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ElectroStatics
Physics NPHS
Spring 2016
Lesson 1
• Essential Question
–
What is an Electric Charge?
• Lesson Objectives
– To understand the motivation behind
electrostatics unit
– To design and collect observations about
interactions and to make conclusions from
data
Do Now
• What do you know about electric charge?
What do you want to know about electric
charge?
Motivation
• Have you ever been shocked touching a
door?
• Have you done laundry and noticed that
your clothes stick together?
• Have you noticed that your hair gets frizzy
when it is a dry day?
• Have you ever wondered why?
Electrostatics Activity 1
• Materials
– Felt/Fur
– Two balloons
– Plastic shopping bag
• Create a data table to show the following interaction
– Two balloons
– Two ballons rubbed with felt
– 1 balloons rubbed with felt and felt
– Two ballons rubbed with plastic shopping bag
– One ballons rubbed with plastic shopping bag, one
with felt
• Identify patterns in your observations.
Describe them carefully!
Building a Data Table
• Decide what you need to record
• Decide how you want to collect your data
(in what order)
• Decide how to organize your materials so
that you can find patterns in your data
• How will we organize our data?
Data Table
Interaction
1
2
3
4
5
Material 1
Material 2
Observation
Data Table
Interaction
Material 1
Material 2
Observation
1
Unrubbed
Foam
Unrubbed
Foam
No
interactions
2
Foam
Rubbed with
felt
Foam
Rubbed with
felt
They repel
each other
3
Foam
Rubbed with
felt
Felt
attract
4
Foam
Rubbed with
plastic bag
Foam
Rubbed with
plastic bag
repelled
5
Foam
Rubbed with
felt
Foam
Rubbed with
Plastic wrap
attracted
Conclusions
• What are your observations?
• What are your conclusions
System
• A system is a collection of two or more
objects
• What is the system in the experiment that
you just completed?
Statics Lesson 2
• Something must have happened when the
objects were rubbed together because
they did not interact that way before
• We know that objects either are attracted
to each other, repelled by each other or
have not effect on each other
• Some objects are able to exert a force on
other objects to pull or push them
Do Now
• Clothing clings together in a dryer. Why
does this happen?
• If you have two socks and a shirt, which
two would you predict will be stuck
together?
• Is this consistent with your observations
yesterday?
ElectroStatics
• Static electricity is the cause of all these
occurrences
• The study of static electricity is called
Electrostatics
• This is the first step in studying areas such
as electric current, power and electricity.
• Static electricity
Static Charge
• Objects we observed that are able to apply
a force to other objects are called charged
objects.
• This was first noticed by ancient Greeks.
Pieces of amber attracted or repelled one
another, similar to your plastic sticks. The
Greek word for amber is “electron”.
Movie Size of an Atom
• Drghonorsphysics.wikispaces.com
Atoms – Building Blocks of Matter
Atoms are composed of Protons
(positively charged particles),
Neutrons (neutral particles) and
Electrons (negatively charged
particles)
The Nucleus is a dense core
composed of protons and neutrons
and is surrounded by “cloud” of
much smaller particles – electrons.
Electrons orbit the nucleus in a
specific configuration
•
Figure 1. The Bohr Model:
A basic (not to scale) model
of an atom displaying the
nucleus, electrons and
orbiting paths of electrons.
copyright
Key to the Periodic Table
Atomic Number
Atomic Weight
• Elements are organized on the
table according to their atomic
number, usually found near the
top of the square.
– The atomic number refers
to how many protons an
atom of that element has.
– For instance, hydrogen has
1 proton, so it’s atomic
number is 1.
– The atomic number is
unique to that element. No
two elements have the
same atomic number.
– Rounding the atomic
weight and subracting the
atomic number gives the
number of neutrons
– If an atom is neutral
#electrons=#protons
http://periodic.lanl.gov/index.shtml - for more information about specific atoms
Atoms
• Atoms may gain or lose electrons.
