Transcript Statics PPT

Electrostatics
1
What is this substance?
Thales of Miletos (600 BC)
discovered that if he rubbed fur
on amber, the amber would
attract feathers. This was referred
to as the “Amber Effect”
3
History
 The
word
electricity comes
from the Greek
elektron which
means “amber”.
 The “amber effect”
is what we call
static electricity.
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ELECTROSTATICS
the study of electric charges, forces and
fields
 Static Electricity is “Stationary Electricity”
or Accumulation of charge

History
Two types of charges
exists, arbitrarily named
POSITIVE and NEGATIVE
By Benjamin Franklin
So what is positive and what is negative?
We know that charged particles
exist in atoms
Electrons are responsible for
negative charge
and Protons for positive charge
Benjamin Franklin did not know about the
existence of these particles but, he did
investigate the behavior of static discharge and
lightning.
Ben knew that if certain electrically neutral objects
are rubbed, they can become charged.
For example; when rubber is
rubbed with a wool cloth, both
become charged.
or a comb through hair
The rubber scrapes electrons
from fur atoms. So the rubber is
negatively charged and the cloth
is positively charged.
Ben also knew that a charge separation occurs
when a glass rod is rubbed with a silk cloth
In the case of the glass and silk, the glass rod
loses negative charge and becomes positively
charged while the silk cloth gains negative
charge and therefore becomes negatively
charged.
Ben experimented with the interactions between
the charge objects. He suspended one and
brought other charged objects near…
repel
attract
Electrons:
Ben observed that
like charged object repel
and unlike charges attract
repel
Fundamental Rule
Opposites attract, Likes Repel
 Things don’t like having a net charge
 If objects don’t like having a net charge,
then how does it happen?

 Silk
on glass  glass  (+)
 Who
loses electrons?
 The
glass
 Fur
on rubber  rubber  (-)
 Who
 The
loses electrons?
fur
When objects get Charged:


Must obey Law of Conservation of Charge
Charges may be transferred among different
atoms, materials, or objects but all charge is
accounted for.
 NO NEW charges are created nor are any
charges destroyed.
 Only electrons can move
Remember:
An excess of electrons results in:
 A negative charge
 A shortage of electrons results in:
 A positive charge
 ONLY ELECTRONS MOVE

How Do Charges Behave in
Materials?

Conductor:
Allows electrons to move easily
 Metals, why?
 Metals lose electrons, (not held tightly)


Insulator:
Does not allow electrons to move easily
 Non metals. Glass, plastic, dry wood. Why?
 Electrons held tightly


Semiconductors – charges only move
freely when certain conditions are met
(i.e., heat, sufficient voltage, etc.) ex
germanium, selenium, and silicon.

Superconductors – charges move
effortlessly and cannot be stopped once
they are moving
Objects become charged by…
Friction Electrons are rubbed off one insulator
onto another insulator
Conduction
Induction
Grounding
With a credit card 
FRICTION: e- rubbed off one
insulator to another
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Objects become charged by…
Friction
Conduction Charging by CONTACT with
a charged object
Induction
Grounding
Charging by
Conduction
Requires Contact
Electrons
transferred.
Results in:
Object with the
same charge as
original charged
object.
Some electrons leave rod
and spread over sphere.
Objects become charged by…
Friction
Conduction
Induction Charging an object WITHOUT
touching a charged object
Grounding
Induction
no contact occurs between charged
object and neutral object..
 Involves temporary rearrangement of
electrons on neutral object
 Neutral Object becomes “polarized”
but net charge remains the same
 If neutral polarized object is
grounded, charge will become
“opposite” of the charged object and
is no longer temporary

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Charging by Induction
Neutral objects can be temporarily attracted to
charged objects by a process called
POLARIZATON.
A negatively charged balloon is brought near a
neutral conducting sphere as shown below. As it
approaches, charge within the sphere will
distribute itself in a very specific manner. Which
one of the diagrams below properly depicts the
distribution of charge in the sphere?
What is grounding?

