Units of Chapter 16 Static Electricity Electric Charge and Its Conservation Electric Charge in the Atom Insulators and Conductors Induced Charge the Electroscope Coulombs Law Solving Problems Involving Coulombs Law and Vectors ID: 273318
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Slide1
Unit 14: ElectrostaticsSlide2
Units of Chapter 16
Static Electricity; Electric Charge and Its Conservation
Electric Charge in the Atom
Insulators and Conductors
Induced Charge; the Electroscope
Coulomb’s Law
Solving Problems Involving Coulomb’s Law and Vectors
The Electric FieldSlide3
Units of Chapter 16
Field Lines
Electric Fields and
ConductorsSlide4
Do Now
How do protons and electrons differ in their electrical charge?
Is an electron in a hydrogen atom the same as an electron in a uranium atom?
Which has more mass – a proton or an electron?
In a normal atom, how many electrons are there compared to protons?
Slide5
Do Now
How do protons and electrons differ in their electrical charge?
Same magnitude, but opposite charge.
Is an electron in a hydrogen atom the same as an electron in a uranium atom?Yes.
Which has more mass – a proton or an electron?
Proton
In a normal atom, how many electrons are there compared to protons? Same number, no net charge.Slide6
Atomic StructureSlide7
16.1 Static Electricity; Electric Charge and Its Conservation
Objects can be charged by rubbing Slide8
Triboelectric Series
Friction
can cause electrons to transfer from one material to anotherDifferent materials have a different degree of attraction for electrons
The
triboelectric
series determines which materials have a greater attraction.When two materials are rubbed together, the one with the higher attraction will end up getting some of the electrons from the other materialSlide9
Triboelectric Series
Human Hands (if very dry)
Leather
Rabbit Fur
Glass
Human Hair
Nylon Wool Fur Lead Silk Aluminum Paper Cotton Steel (neutral) Wood Amber
Hard Rubber
Nickel, Copper
Brass, Silver
Gold, Platinum
Polyester
Styrene (Styrofoam)
Saran Wrap
Polyurethane
Polyethylene (scotch tape)
Polypropylene Vinyl (PVC)
Silicon
Teflon
MORE POSITIVE
MORE NEGATIVE
If two materials are rubbed together, the one that is higher in the series will give up electrons and become more positive.Slide10
Question
If fur is rubbed on glass, will the glass become positively charged or negatively charged?
Glass – positive
Fur -negativeSlide11
16.1 Static Electricity; Electric Charge and Its Conservation
Charge comes in two types, positive and negative; like charges repel and opposite charges attract Slide12
Interaction Between Charged and Neutral Objects
Any charged object - whether positively charged or negatively charged - will have an attractive interaction with a neutral object.Slide13
16.1 Static Electricity; Electric Charge and Its Conservation
Electric charge is conserved – the arithmetic sum of the total charge cannot change in any interaction.Slide14
16.2 Electric Charge in the Atom
Atom:
Nucleus (small, massive, positive charge)
Electron cloud (large, very low density, negative charge)Slide15
16.2 Electric Charge in the Atom
Atom is electrically neutral.