• In these cases, the atom as a whole is no longer neutral,
and we call it an ion.
• A Positive Ion has a net positive charge because the
atom has lost one or more electrons.
• A Negative Ion occurs if an atom gains one or more
electrons and it has a net negative charge, and is
therefore called a negative ion.
• Like charges repel each other, while opposite charges
attract each other.
• In physics, we represent the charge on an object with the
symbol q.
Fundamental properties of
electric charge
Phet Balloon Simulation
Lesson 3
• Phet_Balloon_Charge.doc
Conduction
• Electrons can flow freely from particle to
particle
• Distribution of charge caused by moving
electrons
• Charge will be re-distributed in an object to
until repulsive forces between electrons is
minimized
Insulators
• Impede the free flow of electrons
Check your understanding
• One of these isolated charged spheres is
copper and the other is rubber. The
diagram below depicts the distribution of
excess negative charge over the surface
of two spheres. Label which is which and
support your answer with an explanation.
Polarization - Conductor
• Negatively charged object induces
electrons to move away
• Can becomes polarized (positive on one
side, negative on the other)
Polarization - Insulator
• An insulator can become polarized as well
but electrons re-distribute around nucleus.
The atom is still neutral
Polarization of an atom
Check your understanding
• Which of the diagrams below best
represents the charge distribution on a
metal sphere when a positively charged
plastic tube is placed nearby?
Grounding a Negatively
Charged Object
• Grounding removes charge by transferring
to a large electron sink (earth, human
body)
Charge Diagrams Conduction
Charge Diagrams Induction
Charge Diagrams Polarization
Conduction/Induction/Polarization
Name that charge (Physics Classroom
Interactive)
Show me your result
Lesson 4
• Essential Question
–
What is an Electric Charge?
Lesson Objectives
• To understand how electric charge is
transferred between objects
• To understand the difference between
positively charged and negatively charged
particles
Charged particle activity
• Take from PUM 4.7 Reason
Do Now
• Write a math statement to describe how the net charge of the
material changes.
– A neutral object gains 6 electrons.
– A neutral object loses 9 electrons.
– An object that has an excess charge of + 8 gains
electrons and becomes neutral.
– An object that has an excess charge of +6 gains 7
electrons.
– An object that has an excess charge of -10 is
neutralized.
– An object that has an excess charge of -7 loses 4
electrons.
Lesson objective
• Investigate Coulomb’s law – continue with
(or start) worksheets
• Understand that charge is quantized
• Example problems using Coulomb’s law
Electric Charge
• Electric charge is a physical quantity
• The unit for electric charge is Coulomb
• 1 Coulomb=amount of charge is 2 point
charges placed 1 meter part to produce a
force of 8.988x109N
Electrostatic Facts
• Electric charge comes in two types – positive and
negative; like charged objects repel and unlike charged
objects attract. Historically, charge accumulated on a
glass rod rubbed with silk was called positive, electric
charge accumulated on a resin rod rubbed with fur –
negative.
• Electrons and protons have basic electric charge of
magnitude
q = 1.6 x 10-19 C
the electron being negatively charged and the proton
positively
charged.
Electric charge is quantized
• Since an object cannot gain or loose a
fraction of an electron, the net charge on
any electron must be a multiple of the
charge of an electron.
• Quantized means that charge can only
change in discreet amounts (ie a multiple
of the charge of an electron 1.6x10-19C)
How many electrons make up a
charge of -20mC?
Do Now
• What is the total charge of 2200
electrons?
– Hint: for each electron q=1.6x10-19 Coulombs
32.2 Conservation of Charge
Principle of Conservation of Charge
Electrons are neither created nor
destroyed but are simply
transferred from one material to
another. This principle is known as
conservation of charge.
In every event, whether large-scale
or at the atomic and nuclear level,
the principle of conservation of
charge applies.
