Electric Forces and Fields Chapter 20. Charges and Forces Experiment 1 Nothing happens Nothing happens The objects are neutral The objects are neutral

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


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


These rods attract each otherThese rods attract each otherPositive and negative chargesPositive and negative charges


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


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


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


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)


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


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


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


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.



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


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


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


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 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



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Electric Forces and Fields Chapter 20. Charges and Forces Experiment 1 Nothing happens Nothing happens The objects are neutral The objects are neutral