge of a proton or electronor electron
e = 1.60 X 10e = 1.60 X 10-19 -19 CC
Fundamental ChargeFundamental Charge
Charge is conservedCharge is conservedThe total amount of charge remains The total amount of charge remains
constantconstant
Coulomb’s LawCoulomb’s Law
Electric force increases for object’s with Electric force increases for object’s with more charge and decreases as charged more charge and decreases as charged objects are moved farther apart. objects are moved farther apart.
Coulomb’s Law EquationCoulomb’s Law Equation
K is the electrostatic constant K is the electrostatic constant K = 8.99 x10K = 8.99 x109 9 N mN m22/C/C22
C are units of electric charge called coulombsC are units of electric charge called coulombs r is the distance the charges are apartr is the distance the charges are apart
221
1221 r
qqKFF onon
Direction of forcesDirection of forces
The forces are directed along the line The forces are directed along the line joining the two particlesjoining the two particles
The forces are repulsive for two like The forces are repulsive for two like chargescharges
The forces are attractive for two opposite The forces are attractive for two opposite chargescharges
These forces are an action/reaction pair These forces are an action/reaction pair equal in magnitude but opposite in equal in magnitude but opposite in direction.direction.
Forces are VectorsForces are Vectors
Electric forces are represented by vectorsElectric forces are represented by vectorsLike other forces they can be Like other forces they can be
superimposedsuperimposed If multiple charges are acting on charge j, If multiple charges are acting on charge j,
the net electric force on charge j is the the net electric force on charge j is the sum of all the individual forces due to each sum of all the individual forces due to each charge.charge.
FFnetnet = F = F1onj1onj + F + F 2on j2on j + F + F3onj3onj +… +…
Adding Electric Forces in 1 Adding Electric Forces in 1 dimensiondimension
Example 20.1Example 20.1 A. Two +10nCcharged particles are 2.0 cm apart A. Two +10nCcharged particles are 2.0 cm apart
on the x-axis. What is the net force on a on the x-axis. What is the net force on a +1.0 nC charge midway between them? +1.0 nC charge midway between them?
FFnetnet =F =F1on31on3 + F + F2on32on3 FFnetnet = 0 = 0 qq11 and q and q22 exert repulsive forces on q exert repulsive forces on q33. Equal in . Equal in
magnitude, opposite in directionmagnitude, opposite in direction
Adding Electric Forces in 1 Adding Electric Forces in 1 dimensiondimension
20.1 b. What is the net force if the 20.1 b. What is the net force if the charged particle on the right is replaced by charged particle on the right is replaced by a -10 nC charge?a -10 nC charge?
Forces are equal in magnitude and Forces are equal in magnitude and direction direction
FFnetnet = 2F = 2F1on31on3
Adding Electric Forces in 1 Adding Electric Forces in 1 dimensiondimension
20.1 B. The 20.1 B. The magnitude is given by magnitude is given by Coulomb’s LawCoulomb’s Law
FF1on31on3= 9.0 x 10= 9.0 x 10-4-4 N N FFnet net = 2(F= 2(F1on31on3) = ) = 1.8 X 101.8 X 10-3-3 N to the right N to the right
221
31 r
qqKF on
2
99229
31010.0
100.11010/100.9
m
CxCxCNmxF on
2
99229
31010.0
100.11010/100.9
m
CxCxCNmxF on
Adding Electric forces in 2 Adding Electric forces in 2 dimensiondimension
Example 20.2 Example 20.2 Three charges particle with qThree charges particle with q11 = -50nC, q = -50nC, q22
= +50nC, and q= +50nC, and q33 = +30nC are placed as = +30nC are placed as
shown. What is the net force on charge qshown. What is the net force on charge q33
due to the other 2 chargesdue to the other 2 charges
Adding Electric forces in 2 Adding Electric forces in 2 dimensiondimension
Define a coordinate system with qDefine a coordinate system with q33 at the at the
origin. origin. Draw the forces on the charge qDraw the forces on the charge q33 with with
direction determined by the signs of the direction determined by the signs of the charges.charges.
