Static Electricity
Section 20.1 Learning Objectives:
Electrostatics
• Recognize that objects which are charged exert
forces, both attractive and repulsive.
• Demonstrate that charging is the separation, not
the creation of electrical charges.
• Describe the differences between conductors and
insulators.
Static Electricity is about charges
that are not moving
What is Charge?
• Charge is a fundamental property of matter.
• Electrostatics:
• It is the cause of all electrical phenomena:
electrical fields, forces, current, etc.
the study of electric forces and fields that
are created by stationary charges.
• It is either “+” or “-”
• It is measured in Coulombs (C)
CHARGE IS…
• ...the stuff that flows during an electric current.
• ...the stuff that appears on a balloon when you rub it on your hair.
• ...charge is the + and - stuff that forms or is carried by subatomic particles..
• ...charge is the stuff that, when it wiggles fast, creates light.
• ...charge is the stuff that, when it flows or spins, creates magnetism.
• ...charge is the stuff that reflects light and makes objects visible.
• ...charge is the stuff that makes metals look metallic or "silvery."
• ...charge is the stuff that holds everyday objects together.
• ...charge is the stuff inside of conductors that is movable, almost fluid.
Properties of Charge:
• Cannot be created or destroyed
• occurs in discrete natural units, equal
to the charge of an electron or proton
• can be ‘positive’ or ‘negative’
• one positive charge can combine with
one negative charge, and the result is a
net charge of zero (neutral)
• ...charge is the stuff inside of nonconductors that is "frozen" in place.
• …charge is a fundamental property of matter
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Unit of Charge:
Coulomb (C)
Who discovered charge?
1 Coulomb =
• Ancient Greeks knew about static
electricity and how to make it
the amount of charge in 6.24 × 1018 electrons
• Ben Franklin coined the term “charge”
and came up with the idea of positive
and negative
• Robert Milikan Discovered the charge
of 1 electron in the famous Milikan Oil
Drop Experiment
Basic charge laws
• + & - attract
• + & + repel
• - & - repel
+
+
-
-
1 electron =
1.60 x 10 -19 Coulombs
conductor vs. insulator
-
+
So, what is the charge of 1 electron?
• Conductors allow electrons to move
freely
Like charges repel & Opposites attract
conductor vs. insulator
• Insulators keep the electrons in place
conductor vs. insulator
Conductors
Insulators
Metals
Plastic
Salt water
Glass
Plasma
Rubber
2
Charging an Object
Forming Charge: Chemistry
-3 for 3 electrons
-4 for 4 electrons
+3
+3
Objects become charged if they have an
imbalance of protons and electrons.

Total Charge = zero
Total Charge = -1
-2 for 2 electrons
Can an object gain or lose protons?
(Think: Can protons MOVE?)

Can an object gain or lose electrons?
(Think: Can electrons MOVE?)
+3
Law of Conservation of Charge:
Total Charge = +1
Forming Charge
the net charge in an isolated system remains constant
Conservation of Charge
Charge cannot be created or destroyed.
• Friction
– Electrons can be removed from a material
and it becomes positive
– Electrons can be added to a material and it
becomes negative
Protons and Electrons cannot be created or
destroyed.
Electrons can be transferred from one
substance to another.
Neutral (grounded) objects have and equal
number of protons and electrons.
Electrostatics
Part 2: Section 20.2
Electrostatics
• Benjamin Franklin (1706 – 1790)
• Proposed “Conservation of Charge”
• First to label charges positive and negative
• What was known about atoms then?
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Models of the Atom
The Atom
• Atoms have positively charged nuclei made of protons (+) and neutrons (0)
• These nuclei are surrounded by electrons (‐)
Electron
Proton
Neutron
More accurate picture of the
atom—the Helium atom
Unit of Charge:
Coulomb (C)
Important Facts
• All electrons are identical; that is, each has the same mass and the same quantity of negative charge as every other electron
• All protons are identical in mass and charge. The charge is equal in magnitude to electrons, but opposite in sign.
• Atoms are neutral (by definition)
What’s a Coulomb
• The coulomb (symbol: C) is the SI unit of electric charge.
• A coulomb is like a dozen, a gallon (128 fl oz), a ton (32 000 ounces), or a mole (6.02×1023)…an arbitrary amount of something used to make a large number more manageable
• To make a coulomb of charge, it takes 6.24×1018 electrons.
1 Coulomb =
the amount of charge in 6.24 × 1018 electrons
So, what is the charge of 1 electron?
1 electron = “elementary charge” =
1.60 x 10 -19 Coulombs
FYI: The charge that produces a large lightning bolt is
about 10 coulombs of charge.
Coulomb
• All charges should be reported in coulombs (C) or units of elementary charge (e) (i.e. that of one electron)
• About 1 C of charge flows through a 100 W incandescent bulb in 1 s
• That is about 6 billion billion electrons
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Static Electricity
Section 20.2 Learning Objectives:
• Explain how to charge by conduction or induction.
• Develop a model of how charged objects can
attract a neutral object.
Types of Charging
• Friction (pulls off electrons)
• Conduction (contact)
• Induction (almost touching)
• Describe how an electroscope detects charge.
• Summarize the relationship between forces and
charges.
(Polarization: an effect produced during induction)
• Use Coulomb’s Law to solve electrical force
problems.
Charging by Friction:


Triboelectric Series


If two different neutral materials are rubbed
together, one of them will give up electrons
and become positively charged.
REMEMBER: ONLY ELECTRONS CAN MOVE!!!

Valence electrons, specifically
Direction of e - movement depends on materials involved.


“Triboelectric
Series”:









Q:
A:
How do we know if a material gains or loses electrons?
A triboelectric series;
(the one higher on the list should give up electrons)
Charging by Conduction
Conduction just means that the two objects will come
into actual physical contact with each other (this is why it
is sometimes called “charging by contact”).
A neutral object becomes charged by a charged object
that touches it.



Asbestos
Fur (rabbit)
Glass
Mica
Wool
Quartz
Fur (cat)
Lead
Silk
Human skin , Aluminum
Cotton
Wood
Amber
Copper, Brass
Rubber
Sulfur
Celluloid
India Rubber
Charging by Conduction
Let's assume we have a negatively charged metal object
and an uncharged metal sphere. The uncharged sphere
is on an insulating stand so that it will not interact with
anything else.
It gains the same charge as the object that charges it.
If the two objects are brought close enough that an arc of
electricity jumps between them, it counts as conduction
also.
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Charging by Conduction
We bring the two objects close together. We will see a
separation of charge happen in the neutral object as
negative electrons are repelled to the right hand side.
Charging by Conduction
We now allow the two objects to touch. Some of the
negative charge will transfer over to the uncharged metal
object.
At this time, they are not touching and no charges have
been transferred.
This happens since the negative charges on the first object are
repelling each other... By moving onto the second object they will be
able to spread away from each other.
Charging by Conduction
Charging by Conduction
When the negative object is removed, it will not be as
negative as it was. Both of the objects have some of the
negative charge… how much depends on the size of the
objects and the materials they are made of.
If they are the
same size,
made of the
same materials,
then the charge
will be the
same on both.
Charging by Induction
Charging by
Conduction
Video
Temporary vs. Permanent Induction
When a charged object is brought close to an object
without physical contact, the charged object will induce a
movement of electrons in the uncharged object.
We call this temporary induction
‘polarization’:
In this case the electrons in the neutral object will will be
repelled from the electrons in the charged object.
If we take the negative rod away, the
sphere would return to normal.
What could we do instead?
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Temporary vs. Permanent Induction
If a grounding wire is attached to one side of the polarized
neutral object, charge will flow into the ground in order to
stabilize the electrical forces surrounding the neutral object.
Temporary vs. Permanent Induction
Once the electrons have been removed from the neutral
object, remove the ground wire and then the negatively
charged object.
It is crucial that the ground wire is removed first, otherwise
the electrons will simply return to the metal sphere.
In this case the charge is negative, so electrons will leave to an object and go to ground.
Thus leaving the originally neutral object with fewer
electrons than protons, and resulting in a net positive charge.
Charging by Induction
Charging by
Induction
Video
How Can We Detect Charge?
Electroscope: a device used to detect charge.
When a charge is present, the straw rotates.
Degree of rotation or separation indicates
strength of charge