Involves Transfer of excess electrons to
and from the ground
Charging by Induction AND Grounding
polarization
grounding
permanent charge
The rod does not touch the sphere. It pushes electrons out
of the back side of the sphere and down the wire to
ground. The ground wire is disconnected to prevent the
return of the electrons from ground, then the rod is
removed.
The charge on the object is opposite if grounded

~ 1750
 “bells”

It consisted of two metal bells, one electrically connected to the earth
(grounded) and the other connected to a lightning rod. Hanging
between the two bells was a metallic ball suspended by an insulating
(dielectric) thread. The lightning rod allows an electric charge to build
up on one bell, which then attracts the metallic ball. When the ball hits
this charged bell it becomes charged to the same potential and is
immediately repelled. Since the grounded bell is charged oppositely,
this attracts the ball towards it. When the ball touches and rings the
grounded bell, the charge is transferred and the process repeats.

Lightening Bells
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How can I tell if something is
charged?
Pith Ball Experiment
Pith Ball and a Charged Rod
 Pith Ball Experiment

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
In the beginning, the negative rod repels
the electrons in the neutral pith ball to
the ight. This causes the left side of the
pith ball to obtain a net positive charge.
Since opposite charges attract, the pith
ball is attracted to the rod. When they
touch, electrons are transferred to the
pith ball giving it a net negative charge.
Since the rod remains negative even
after the transfer, it repels the negatively
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Electroscope
-detects the
presence of
charge
When the leaves are charged, what will
happen?
 When the leaves are not charged, what
will happen?

An electroscope is a device
that detects static charge.
Negatively charged
Positively charged
The metal leaves of the electroscope move apart
if a charged object is brought near the knob.
Benjamin Franklin used a similar device when he
investigated charges.
To review…
Induction results in an OPPOSITE CHARGE
Conduction results in the SAME CHARGE
Grounding is allowing charges to move freely
along a connection between a conductor and
the ground.
The Earth (the ground) is a practically
infinite reservoir of electric charge.
Here a positively charge rod
attracts electrons from the
ground into the electroscope
Here a negatively charge rod
repels electrons into the
ground from the sphere
Static Electricity in our lives

Why are there more problems with static during the
dry winter months?
 The net charge of a water molecule is neutral,
however it is a polar molecule. It can attract a build up
of excess electrons
 Some Static Electricity is due to friction
 Clothes stick to each other in the dryer
 How to solve this problem?
 Dryer sheets. How do they work?
 Dryer sheets contain lots of polar molecules to absorb
the excess electrons and keep clothes neutral
Applications of Electrostatic Charging
Fine mist of negatively charged
gold particles adhere to
Negatively charged paint
positively charged protein on
adheres to positively
fingerprint.
charged metal.
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
How the Microfiber Works: Proper use of our microfiber cloths means that 99% of the bacteria are removed from surfaces. This is
important in areas such as your bathroom and kitchen. The microfiber effectively removes dust, dirt, grease, chemical residues, and
micro-organisms. The dry cloth employs static electricity (created when the synthetic fibers rub together) to attract dust and pull it
into the fibers. The wet cloth removes dirt and moisture by capillary action, breaking the surface tension of water and grease,
wicking them up. You're left with a clean, microorganism-free surface! By following our care instructions, these cloths will stand up to
the rigors of every day use and washing, in both home and commercial settings. Microfiber cloths are very easy to use:Simply
fold them, and wipe smooth surfaces with a flat section of cloth in contact with the surface. This means that the palm of your hand
covers as much of the cloth as possible. When the cloth gets dirty, refold the cloth so a clean section will be in contact with the
surface you are cleaning. Continue until all surfaces of the cloth are filled. This technique works for both wet and dry cleaning. To
use the cloths wet, simply wet them in warm water, squeeze out excess water, and fold the cloth to wipe.*** Remember to not use
chemical cleaners or soaps with the microfiber - use only water!


To clean the cloths:
If the cloths are only slightly dirty, they can be rinsed with some dish detergent in warm water. Rinse well, hang to dry, and they are
ready to be used again! The microfiber can be used every day, washed many times, and still retain its effective cleaning properties.






