Essential Knowledge 1B2 There are only two kinds of electric charge Neutral objects or systems contain equal quantities of positive and negative charge with the exception of some fundamental particles that have no electric charge ID: 530860
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Slide1
ElectrostaticsSlide2
Essential Knowledge
1.B.2
: There are only two kinds of electric charge. Neutral objects or systems contain equal quantities of positive and negative charge, with the exception of some fundamental particles that have no electric charge.
Like-charged objects and systems repel and unlike-charged objects and systems attract.
Charged objects or systems may attract neutral systems by changing the distributions of charge in neutral systems.
1.B.3
: The smallest observed unit of charge that can be isolated is the electron charge, also known as the elementary charge.
The magnitude of the elementary charge is equal to1.6 x1 10
-19
Coulombs.
Electrons have a negative elementary charge of equal magnitude, although the mass of the proton is much larger than the mass of an electron.
3.C.2:
Electric force results from the interaction of one object that has an electric charge with another object that has an electric charge.
Electric force dominates the properties of the objects in our everyday experiences. However, the large number of particles interaction that occur make it more convenient to treat everyday forces, such as normal force, friction, and tension.
Electric force may be attractive or repulsive, depending upon the charges on the objects involved.
5.A.1
: A system is an object or collection of objects. The objects are treated as having no internal structure.Slide3
Learning Objectives
1.B.2.1
: The students is able to construct and explanation of the two charge models of electric charge based on evidence produced through scientific practices.
1.B.3.1
: The student is able to challenge the claim that an electric charge smaller than elementary charge has been isolated.
3.C.2.1
: The students is able to use Coulomb’s law qualitatively and quantitatively to make predictions about the interaction between two electric point charges (
interaction between collections of electric point charges are not covered in Physics 1 and instead are restricted to Physics 2
)
3.C.2.2
: The student is able to connect concepts of gravitational forces and electric force to compare similarities and differences between the forces. Slide4
Science Practices
1.5:
The student can re-express key elements of natural phenomena across multiple representations in the domain.
2.2:
The student can
apply mathematical
routines to quantities that describe natural phenomena.
6.1:
The student can
justify claims
with evidence.
6.2:
The students can
construct explanations
of phenomena based on evidence produced through scientific practices.
6.4:
The student can
make claims and predictions about natural phenomena
based on scientific theories and models.
7.2:
The students can
connect concepts
in and across domain(s) to generalize or extrapolate in and/or across enduring understanding and/or big ideas.Slide5
Electric Charges
1. Two kinds of charges: Positive and Negative
2. Like charges repel, unlike charges attract
3. Charge is conserved
4. Charge is quantized
LINKSlide6
The
total charge in a closed system remains constant.
Charges are transferred.
The total charge in a closed system remains constant.
Neutral objects have equal amounts of positive and negative charge.
Only
electrons
are transferred in solids.
Single charges may not be created nor destroyed.Pairs of opposite charges may be created or destroyed.
Examples:
Charge separation by friction
Chemical equations
Beta Decay Pair production / pair annihilation
Law of Conservation of ChargeSlide7
SI Unit for Charge
Coulomb [C] is the SI unit for charge.
Coulomb is a derived unit based on the fundamental unit for current, Ampere.
Coulomb is a humongous amount of charge.Slide8
Natural unit for charge
On
the atomic level, the unit of charge is
the elementary charge, e.
+
e is the charge of a proton
-e is the charge of an electron+2e is the charge of an alpha particleSlide9
Balloon Charge Tester
Item
Attract
RepelSlide10
Natural unit for charge
On the sub-atomic level, fractional charges exist
.
Quarks have
+
(1/3) e or + (2/3) eSlide11
Charge is Quantized
In 1909 Robert Millikan confirmed that electric charge always
occurs in integral multiples of the fundamental unit of charge, e.
q is the standard symbol for charge (units - Coulombs)
Money is quantized, the smallest unit of US currency is the penny!
Total Charge
Number of fundamental charges
Elementary
Charge
1.6 x 10
-19
CSlide12
Particle
Mass
Charge
electron
9.11 x 10
-31
kg
-1.6 x 10
-19
C
-1e
proton
1.672 x 10
-27
kg
+1.6 x 10
-19
C
+1e
neutron
1.674 x 10
-27
kg
0
Fundamental particle propertiesSlide13
An object has a net charge of +3 Coulombs
How many more protons than electrons are on the object?
