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Describe two examples of electric forces in everyday life.

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Describe two examples of electric forces in everyday life.
Static
Electricity and
Coulomb’s Law
Objectives
•
Describe the historical development of the concepts of electrostatics.
•
Identify examples of electric forces in everyday life.
•
Identify the two types of electric charge and how they interact.
•
Describe the historical development of the concepts of
electromagnetic force.
•
Describe and calculate how the magnitude of the electric force
between two objects depends on their charges and the distance
between them.
Physics terms
•
Coulomb’s law
•
coulomb
Equations
The force between point charges is
proportional to the product of the
charges, divided by the square of
the distance between them.
Static electricity
Benjamin Franklin was America’s first famous scientist.
He and many others were studying the properties of
static electricity even in the 1700’s.
•The study of electric charges at rest is called
electrostatics.
•Static electricity results from a tiny excess of positive
or negative electric charge on an object.
An example
Rub a balloon against a wool sweater.
This transfers some negative charges
from the sweater to the balloon.
The sweater now has a net positive
charge and the balloon has a net
negative charge.
They will attract and stick together.
What makes the zap?
Shuffling your feet on the carpet
can also transfer charges.
Touching a metal conductor, like
a doorknob, causes the excess
charge you’ve collected to be
attracted to its opposite in the
metal.
A tiny electric current flows …
and that’s what makes the zap!
Electric charge
There are two kinds of electric charge:
positive (+) and negative (–).
In electrically neutral objects, positive
(+) charges are balanced by an equal
number of negative (–) charges.
Most objects are electrically neutral …
otherwise there would be lightning
everywhere all of the time!
Opposites attract
As with magnetic poles,
unlike charges attract:
Like electric charges repel:
Neutral particles feel no
electric force at all from
positive or negative charges.
Electric force
Electric forces of attraction and
repulsion can be enormously
strong.
These are the forces hold atoms
and molecules together.
Electric forces are so strong that,
outside of atoms, positive and
negative charges are rarely
separated for long!
The Van de Graaff generator
A Van de Graaff generator
is a device that can
separate electric charge.
Its sphere can reach
thousands or even millions
of volts, but it can only
create a big spark – not a
large electric current – if
you touch it.
Charge distribution
Like charges repel, so charges
will push each other apart as far
as possible.
If the charges are on a Van de
Graaff generator, repulsive
forces will push them to the
outside edge of the sphere.
Electrostatic induction
It is also possible to charge an
object without touching it.
Here, a negatively-charged rod
repels negative charges and
attracts positive charges. The
sphere is now “charged”.
This process of separating charge
is called electrostatic induction.
Demo: the electroscope
An electroscope uses induction to detect electric charge.
Summary: charging an object
There are three ways that objects typically become charged.
• conduction: A charged object contacts an uncharged
object and transfers charges to it.
•
friction: Different materials have different affinities for
charge. Rubbing these materials together causes
charges to transfer between the materials.
•
electrostatic induction: A charged object is brought
close to a neutral object, resulting in charge separation
in the neutral object.
Assessment
1. Name one of the founding fathers of the United States who
also contributed to our understanding of static electricity.
Assessment
2. Describe two examples of electric forces in everyday life.
Assessment
3. Which statement about electric forces is true?
A. A negative charge attracts a negative charge.
B. A positive charge repels a negative charge.
C. A negative charge repels a negative charge.
D. A positive charge attracts a positive charge.
Where does charge come from?
All ordinary matter is made of
atoms, and all atoms contain
charged particles.
• The atomic nucleus contains
positively-charged protons.
• Outside of the nucleus are tiny
fast-moving, negatively charged
particles called electrons.
Where does charge come from?
The charge of an electron and
proton are exactly equal and
opposite.
A complete atom has equal
numbers of protons and electrons.
Therefore, a complete atom has
no net electric charge.