Wash dirty cloth in the washing machine with detergent. We recommend the Norwex Laundry Detergent, because it is free from
fillers. However, any detergent can be used, as long as it does not contain bleach, fabric softeners, or chemical additives. It is also
important to wash the cloths with lint-free laundry (not sweaters or bath towels).
Hang the cloths to dry, or use the dryer. If the cloths are not working well, they may be holding chemical residues or bacteria. Wash
in the hottest cycle in your machine, and dry at the hottest dryer setting. Alternatively, they can be boiled on the stove top for several
minutes.
To clean the dry mop, use our Rubber Static Brush to release dust and dirt from the mop into the garbage or outdoors. The mops
can be machine washed as above.
Lightning
Becomes
very
“negative.”
Becomes
very
“positive.”
WHO DETERMINED
POSITIVE AND
NEGATIVE?
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How lightning forms – YouTube
 how lightning works – YouTube
 The Birth of a Lightning Bolt - YouTube

Four fundamental forces in nature
Gravity
 Weak nuclear
 Electromagnetic (electricity and
magnetism)
 Strong nuclear

EM force is a billion times stronger than
gravity, why don’t we notice it?
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Electrical Forces
Why don’t we notice
them?
 The attractive and
repulsive forces
between the charges
in Earth and the
charges in your body
balance out!!!!

Electromagnetic force is significant
at atomic level. Things you know:





Atom has positively charged nucleus
surrounded by negatively charged electrons
All electrons are identical (same mass; same
quantity of negative charge)
All protons are identical (same mass; same
quantity of positive charge)
Nucleus composed of protons and neutrons.
Neutrons are neutral
Neutral atoms have equal protons and
electrons so zero net charge
What exactly is CHARGE?
It
is physical property of
matter.
It
comes in two flavors:
“plus” and “minus.”
What is the unit for charge?
Coulombs (C)
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What is the smallest charge possible?
Millikan Oil Drop Experiment
 In 1910, Millikan was able to measure
the charge of an electron.
 The smallest charge possible is:
-1.602 x 10-19 Coulombs (C).

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Definition of Coulomb





Abbreviation: C
SI unit for charge
One coulomb is NOT equal to the charge of 1
electron!!!!
1C ~ the charge of 6.25 x 1018 electrons
It is the amount of charge to pass through a
cross-section of wire in 1 second when 1
Ampere (A) of current is applied.


(We’ll cover the amp later.)
Likewise the + charge of protons is associated
with 6.25 x 1018 protons
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Elementary Particles
Particle
# of particles
in a Coulomb
electron
Charge,
(Coulombs
per particle)
-1.6 x 10-19
proton
+1.6 x 10-19
6.25 x 1018
6.25 x 1018
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Ex 1
A strong lightning bolt transfers 35 C to
Earth. How many electrons were
transferred?
 (35 C) (6.25 x 1018 electrons)
 2.19 x 1020 electrons

Coulomb’s Law
Charles-Augustin de Coulomb used a
torsion pendulum to establish his law.

q1q2
F  k 2 rˆ
d
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Electric Force

q1q2
F  k 2 rˆ
d
q  charge, C (coulombs)
 d  distance between charges, m
 F  electric force, N
 k  electrostatic constant 9.00 x 109 Nm2/C2

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
What happens to F as charge increases?
 Increase
 What happens to F as r increases?
 Decreases by inverse square
 Look at kc. Is this a large or small value?
 large
 How is q described for a proton?
 positive
 For an electron?
 negative
The Product of q1and q2
If
the product, q1q2 ,is
negative then the force is
attractive.
If the product, q1q2 ,is
positive then the force is
repulsive.
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Ex 2: Two negatively charged balloons are 0.70m apart.
If the charge of each is 2.0 x 10-6C, What is the
electric force between the two balloons?
q1 = q2 = 2.0 x 10-6 C
d = r = 0.70 m
q1q2
Fk 2
r
F = 9.0 x 10
F = 0.073 N
9
N m2/C2 (-2.0 x 10-6 C)2
(0.70m)2
An attracting or repelling force?
Ex.3: Two equally charged balloons repel each other
with a force of 4.0 x 10-3 N. If they are 0.015 m
apart, what is the charge of the each balloon?
F = 4.0 x 10-3 N
d = 0.015 m
q1q2
Fk 2
r
q2 = Fd2
k
q2 = (4x10-3N)(0.015m)2
(9x109Nm2/C2)
q1 = q2 = 1.0 x 10-8C
Ex 4:
How many Coulombs are in a µC?
 1 x 10-6
 Two charges are separated by 3.0 cm.
Object A has a charge of +6.0 µC.
Object B has a charge of -6.0 µC. What
is the force on Object A? Is the force
attractive or repelling?
 -360N, attractive