Can you determine the total number of protons on the object?Slide14
Object
# of Excess Protons/Electrons
Quantity of Charge (Q)
in Coulombs (C)
A
1 x 10
6
excess electrons
- 1.6 x 10
-13
C
B
2 x 10
8
excess protons
+ 3.2 x 10
-11
C
C
2 x 10
10
excess electrons
- 3.2 x 10
-9
C
Ex: Find the total charge on the object in each caseSlide15
Conductors and Insulators
Good conductors have many “free” electrons
EX: Metals
Insulators have few “free” electrons
Ex: Rubber, woodSlide16
Insulators and Conductors
Electrical Conductibility
Insulators
Conductors
No movement of charges within the object
Free
movement
of charges
Semi-conductors
Limited number of free carriers
Wood, plastics, glass
Silicon, Germanium
Metals (Cu, Ag, Al)
Grounding: The “Earth” is considered an infinite sink of chargesSlide17
Ground (
Earthing
)
Grounding
: The “Earth” is considered an infinite source or sink for excess charge.
Grounding
prevents charge from building up on the chassis of appliances.Mutual grounding provides a common reference point.Slide18
Coulomb’s Law
In 1785 Charles Coulomb established a law of electric force
between two
stationary
charged particles.
Force inversely proportional to square of distance
Force along the line joining the particles
Force proportional to the product of the charges
Force attractive between opposite sign charges.
Force repulsive between charges of the same sign
k = Coulomb constant
= 8.99 x 10
9 Nm
2/C2Slide19
Direction of the Coulomb Force
Force can be attractive or repulsive
Equal in magnitude
Opposite in directionSlide20
Ex:
If q
1
=-3uC, q
2
= +4uC, and d = 2 m,
find the electric force between the charges.
+
-
q
1
q
2
d
F
12
F
21Slide21
Coulomb Force is proportional to 1/r
2
Hyperbolic relationship between force and distance
linkSlide22
Analogy to
Gravitational
Force
Coulomb Force
Gravitational Force
The gravitational force can only be attractive.
Example: Compare the gravitational force in the hydrogen atom to the Coulomb (electric) force. Which is stronger?Slide23
Atomic
Radius:
10
-10
meters
Nuclear
Diameter:
10-15 meters
Mass of electron: me = 9.11 x 10
-31 kgMass of proton:
m
p = 1.67 x 10-27 kg
Compare the gravitational force in the hydrogen atom to the Coulomb (electric) force. Which is stronger? How much stronger?Slide24
Difference and Similarities between Electricity and Gravity
Coulomb Law and Law of Gravitation similarities
Gravitation is always attractive
Electrical force can be attractive or repulsive
Electric force dominates the atomic world
Gravitational forces dominates the macroscopic scale: people, planets, galaxies
Electric forces are stronger !!!
Slide25
A metal sphere is charged by losing 5.18 x 10
13
electrons while a second sphere is charged by losing 15.54 x 10
13
electrons. The two spheres are 25 cm apart. Determine the force between the two spheres. Slide26
Two charged objects have a repulsive force of .080 N. If the charge of one is doubled, and the distance separating them is doubled what is the new force?
Two charged objects have a repulsive force of .080 N. If the charge of both of the objects is doubled and the distance separating the objects is doubled what is the new force?
Two charged objects have an attractive force of .080 N. If the charge of one of the objects is quadrupled, and the distance separating the objects is doubled what is the new force?
4.
Two charged objects have an attractive force of .080 N. If the charge of one is tripled and the distance separating the objects is tripled what is the new force?Slide27
Two uniformly charged spheres are firmly fastened to and electrically insulated from a table. The charge on sphere 2 is three times the charge on sphere 1. Which diagram correctly shows the magnitude and direction of the electrostatic forces:Slide28
Alternate Form of Coulomb’s law
Coulomb’s constant k is often written in terms of the permittivity of free space e
0
Coulomb’s Law can then be written as:Slide29
Superposition Principle
When more than two charges are present, the resultant force on any one of them is equal to the
vector
sum of the forces exerted by each of the individual charges.Slide30
Solution:
Note the direction of forces
Resolve F
32
and F
31
into their x and y components
Add the x and y components of F32 and F
31 to find x and y components of F
3Find the magnitude and direction of F
3
45
0
F
32
F
31
45
0
Three point charges located at corners of triangle as shown.
