Download Unit 3 Electricity & Magnetism Electric Fields

Unit 3 Electricity &
Magnetism
Electric Fields
Physics 5e.
Students know charged particles are
sources of electric fields and are subject
to the forces of the electric fields from
other charges.
Electric Charge
Key Question:
How do electric
charges interact?
Electric Charge
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All ordinary matter contains both positive
and negative charge.
You do not usually notice the charge
because most matter contains the exact
same number of positive and negative
charges.
An object is electrically neutral when it has
equal amounts of both types of charge.
Electric Charge
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Objects can lose or
gain electric charges.
The net charge is also
sometimes called
excess charge
because a charged
object has an excess of
either positive or
negative charges.
A tiny imbalance in
either positive or
negative charge on an
object is the cause of
static electricity.
Electric Charge
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Static electricity video
http://glencoe.mcgrawhill.com/sites/dl/free/0078778069/16138
3/00053407.html
Electric Charge
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The innermost electrons in an atom
are attracted strongly to positively
charged atomic nucleus.
The outermost electrons (valence
electrons) of many atoms are
attracted loosely, therefore these
electrons can be removed more
easily.
Electric Charge
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When two neutral objects are rubbed
together, charge is transferred from
one to the other and the objects
become oppositely charged.
This is called charging by friction.
Objects charged by this method will
attract each other.
Electric Charge
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Example:
When you rub a comb against your
hair, electrons transfer from the hair
to the comb.
– The comb then has excess electrons
and is said to be negatively charged.
– Your hair, has a deficiency of electrons
and is said to be positively charged.
Practice Question
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Beth rubs an ebonite (polished
rubber) rod with wool. She finds that
the wool acquires a positive charge.
Which conclusion can BEST be
drawn from her observation?
Practice Question

Beth rubs an ebonite (polished
rubber) rod with wool. She finds that
the wool acquires a positive charge.
Which conclusion can BEST be
drawn from her observation?
Answer:
C. Many negatively charged particles
moved from the wool to the ebonite
rod.
Practice Question

If James were to pick up a negatively
charged metal ball and touch it to a
neutral metal disk, what would
happen?
Practice Question

If James were to pick up a negatively
charged metal ball and touch it to a
neutral metal disk, what would
happen?
Answer:
D. The metal disk would become negatively
charged.
Charge Polarization
Since there are no free electrons to
move throughout the material, there is
a rearrangement of the positions of
charges within the atoms and
molecules themselves.
Charge Polarization
This charge polarization does not last. Once the
balloon is removed, the molecules go back to
their usual positions.
Electric Charge
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Conservation of charge:
– Whenever something is charged, no
electrons are created or destroyed.
– Electrons are simply transferred from
one material to another.
Electric Charge
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Electric charge is a
property of tiny
particles in atoms.
The unit of electric
charge is the coulomb
(C).
1 charge of coulomb
is 6.24x1018 electrons
A quantity of charge
should always be
identified with a
positive or a negative
sign.
Electric Forces
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Electric forces are created between ALL
electric charges.
Because there are two kinds of charge
(positive and negative) the electrical
force between charges can attract or
repel.
Electric Forces
QuickTime™ and a
decompressor
are needed to see this picture.
Electric Forces
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Charge can be transferred by
conduction.
– Electrical Conduction: is the movement of
electrically charged particles through a
transmission medium.
Practice Question

The diagram below shows the electric force
between the electrons and the nucleus of an
atom.
-Which conclusion can be drawn about the
electric force?
QuickTime™ and a
decompressor
are needed to see this picture.
Practice Question

The diagram below shows the electric force
between the electrons and the nucleus of an
atom.
-Which conclusion can be drawn about the
electric force?
QuickTime™ and a
decompressor
are needed to see this picture.
Answer:
B. The electric force between the electrons
and the nucleus acts at a distance.
Coulomb’s Law
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Coulomb’s law relates the force
between two single charges
separated by a distance.
Constant
9 x109 N.m2/C2
Force
(N)
F = K q1 q2
r2
Charges (C)
Distance (m)
Coulomb’s Law
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The force
between two
charges gets
stronger as the
charges move
closer together.
The force also
gets stronger if
the amount of
charge becomes
larger
Coulomb’s Law
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The force between
two charges is
directed along the
line connecting
their centers.
Electric forces
always occur in
pairs according to
Newton’s third law,
like all forces.
Coulomb’s Law
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The force between
charges is directly
proportional to the
magnitude, or
amount, of each
charge.
Doubling one
charge doubles the
force.
Doubling both
charges quadruples
the force.
Coulomb’s Law
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The force between charges
is inversely proportional to
the square of the distance
between them.
Doubling the distance
reduces the force by a
factor of 22 = (4),
decreasing the force to
one-fourth its original value
(1/4).
This relationship is called
an inverse square law
because force and distance
follow an inverse square
relationship.
Practice Question
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The electrostatic force between two
charged objects is influenced by the
size of the charges and the distance
between the objects. Which
expression correctly describes the
magnitude of the force between two
charged objects?
Practice Question
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The electrostatic force between two
charged objects is influenced by the size
of the charges and the distance between
the objects. Which expression correctly
describes the magnitude of the force
between two charged objects?
Answer:
C. the electrostatic constant (k) multiplied by
the product of the charges divided by the
square of the distance between them
Fields & Forces
Charge creates an electric field that
creates forces on other charges.
Fields & Forces
Mass creates a gravitational field that
exerts forces on other masses.
Electric Field Lines
Electric Field Lines
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Show the direction of electric field
Show the intensity of the electric field
Fields & Forces
Practice Question
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Which diagram shows opposing
positive electric field lines?
QuickTime™ and a
decompressor
are needed to see this picture.
QuickTime™ and a
decompressor
are needed to see this picture.
Practice Question

Which diagram shows opposing
positive electric field lines?
Answer:
QuickTime™ and a
decompressor
are needed to see this picture.
Conductors
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All materials contain electrons.
Conductors are materials that allow charged
particles to pass through them easily.
The electrons are what carry the current in a
conductor.
Examples:
– Copper, silver, and other metals
Insulators
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The electrons in insulators are not
free to move—they are tightly bound
inside atoms.
Examples:
– Rubber, glass
Conductors & Insulators
 A semiconductor has a few free electrons
and atoms with bound electrons that act
as insulators.