Ex 5
Two electrons exert an electrical force of
1.0 x 10-8 N on one another.
Is this an attractive or repelling force?
Repelling
Calculate the distance between them.
Rearrange formula to solve for d
Use known charge for an electron
1.5 x 10-10 m

Two charges create a
force on one another. If
the charge of one object
is doubled, how does
the resulting force
change?
 F will double
 What if charge of one
object is tripled?
 F will triple

q1q2
F  k 2 rˆ
d




Two charges create a
force on one another. If
the distance between
the objects is increased
by a factor of 2, the
force changes by a
factor of?
F will decrease by a
factor of 4
What if distance
between the objects is
tripled?
F will decrease by a
factor of 9

q1q2
F  k 2 rˆ
d
Review……….
How many electrons in one Coulomb?
 6.25 x 1018 electrons
 What is the charge of one electron
 -1.6 x 10-19 Coulombs (C)
 How many protons in one Coulomb?
 6.25 x 1018 protons
 What is the charge of one proton
 +1.6 x 10-19 Coulombs (C).

Review……….
How many electrons in one Coulomb?
 6.25 x 1018 electrons
 Calculate the charge of one electron
 -1.6 x 10-19 Coulombs (C)
 How many protons in one Coulomb?
 6.25 x 1018 protons
 Calculate the charge of one proton
 +1.6 x 10-19 Coulombs (C).

Force and Fields

Contact forces
What we mostly
deal with
 Objects touch each
other directly
 Ex. A tennis racket
hits a tennis ball
 F=ma

www.CartoonStock.com.
Forces can occur without
contact!

Action at a distance

Can you think of anything that applies a
force without touching?
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Gravity demonstrates action at a
distance
What happens if you get too far away
from the mass exerting the force?
 The effects are less

67
What else applies an action at a
distance?
Magnets!
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What else applies an action at a
distance?
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Attracting and repelling forces of
charges
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The space that surrounds these things is
altered
 Examples:

Magnets
 Sun
 Planets
 Electric charge

Action at a distance depends on
a field of influence

An object within the field may be affected by it
 Can be scalar or vector
 Magnitude only
 Ex. Heat
 Can be vector
 Magnitude and direction
 Ex. Gravity (one direction only since only
attracts)
 Ex. Electric (more than one direction; attracts
and repels
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Fields are NOT Force, they exert
the force
Ex. A person pushes a box.
 The person is not the force, he exerts the
force!

73
Electric field
A field that exerts force that surrounds
an electric charge or group of charges
 Magnitude and direction (vector)

Electric field
How would you detect and measure an
electric field around a charge?
 Place another one nearby and see what
happens!
 Since all charges produce fields, come
up with a model

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Electric field model
Source charge: charge producing the
field. Usually designated with a capital
Q
 Test charge: a mathematical creation

Always positive
 Symbol: q’
 Doesn’t exist
 Infinitely small, thus produces no field of its
own

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What is the source charge if
The test charge q moves towards it?
 Negative (attracts)
 The test charge q moves away from it?
 Positive (repels)
 How would I draw these?