Rubbing charges objects by moving electrons from one to the other.Slide16
16.2 Electric Charge in the Atom
Polar molecule: neutral overall, but charge not evenly distributedSlide17
16.3 Insulators and Conductors
Conductor:
Charge flows freely
Metals
Insulator:
Almost no charge flows
Most other materials
Some materials are semiconductors.Slide18Slide19
How Charge Is Transferred
Objects can be charged by rubbingMetal objects can be charged
by conduction
Metal objects can be charged byinductionSlide20
16.4 Induced Charge; the Electroscope
Metal objects can be charged by conduction:Slide21
Charging by Induction
When an object gets charged by induction, a charge is created by the influence of a charged object but
not by contact
with a charged object. The word induction means to influence without contact. If a rubber balloon is charged negatively (perhaps by rubbing it with animal fur) and brought near (without touching) the spheres, electrons within the two-sphere system will be induced to move away from the balloon.Slide22
Charging By InductionWith Negatively
Charged Object
In step iii, why is the charge on the right
sphere almost
uniformly distributed?Slide23
Charging By InductionWith Negatively Charged Object
What was the source of
positive charge that ended
up on sphere B?Slide24
Source of charge in induction
In induction, the source of charge that is on the final object is not the result
of movement
from the charged object to the neutral object.Slide25
Ground
An infinite source or sink for
chargeSlide26
16.4 Induced Charge; the Electroscope
They can also be charged by induction:Slide27
16.4 Induced Charge; the Electroscope
The electroscope can be used for detecting charge:Slide28
16.4 Induced Charge; the Electroscope
The electroscope can be charged either by conduction or by induction.Slide29
16.4 Induced Charge; the Electroscope
The charged electroscope can then be used to determine the sign of an unknown charge.Slide30
16.4 Induced Charge; the Electroscope
Nonconductors won’t
become charged by conduction or induction, but will experience charge
rearrangement.
The
atoms or molecules become polarized.
:Slide31
16.4 Induced Charge; the Electroscope
Nonconductors won’t become charged by conduction or induction, but will experience charge separation:Slide32
Do Now
True or False? Explain your reasoning
.
An object that is positively charged contains all protons and no electrons.False
Positively charged objects have electrons; they simply possess more protons than electrons.
2. An object that is electrically neutral contains only neutrons.
FalseElectrically neutral atoms simply possess the same number of electrons as protons.Slide33
Do Now
True or False? Explain your reasoning
.
An object that is positively charged contains all protons and no electrons.
2. An object that is electrically neutral contains only neutrons.Slide34
Coulomb’s Law
The French physicist Charles Coulomb (1736 – 1806) investigated electric forces in the 1780s using a torsion balance.Slide35
16.5 Coulomb’s Law
Experiment shows that the electric force between two charges is proportional to the product of the charges and inversely proportional to the distance between them.Slide36
Coulomb’s Law
1. If two point charges and are a distance r apart, the charges exert forces on each object of magnitude:
These forces are an action/reaction pair, equal in magnitude but opposite in direction.
Slide37
16.5 Coulomb’s Law
2. The forces are
along the line connecting the charges, and is attractive if the charges are opposite, and repulsive if they are the same.Slide38
16.5 Coulomb’s Law
Unit of charge: coulomb, C
The proportionality constant in Coulomb’s law is then:
Charges produced by rubbing are typically around a
microcoulomb
:
When we only need two significant figures:Slide39
16.5 Coulomb’s Law
Charge on the electron:
Electric charge is quantized in units of the electron charge.
This is the smallest charge in
nature –
fundamental or elementary charge.
The net charge of any object must be a
multiple
of that charge.Slide40
16.5 Coulomb’s Law
The proportionality constant
k
can also be written in terms of , the permittivity of free space:
(16-2)Slide41
16.5 Coulomb’s Law
Coulomb’s law strictly applies only to point charges.
Superposition: for multiple point charges, the forces on each charge from every other charge can be calculated and then added as vectors.Slide42
16.6 Solving Problems Involving Coulomb’s Law and Vectors
The net force on a charge is the vector sum of all the forces acting on it.Slide43
16.6 Solving Problems Involving Coulomb’s Law and Vectors
Vector addition review:Slide44
Example 1
Suppose that two point charges, each with a charge of +1.00 Coulomb are separated by a distance of 1.00 meter. Determine the magnitude of the electrical force of repulsion between them
.