Adding Electric forces in 2 Adding Electric forces in 2 dimensiondimension
Adding Electric forces in 2 Adding Electric forces in 2 dimensiondimension
Determine the net Determine the net force on charge q3 force on charge q3 using Coulomb’s law:using Coulomb’s law:
=2.7 x 10=2.7 x 10-3-3 N N
221
31 r
qqKF on
2
99229
310707.0
10301050/100.9
m
CxCxCNmxF on
Adding Electric forces in 2 Adding Electric forces in 2 dimensiondimension
Determine the net force Determine the net force on charge qon charge q33 using using
Coulomb’s law:Coulomb’s law: Magnitude and distance Magnitude and distance
are the same for Fare the same for F2on32on3
=2.7 x 10=2.7 x 10-3-3 N N
221
32 r
qqKF on
2
99229
320707.0
10301050/100.9
m
CxCxCNmxF on
Adding Electric forces in 2 Adding Electric forces in 2 dimensiondimension
Compute values for the componentsCompute values for the componentsusing trig.using trig.
(F(F1on31on3))x x =-1.9x10=-1.9x10-3-3 N N
(F(F1on31on3))y y =-1.9x10=-1.9x10-3-3 N N
(F(F2on32on3))x x =-1.9x10=-1.9x10-3-3 N N
(F(F2on32on3))y y =-1.9x10=-1.9x10-3-3 NN
Adding Electric forces in 2 Adding Electric forces in 2 dimensiondimension
Next add the components of the net forceNext add the components of the net forceFF3x3x =F =F(1on3)x(1on3)x + F + F(2on(2on 3)x3)x = = -1.9 X 10-1.9 X 10-3-3 N -1.9 x 10 N -1.9 x 10-3-3 N = 3.8 x10 N = 3.8 x10-3-3NNFF3y3y = F = F(1on3)y(1on3)y + F + F(2on3)y(2on3)y = =+1.9 X 10+1.9 X 10-3-3 N -1.9 x 10 N -1.9 x 10-3-3 N = 0 N = 0Net force is F3 = Net force is F3 =
3.8 x103.8 x10-3-3N, -x directionN, -x direction
The Electric FieldThe Electric Field
Grass seed in a pan of oilGrass seed in a pan of oil When charged spheresWhen charged spheres (+ and -) touch the surface,(+ and -) touch the surface, the grass seeds line up the grass seeds line up in a regular pattern.in a regular pattern.The pattern suggests that some kind of electric The pattern suggests that some kind of electric
influence fills the space around the charges.influence fills the space around the charges.This alteration of space could be the mechanism This alteration of space could be the mechanism
by which the long range Coulomb’s Law force is by which the long range Coulomb’s Law force is exerted.exerted.
The Field ModelThe Field Model
The alteration of space around charge is The alteration of space around charge is the agent that exerts a force on charge Bthe agent that exerts a force on charge B
This alteration of space is called a fieldThis alteration of space is called a fieldThe charges make an alteration The charges make an alteration
everywhere in spaceeverywhere in spaceOther charges then respond to those Other charges then respond to those
alteration at their positionalteration at their position
The Field ModelThe Field Model
The Field ModelThe Field Model
The field model applies to many branches The field model applies to many branches of scienceof science
Electric field-alteration of space around a Electric field-alteration of space around a chargecharge
Gravitational Field-alteration of space Gravitational Field-alteration of space around a massaround a mass
Magnetic Field-alteration of space around Magnetic Field-alteration of space around a magneta magnet
Field ModelField ModelSource Charges alter the space around Source Charges alter the space around
them by creating an electric field E.them by creating an electric field E.A separate charge in the electric field then A separate charge in the electric field then
experiences a force F exerted by the fieldexperiences a force F exerted by the field
Electric Field DiagramElectric Field Diagram
Electric Field DiagramElectric Field Diagram
Electric Field DiagramElectric Field Diagram
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