The unit of electric charge
The unit of electric charge is the
coulomb (C) in honor of CharlesAugustin de Coulomb (1736-1806).
Coulomb was a French physicist
who made the first accurate
measurements of electric force
between charges.
How big is a Coulomb?
A coulomb is a very large amount
of charge.
One coulomb equals the charge
of 6 × 1018 electrons or protons.
Ordinary static electricity results
from net charges of less than a
millionth of a coulomb.
Think:
How can you increase the repulsive force
between two positive charges?
What does this force depend on?
q1
Fe
q2
r
Fe
It depends on the magnitude of each charge,
and the distance between them.
Can you guess the formula for Fe?
Coulomb’s law
Coulomb’s law quantifies the electric force:
where:
Fe = electrostatic force (N)
ke = Coulomb constant = 9.0×109 Nm2/C2
q1 = electric charge of object 1 (C)
q2 = electric charge of object 2 (C)
r = distance between the two objects (m)
Coulomb’s law
Stating Coulomb’s law in words:
The force between two charges equals
the product of the charges divided by
the square of the distance between
them, all multiplied by a constant, ke.
Engaging with the concepts
Two silver balls with charges
of +7.0 μC and +1.2 mC, are
located 80 cm apart.
Electric force
What is the force between
them?
7 e-6 7.0e-6
1.2 e-3 1.2e-3
0.80
8.99e+9
0.80
In what direction does it act?
Hint: make sure you use the right units!
Engaging with the concepts
Two small metal spheres are
separated by 25 mm. One has
a charge of +2.0 μC, the other
−2.0 μC.
What is the force between
them?
Electric force
7 e-6 2.0e-6
1.2 e-3 -2.0e-6
0.80
8.99e+9
0.025
Engaging with the concepts
Two small metal spheres are
separated by 25 mm. One has
a charge of +2.0 μC, the other
−2.0 μC.
What is the force between
them if the distance doubles?
What is the force between
them if the charges ALSO
double?
Electric force
7 e-6 2.0e-6
1.2 e-3 2.0e-6
0.80
8.99e+9
50e-3
Fe increases with charge
The electric force increases with increasing charge:
Doubling one charge
doubles the force:
Doubling both charges
quadruples the force:
Fe obeys an inverse square law
The electric force varies inversely with the
square of the distance between charges.
DOUBLING the distance reduces
the force by a factor of ¼.
HALVING the distance increases
the force by a factor of 4.
Test your knowledge
Two +1.0 μC charges are separated by a distance of 1.0 cm.
What is the magnitude and direction of their mutual electric force?
Asked:
Given: r
Relationships:
Solution:
The electric force, Fe
The electric force is enormously strong:
One drop of water contains about 2,000 C
of positive and negative charges.
If you could separate these charges by one
meter . . .
The electric force, Fe
The electric force is enormously strong:
One drop of water contains about 2,000 C
of positive and negative charges.
If you could separate these charges by one
meter, they would attract each other with a
force of 36,000,000,000,000,000 N.
Most applications in electrostatics involve small amounts of
charge, often expressed as micro-coulombs (μC = 10-6 Coulombs).
Comparing forces: Fe and Fg
The electric forces between
elementary particles are
much much greater than
their gravitational attraction.
The force of gravity is so
much smaller in these
situations that we say it is
negligible—meaning you can
ignore it.
Assessment
1. For whom was the unit of charge, the coulomb, named and in
what way(s) did this person contribute to our understanding
of the electric force?
Assessment
2. Two charged particles are located 1.0 m apart. Describe
the direction for the electric force between them if the
two charges are:
a. +1 μC and +1 μC
b. +1 μC and -1 μC
c. -1 μC and -1 μC
Assessment
3. Two charged particles are located 1.0 m apart.
a) Calculate the magnitude of the electric force between
them if the two charges are +1.0 μC and +1.0 μC.
a) What would be the magnitude of the force if the charges
were 50 centimeters apart?
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