Given: Find: F - ?IQI I= 1.00 C
Q
2
I= 1.00 Cr = 1.00 mSlide45
Solve:
The force of repulsion of two +1.00 Coulomb charges held 1.00 meter apart is 9 billion Newton. This is an incredibly large force that compares in magnitude to the weight of more than 2000 jetliners. Slide46
Example 2
Determine the magnitude and direction of the electric force on the electron of a hydrogen atom exerted by the single proton that is the atom’s nucleus. Assume the average distance between the revolving electron an the proton
Given: Find: F-?Slide47
Solution:Slide48
Problem 1
A balloon with a charge of is held a distance of 0.10m from a second balloon having the same charge. Calculate the magnitude of the electrical force between the charges. Draw a diagram.Slide49
Problem 2
Calculate the electrical force exerted between a 22-gram balloon with a charge of -2.6
μ
C and a wool sweater with a charge of +3.8μC; the separation distance is 75cm. (Note: ) Slide50Slide51
Neutral vs. Charged Objects
if an atom contains equal numbers of protons and electrons, the atom is described as being
electrically neutral
.if an atom has an unequal number of protons and electrons, then the atom is electrically charged and referred to as an ion.
Any particle, whether an atom, molecule or ion, that contains less electrons than protons is said to be
positively charged
.Conversely, any particle that contains more electrons than protons is said to be negatively charged.Slide52
Charged Objects as an Imbalance of Protons and Electrons
Electrons can move to the electrons’ shells of other atoms of different materials.
F
or electrons to make a move from the atoms of one material to the atoms of another material, there must be an energy source and a low-resistance pathway.rubbing your feet on carpet
clothes tumble in the dryerSlide53
Charged Objects as an Imbalance of Protons and Electrons
The principle stated earlier for atoms can be applied to objects.
Objects with more electrons than protons are charged negatively; objects with fewer electrons than protons are charged positively.Slide54
True or False?
An object that is positively charged contains all protons and no electrons.
False
Positively charged objects have electrons; they simply possess more protons than electrons.2. An object that is electrically neutral contains only neutrons.
False
Electrically neutral atoms simply possess the same number of electrons as protons.Slide55
Do Now
Suppose you rub a glass
rod with a
silk cloth and a second glass rod with rabbit fur. The silk clothwill acquire a __________ (+ , -) charge; the rabbit fur will acquire a __________ (+ , -) charge.
The
rabbit
fur and the silk cloth will then be observed to ______________________ (attract, repel, notinteract with) each other.Slide56
Three Ways to Charge an Object
Friction (by rubbing)
2.
Conduction(with contact) 3
.
Induction(without contact)
Slide57
Ground
An infinite source or sink for charge
Charge always distributes itself evenly around a conducting sphere
We can think of ground as a conductor that is so large that it can always accept more charge (or provide more charge).
SymbolSlide58
Charging by Conduction
When charging something by contact it is important to note the following properties
The objects must actually touch and transfer some electrons.
The objects become charged alike.The original charged object becomes less charged because it actually lost some charge. Slide59
The
Electroscope
The electroscope can be used for detecting charge:Slide60
Electroscope Can be Charged by Induction Slide61
Electroscope Can be Charged by Conduction
Once the
contact
of the rod to the electroscope is made, the electrons move from the electroscope to the rod. The electroscope is positively charged.Slide62
Nonconductors
Nonconductors
won’t
become charged by
conduction or induction, but will experience
charge
rearrangement. The atoms or molecules become polarized.Slide63
Do Now
Object A is rubbed with object B. Object C is rubbed withobject D. Objects A and D are observed to repel each other.
Object B is observed to repel a negatively charged balloon.
This is conclusive evidence that …… object A acquired a __________ (+ , -) charge.… object B acquired a __________ (+ , -) charge.… object C acquired a __________ (+ , -) charge.… object D acquired a __________ (+ , -) charge.Slide64
Law of Universal Gravitation Analogy
Coulomb’s Law
Law of Universal Gravitation Slide65
Questions:
Two charged objects have a repulsive force of .080 N. If the charge of one of the
objects is
doubled, then what is the new force?2. Two charged objects have a repulsive force of .080 N. If the distance separating
the objects
is doubled, then what is the new force?Slide66
Do Now
Object A is rubbed with object B.