In class activity HONORS
• Print pages 17-20 PUM
• Phyz worksheet CP
Coulomb’s Law
• 𝐹=
𝑘𝑄1 𝑄2
𝑟2
• Tells you the MAGNETUDE of force
• The direction of the force is always along a line
connecting two objects
• K = constant = 9.0x10 Nm2/C2 constant
• This equation applies to objects whose size is
much smaller than the distance between them.
• You have 2 objects. One has twice as much
charge on it than the other. When brought
together, which exeriences the greater force?
Example Problem 16-1
• Determine the magnitude and direction of electric force on the
electron of a hydrogen atom exerted by the single proton that is the
atoms nucleus. Assume the average distance between the
revolving electron and the proton is r=0.53x10-10m
What is the force due to gravity between the
proton and electron in a hydrogen atom?
• Assume the average distance between the revolving electron and
the proton is r=0.53x10-10m
• Mass Proton =1.6726 x10-27 kg
• Mass Electron (at rest) =1.6749x10-27 kg
Similarities?
A comparison between Electric
and Gravitational forces
Equation
Force
Characteristics
Constant
Magnitude
Electric Force
Gravitational Force
𝑘𝑄1 𝑄2
𝐹=
𝑟2
Attract and
Repel
Non-contact
𝐺𝑚1 𝑚2
𝐹=
𝑟2
Only Attract
K=9x109
Nm2/C2
Much Stronger
Force
G=6.67x10-11Nm2/kg2
Non-contact
Example problems using charge
• What is the force on a +2.3mC charge that
lies 3.7 m to the left of a -5.1mC charge?.
How far is a +4.5 x10-3 C charge from a -8.2x10-3
C charge if there is a force of 13N between them?
Thought Question
• You are given 2 balloons. One has twice
as much charge on it than the other.
– When brought together, which experience the
larger force and why?
– If the amount of charge on one is doubled,
what do you think happens to the forces
between them?
Another thought question
• Which graph best represents the electrostatic force between an
alpha particle (net charge of +2) and a positively charged nucleus as
a function of their distance of separation?
Conductors and Dielectrics
• Conductors: These are materials in which charged
particles can move freely from one place to another. In
metals these particles are electrons.
• Dielectrics: These are materials in which electrically
charged particles cannot move freely. However, the
electric charge in the particles (atoms and molecules) of
which the material is made can shift slightly within the
particles. This small redistribution can cause an excess
of one type of charge on one side of the object and an
excess of the other type of charge on the opposite side
of the object.
32.4 Conductors and Insulators
A substance is classified as a
conductor or an insulator based on
how tightly the atoms of the
substance hold their electrons.
The conductivity of a metal can be
more than a million trillion times
greater than the conductivity of an
insulator such as glass.
In power lines, charge flows much
more easily through hundreds of
kilometers of metal wire than through
the few centimeters of insulating
material that separates the wire from
the supporting tower.
32.4 Conductors and Insulators
Semiconductors are materials that can be made to behave
sometimes as insulators and sometimes as conductors.
Atoms in a semiconductor hold their electrons until given
small energy boosts.
This occurs in photovoltaic cells that convert solar energy
into electrical energy.
Thin layers of semiconducting materials sandwiched
together make up transistors.
Do Now
• Two conducting spheres have an excess
of negatively charged particles and are
placed very far apart from one another.
– Draw a picture of the spheres when they are
far apart
– Draw a picture of the spheres when they are
close together but not touching
– Draw a picture of the spheres when they are
moved far apart again.
Principal of Superposition
• What if several charges exist?
– The net force on any one of the charges is a
vector sum of the forces on that charge due
to each other
– Determined experimentally
Review of Vector Addition
1. Pythagorean theorem
Trigonometry Review
SohCahToa
Example
• Three charged particles are arranged in a
line. Calculate the net electrostatic force
on particle 3 due to the other two charges.
-
+
D=0.3m
Q1=-8.0 mC
D=0.2 m
Q1=3.0 mC
Q1=-4.0 mC
Example 2