77
Where do you think the field is
strongest?
78
What if I had more than
one source charge?
What would the field lines
look like?
79
Think: Where is the electrical potential energy of a
positive test charge (q+) higher, at the point A or B?
Why?
Point A. Because of it’s
location, it is not where
it “wants” to be. It took
work to get it there!
The electric field is strongest in regions where
the lines are close together and weak when the
lines are further apart.
These fields can be detected in lab…
Threads floating on oil bath become polarized
and align themselves with the electric field.
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85
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How do I measure the strength of the electrical field
around a source charge (Q)?
What factors do you think the
electrical field strength is dependent
on?
1. Force (Push or Pull) of Source Charge
on Test Charge
2. Distance Between Source Charge
and Test Charge

First let’s consider effect of force
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Electric Field Intensity (Strength)
F
E
q'

E - Electric Field Strength or Intensity (N/C)
 F - Force experienced by a test charge at that
location (N)
 q’ - magnitude of the test charge placed at that
location (C).
Ex 6
A test charge has a magnitude of 1 x 10-10C.
It experiences a force of 2N in an electrical
field. What is the Intensity of the field?
Variables:
F = 2N
q’ = 1 x 10-10C
E = F/q’
E = 2N/ 1 x 10-10C
E = 2 x 1010 N/C
Ex 7
A test charge of 6 x 10-26 uC is placed 200 mm
from a proton (this is the source charge). What is
the electrical force between them? What is the
Field strength at this point? What is the direction
of the field?
Variables:
q’ = 6 x 10-26µC
p+= 1.6 x 10-19C
d = 200 mm
Variables:
q’ = 6 x 10-32C
p+= 1.6 x 10-19C
d = 0.2m
Now we can solve. This is a 2 step problem.
Step 1: Solve for force using Coulombs Law
Step 2: Use the calculated force and solve for Field Intensity
Ex 7 cont
A test charge of 6 x 10-26 µC is placed 200 mm
from a proton (this is the source charge). What is
the electrical force between them? What is the
Field strength at this point? What is the direction
of the field?
F = kqq
2
d
Variables:
q’ = 6 x 10-32C
F = (9 x 109C)(6 x 10-32C)(1.6 x 10-19C)
p+= 1.6 x 10-19C
0.2m2
d = 0.2m
F = 2.16 x 10-39N
E = F/q’
E = 2.16 x 10-39N / 6 x 10-32C
E = 3.6 x 10-8 N/C
Remember: The direction of the electric field at a point in space is
the same as the direction in which a positive charge would move if it were
placed at that point. The electric field lines or lines of force indicate the
direction.
+
-Q
Electric field line flow Out of positive charges
and into Negative charges.
The electric field intensity E at a distance d from a
source charge Q can be found without knowing the test charge!:
Units: N/C
EX 8: What is the electric field intensity at a distance of 2 m
from a source charge of -12 μC? Include direction.
d = 2 m
q = -12 μC
q = -12μC
To determine the direction of the field, ask
If the source charge is negative do the
field lines go out or in?
kQ
9 x109 (12 x106 )
E 2 
2
2
d
= 2.7x104 N/C, towards q
or to the left
How do I determine the field strength if
there are multiple charges?
96
When more than one charge contributes to the field, the
resultant field is the vector sum of the contributions from
each charge.
kQ
E  d2
Where k : 9x109Nm2/C2
Units: N/C
Note we will look at direction of the field to know
whether fields add or subtract at a point.
Remember this?
+
-Q
Electric field line flow Out of positive charges
and into Negative charges.
Ex 9: Two charges q1=-8 μC and q2=+12 μC are placed
120 mm apart in the air. What is the electric field at the
midpoint between them?
q1 = -8 μC
q2 = +12 μC
r = 0. 120m
ET
E1
q1
X
E2
+
q2
kQ = kq1 + kq2
E  d2
r2
r2
= (9 x 109)(8 x 10-6) + (9 x 109)(12 x 10-6)
(0.06)2
(0.06)2
E= 2.0 x 107 + 3 x 107 = 5.0 x 107 N/C
to the left
Ex.10: Two charges q1=+8 μC and q2=+12 μC are placed 120 mm
apart in the air. What is the electric field at the midpoint
between them?
q1 = + 8 μC
q2 = +12 μC
r = 0. 120m
ET
+
q1
kq
E 2
r
E1
X
E2
The fields are in opposite
directions so they subtract
+
q2
= kq1 - kq2
r2
r2
= (9 x 109)(8 x 10-6) - (9 x 109)(12 x 10-6)
(0.06)2
(0.06)2
E= 2.0 x 107 - 3 x 107 = -1.0 x 107 N/C
E = 1.0 x 107 N/C to the left