Object
C is rubbed with object D. Objects A and D are observed to repel each other.Object B is observed to repel a negatively charged balloon.This is conclusive evidence that …
… object A acquired a __________ (+ , -) charge.
… object B acquired a __________ (+ , -) charge.
… object C acquired a __________ (+ , -) charge.… object D acquired a __________ (+ , -) charge.Slide67
Six Flags Trip
Meet in the auditorium after Pd.2.
1PM Check in at buffet in Old Country Picnic Grove.
5 PM Meet at fountain.
Cell phone.
Sunscreen.Slide68Slide69Slide70Slide71Slide72Slide73Slide74
Do Now
Find the force exerted on the test charge. Indicate the direction of that force
(Hint: Calculate individual forces on the test charge and add them as vectors.)
qA= +2
nC
q
test=-1C qB=+3nC 1m 1mSlide75
Solution
t
o the left
t
o the right
t
o the rightSlide76
Gravitational Force
Objects near surface of Earth
Gravitational force always directed towards the center of Earth.
r
ForceSlide77
Gravitational Force
Force depends on mass of object.
Gravitational
force always directed
towards
center of earth.
Even when there is no mass nearby the earth, we can still talk about a gravitational field near the earth- pointing towards earth.Slide78
Electric Field
Space around Earth and every mass is filled with gravitational field.
Space around every electric charge is filled with electric field.Slide79
Strength of Electric Field
An electric field is a vector. It has both magnitude and direction.
Its magnitude can be measured by its effect on a small positive test charge q placed in it.
The greater the force acting on the charge, the stronger the electric field.E – strength of electric field
Units – N/CSlide80
16.7 The Electric Field
The electric field is the force on a
small (point)
charge, divided by the charge:
•
If you know the direction
and magnitude of the
electric field, you can
determine the direction of
the
force.
• Negatively charged
particles will have
opposite direction of
force.Slide81
The Electric Field
General Expression for point charge:
q- test charge
Q – source chargeSlide82
Gravitational Field Analogy
Strength of electric field:
Strength of gravitational field:
F=mgSlide83
Example 1
Jack pulls his wool sweater over his head, which charges his cotton t-shirt as the sweater rubs against it.
a) What is the magnitude and direction of the electric field at a location where a -piece of lint experiences a force of as it floats near Jack?
Given:Slide84
Solution:Slide85
Example 2
Charge Q acts as a point charge to create an electric field. Its strength, measured a distance of 30 cm away, is 40 N/C. What would be the electric field strength
...
a)30 cm away from a source with charge 2Q? b)60 cm away from a source with charge
Q?
80N/c
10N/CSlide86
Example 3
What is the magnitude and direction of the electric field 0.25 meters away from a
source
charge with -5.0 μC. Draw a diagram. Slide87
16.7 The Electric Field
Force on a point charge in an electric field:
(16-5)
Superposition principle for electric fields:Slide88
16.7 The Electric Field
Problem solving in electrostatics: electric forces and electric fields
Draw a diagram; show all charges, with signs, and electric fields and forces with directions
Calculate forces using Coulomb’s law
Add forces vectorially to get resultSlide89
16.8 Field Lines
The electric field can be represented by field lines. These lines start on a positive charge and end on a negative charge.Slide90
16.8 Field Lines
The number of field lines starting (ending) on a positive (negative) charge is proportional to the magnitude of the charge.
The electric field is stronger where the field lines are closer together.Slide91
16.8 Field Lines
Electric dipole: two equal charges, opposite in sign:Slide92
16.8 Field Lines
The electric field between two closely spaced, oppositely charged parallel plates is constant.Slide93
16.8 Field Lines
Summary of field lines:
Field lines indicate the direction of the field; the field is tangent to the line.
The magnitude of the field is proportional to the density of the lines.