F
ind
The resultant force on
q
3
q
1
= q
3
= 5
C
q
2
= -2 C
a = .1 mSlide31
Two 2 gram balloons are suspended by strings that are 60 cm long. The two balloons establish equilibrium with an angle between the two strings of 25
0
. Determine the charge on each balloon. Assume the same amount of charge is on each. Slide32
Methods of Charging
Ouch!
Friction -
Transfer of electrons between
neutral
objects.
Induction
-
A neutral object becomes charges without ever contacting the charged object.
Conduction -
A
charged
body comes in contact with another body and charge is transferred between them. Slide33
1. Friction
-
When two neutral objects are rubbed together. One gives up its negative charges to the other. One becomes positively charged while the other becomes equally negative.
Hair gives up electrons to the balloon.Slide34
Frictional
charging is a result of
transfer
of electrons
Some materials are greedy and steal electrons, they have a high
electroconductivity
, while others are willing to give them up.Slide35
2. Induction
- When an object is charged by the influence of a charged object near, but not in contact with it. The word induction means to influence
without contact
.
Positively charged object brought near, does not touch the electroscope.
Ground’s attached and electrons are drawn up.
Ground is removed trapping electrons on the electroscope.
Electroscope ends up oppositely charged to the object brought near. Slide36
Electrons attracted by the positive object toward the top of the electroscope. The foil leaves at the bottom have a positive charge so they repel each other.
Electrons pushed by negative object toward the bottom of the electroscope. The foil leaves at the bottom have a negative charge so they repel each other.
Temporary polarization by inductionSlide37
Electrostatic Induction
occurs only in conductors.
Ground -
Is an infinite source or sink for electrons. Slide38
InductionSlide39
InductionSlide40
Polarization and InductionSlide41
A negatively charged rubber rod is brought near an uncharged sphere
The charges in the sphere are redistributed.
After the sphere is grounded they leave the sphere
The positive charge on the sphere is evenly distributed
Charging by induction requires no contact with the object inducing the charge
InductionSlide42
3. Conduction
– Charging by contact
When charging something by contact
:
A charged objects must touch and transfer some electrons.
The objects become charged
alike
.
The original charged object becomes less charged.Slide43
Conduction
A charged object (the rod) is placed in contact with another object (the sphere)
Some electrons on the rod can move to the sphere
When the rod is removed, the sphere is left with a charge
The object being charged is always left with a charge having the same sign as the object doing the chargingSlide44
Grounding neutralizes a charged objectSlide45
Polarization and charging by contactSlide46
Polarization-Induced Attractions
Attraction is more common than repulsion
Charged objects can attract uncharged ones
A charged rod attracts a neutral metal ball
It redistributes the charge
separation of charge in the uncharged object. The attractive force is then greater.
Water faucet comb demoSlide47
Conduction or Induction?Slide48
After rubbing the balloon, why does balloon stick to wall?
How do you know that this force is stronger than gravity?Slide49
Negatively charged paint adheres
to positively charged metal. Slide50
Van
def
Graff generator and charging by contactSlide51Slide52Slide53
Charge Distributions
The excess charge on a conductor resides on the outer surface concentrating on rough edges and corners.
Automobiles are a safe haven from lightening.
Lightening rods and point dischargeSlide54
Gravitational Fields
Surround anything with mass
Vector fields (have magnitude and direction)
Weaken as you move away from a single mass
Magnitude of field can be calculated by:Slide55
q
0
is the test charge
Q is the charged object in the area
E is the electric field experienced by q
0
due to Q
Electric Fields
Surround charged objects
Vector fields (magnitude and direction)
Direction depends on the charge
Weaken as you move away from isolated one charge
Magnitude of field calculated by:Slide56
Electric Field Strength
The Electric Field Strength at a point in an Electric Field is the Force per unit positive test charge exerted on a charge at that point.