More rotation, more charge
Less rotation, less charge
Detects both positive and negative charge, but
cannot tell the difference.
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Electroscope
When the scope is
charged, the foil leaves
at the bottom inside
spread apart.
Types of Charging
Electroscope pictures
Charging by
conduction
The scope is touched
by the charged rod
Types of Charging
-
-
+
+
- ++
-
+
Polarization
Molecules in a neutral
material spin around to
have the opposite charge
pointed to the charged
object outside.
Normal uncharged
Charging by
induction
The rod only comes
close but does not
touch
Static Electricity
Section 20.2 Learning Objectives:
• Explain how to charge by conduction or induction.
• Develop a model of how charged objects can
attract a neutral object.
• Describe how an electroscope detects charge.
• Summarize the relationship between forces and
charges.
-+
-+
-
-
• Use Coulomb’s Law to solve electrical force
problems.
+
+
Coulomb’s Law
Coulomb’s Law
• Charles‐Augustin de Coulomb (1736 ‐1806)
• Defined the electrostatic force of attraction and repulsion • The SI unit of charge, the coulomb (C), was named after him
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Coulomb’s Law
We’ve seen this before…
looks a lot like this…
Comparison
+
• Similarity
– Both are inverse – square laws: force is inversely proportion to the square of the separation
+
–
• Differences
– Electrical force is ≈1,000,000,000,000 (1018)times stronger than gravity
– Electrical forces can be both attractive and repulsive while gravity is always attractive
–
+
–
Direction
k?
• With electrical forces, the direction of force must be specified…
• A negative number implies attraction
• A positive number implies repulsion
• Often, the direction is specified with words and the signs of the charges are ignored:
A hydrogen’s electron feels an attractive force of 8.2 × 10‐8 N toward the nucleus
• The letter k (or kc or ke) in Coulomb’s Law is a proportionality constant, known as Coulomb’s Constant
• If charge is reported in units of coulombs and the distance is in meters…
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Computational Example
The hydrogen atom has the simplest structure of all atoms. Compare the electrical and gravitational forces between the proton and the electron in a hydrogen atom.
Coulomb’s Law
G = 6.7×10‐11
d
m
‐19
qp = 1.6×10 C
qe = – 1.6×10‐19 C
(9.0 x 10 9 N·m2/C2)
q2 = charge #2 (C)
r = the distance between
the charges (m)
r
Coulomb’s Law Example Problem # 1:
Two charges are separated by 3.0 cm. Object
A has a charge of +6.0 x 10-6 C, while object
B has a charge of +3.0 x 10-6 C. What is the
force on object A?
REMEMBER: When working with Coulomb’s law a negative
solution means that there is an attractive force.
This would be the case for finding the force between two
opposite charges such as a positive and negative charge.
+&-=A positive solution means that there is a repulsive force.
Such as two like charges.
Given: d = 3.0 cm = 0.030 m
K = 9.0 x
q1 = charge #1 (C)
-
N∙m2/kg2
mp = 1.7×10‐27 kg
me = 9.1×10‐31 kg
qA = +6.0 x 10-6 C
K = Coulomb’s constant
r2
+
= 5.3×10‐11
F = electrical force
• F = Kq1q2
qB = +3.0 x 10-6 C
+&+=+
-&-=+
109
For a two point source there is actually two forces at play.
q1
Example # 2
q2
For example, you have the force that the
first charge exerts on the second (F12)
F12
and the force that the second
charge exerts on the first (F21)
F21
Two balloons with charges of +3.37 µC and -8.21 µC attract
each other with a force of 0.0626 N. Determine the separation
distance between the two balloons.
d = 1.99 m
Each charged object exerts an equal but opposite force
on the other charged object.
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Example # 3
10-6
A small sphere, carrying a charge of –8.0 x
C, exerts an
attractive force of 0.50 N on another sphere carrying a charge
with a magnitude of 5.0 x 10-6 C.
Example # 4
Three charges A(+5.0 x 10-6 C), B(-2.0 x 10-6 C), and
C(+3.0 x 10-6 C), are arranged at the corners of a right
triangle as shown. What is the net force on charge C?
a) What is the sign of the second charge?
b) What is the distance of separation of the center of the spheres?
150 N @ 22° clockwise
from the horizontal
THE END
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