Field lines start on positive charges and end on negative charges; the number is proportional to the magnitude of the charge.Slide94
16.9 Electric Fields and Conductors
The static electric field inside a conductor is zero – if it were not, the charges would move.
The net charge on a conductor is on its surface.Slide95
Faraday’s Cage, Shielding
A conducting box used for shielding delicate instruments from unwanted external electric fields.Slide96
16.9 Electric Fields and Conductors
The electric field is perpendicular to the surface of a conductor – again, if it were not, charges would move.Slide97
16.12 Photocopy Machines and Computer Printers Use Electrostatics
Laser printer is similar, except a computer controls the laser intensity to form the image on the drumSlide98
Two kinds of electric charge – positive and negative
Charge is conserved
Charge on electron:
Conductors: electrons free to move
Insulators: nonconductors
Summary of Chapter 16Slide99
Summary of Chapter 16
Charge is quantized in units of
e
Objects can be charged by conduction or induction
Coulomb’s law:
Electric field is force per unit charge:Slide100
Summary of Chapter 16
Electric field of a point charge:
Electric field can be represented by electric field
lines
Static electric field inside conductor is zero; surface field is perpendicular to
surfaceSlide101
Chapter 17
Electric PotentialSlide102
Units of Chapter 17
Electric Potential Energy and Potential Difference
Relation between Electric Potential and Electric Field
Equipotential
Lines
CapacitanceSlide103
17.1 Electrostatic Potential Energy and Potential Difference
The electrostatic force is conservative – potential energy can be defined
Change in electric potential energy is negative of work done by electric force:
(17-1)Slide104
Electric PotentialSlide105
17.1 Electrostatic Potential Energy and Potential Difference
Electric potential is defined as potential energy per unit charge:
(17-2a)
Unit of electric potential: the volt (
V
).
1
V
= I
J
/
C
.Slide106
17.1 Electrostatic Potential Energy and Potential Difference
Only changes in potential can be measured, allowing free assignment of
V
= 0.
(17-2b)Slide107
Example 4
What is the potential difference between the terminals of a battery if 60.J of work are done when 3.0 C are pushed trough a wire from one terminal to the other?
Given: Find: V-?
W=60Jq= 3.0C Slide108
Solution:Slide109
17.1 Electrostatic Potential Energy and Potential Difference
Analogy between gravitational and electrical potential energy:Slide110
Electrical Potential Energy
Work is done when you move an object in the direction of the force.
When you raise an object, you move it against gravitational field, you increase its potential energy.
When you move electric charge against electric field, you increase its potential energy.Slide111
Diagram A.
Moving
a positive test charge against the direction of an electric field is like moving a mass upward within Earth's gravitational field.
Both movements would be like going against nature and would require work by an external force.
Diagram
B.
Work would not be required to move an object from a high potential energy location to a low potential energy location. Slide112
17.2 Relation between Electric Potential and Electric Field
Work is charge multiplied by potential:
Work is also force multiplied by distance:Slide113
17.2 Relation between Electric Potential and Electric Field
Solving for the field,
(17-4b)
If the field is not uniform, it can be calculated at multiple points:Slide114
17.3 Equipotential Lines
An equipotential is a line or surface over which the potential is constant.
Electric field lines are perpendicular to equipotentials.
The surface of a conductor is an equipotential.Slide115
Equipotential Lines
Equipotential lines are like contour lines on a map which trace lines of equal altitude
.
In this case the "altitude" is electric potential or voltage. Equipotential lines are always perpendicular
to the electric field.
Movement
along an equipotential line requires no work because such movement is always perpendicular to the electric field. Slide116
17.3 Equipotential LinesSlide117
Equipotential Lines
Dashed lines are equipotential lines
Solid lines are electric field linesSlide118
Summary of Chapter 17
Electric potential energy:
Electric potential difference: work done to move charge from one point to another
Relationship between potential difference and field:Slide119
Summary of Chapter 17
Equipotential: line or surface along which potential is the same