E = F/q
*
Vector Quantity
*[N/C]Slide57
Field of an isolated point chargeSlide58
Coulomb Force is proportional to 1/r
2
Hyperbolic relationship between force and distanceSlide59
Calculate the force exerted by E = 4 N/C
to
the right(
→) on each of the following:
q
1
= +1 C q2 = +4 Cq3 = - 4 Cq4 = + 1.5 Cq
5 = - 1.5 Cq6 = 6 μCq
7 = p+
q8 = e-
q9 = 2e-Slide60
What is the strength of the electric field 2 cm fron a +3uC charge?
If you double the distance from the charge what will be the new electric field strength at that point?
Ans
: ¼ the original strengthSlide61
The number of lines per unit area through the surface is proportional to the magnitude of the electric field
The closer the lines the stronger the field, ‘E’.Slide62
Drawing Electric Field Lines
Lines begin on positive and end on negative charges.
No two field lines can cross.
Number of field lines leaving is proportional to the charge.
Strength of the field is proportional to the density of lines.Slide63
Electric field lines are
proportional to magnitude of the charge
Electric field is tangent at any pointSlide64
If charges are not equal in magnitude the greater charge will
have more field lines
Twice the charge, twice the field linesSlide65
Double the charge means double the field linesSlide66
Field the same strength at every point along the circleSlide67
The diagram below is a representation of the electric field
arising
from?
a single negative charge
two unlike charges
a single positive charge
a pair of positive charges
a pair of negative chargesSlide68
Must Know what the electric fields looks like around
A positive point charge
A negative point charge
A positive and negative point charge
Two positive point charges
Two negative point charges
Around and inside a conducting sphere
Between 2 parallel plates
NOTE:
Point
Charges have no dimensionsSlide69
Positive Point Charge
+
+
Field emanates from positive charge
Perpendicular to the surface
Field lines never cross
Field weakens as 1/r
2
Slide70
Field terminates on it
Field perpendicular to the surface
Field lines never cross
Field weakens as the distance increases
Negative Point ChargeSlide71
Inverse square law: 1/r
2
Double the distance the field is a ¼ the original strength.
Less field lines per unit area.
Positive Point ChargeSlide72
Out of the positive and into the negative
Strongest between the charges
Field lines are perpendicular to the surface
Field lines never cross
One Positive and one Negative Point ChargeSlide73
Field zero between the charges
Field lines diverge
Field lines never cross
Field lines perpendicular to surface
Two Positive Point ChargesSlide74
Field 0 between the charges
Field lines diverge
Field lines never cross
Field lines perpendicular to surface
Two Negative Point Charges
Same as positive charge diagram except field lines go into the chargesSlide75
Negatively Charged Conducting Sphere
E=0
_
_
_
_
_
_
_
_
Field ends on the charge
Field
ZERO
inside the sphere
All excess charge is on the surface
More charge equals more field lines
It’s NOT dimensionlessSlide76
E=0
Everywhere
inside
Field lines perpendicular to surface
Double the distance, field is ¼ original strength.
Inverse square lawSlide77
Field begins on the charge
Field
ZERO
inside the sphere
All excess charge is on the surface
More charge equals more field lines
Positively Charged Conducting SphereSlide78
Edge effects-
Electric field lines bulge out slightly around the edges of Parallel Plates – Field weakerSlide79
Magnitude - The force on a charge placed in the field divided by the charge itself.
Direction
- The direction that the force would be on a positive test charge placed at that point.
All fields are vectors:Slide80
The field lines for a large positively charged plate. The field lines flow away from the plate on both sides. (Note: this is a small section near the center of a large plate. This is why the field lines are not coming from the outside rim of the plate.)Slide81
A uniform electric field is created between two parallel
metal plates if the plates are connected to a battery.
The way the terminals are connected
determines the direction of the fieldSlide82
Field around and between charged parallel plates
Field comes out of the positive plate goes into the negative plate
Field is
UNIFORM,
same strength everywhere
Field above and below the plates is zero.Slide83
+ + + + + + + + + + + + + + + + + +
- - - - - - - - - - - - - - - - - - - - - - - - -
Positive plate
Negative
Plate
E
=const
. Uniform field
Parallel Plate Capacitor
Everywhere outside the plates the field is zeroSlide84
A charged particle introduced perpendicular to the electric field will follow a parabolic path
Like a projectile in a gravitational field