Download Ch. 22 (Electrostatics)

Atomic Structure
Physics 10
Electrostatics
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electric charge ⇒ that property to which
the attractive and repulsive behavior of
protons and electrons is attributed
⇒ We do not know what charge is, we only
know that there are two types of charge.
⇒ The two types of charge were called
positive (+) and negative (-) by Benjamin
Franklin in 1747.
nucleus: consists of
protons (+ charge) and
neutrons (no charge)
surrounding the
nucleus are
electrons (- charge)
Important Facts About Atoms
⇒ Every atom is composed of a positively charged
nucleus surrounded by negatively charged electrons.
⇒ The electrons, of all atoms are identical; they have
the same mass and the same charge (also true of protons
and neutrons)
⇒ Protons have the same charge as electron (but
opposite signs) but have about 1800 times more mass.
Neutrons have a little more mass than protons but they
have no charge.
⇒ Most atoms are neutral because they have the same
amount of protons as electrons.
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Exercise 15
opposite charges attract
like charges repel
Conductors
It is relatively easy to strip the outer electrons
from a heavy atom like that of uranium but
very difficult to remove inner electrons. Why
do you suppose this is so?
⇒ For the outer electrons, the attractive force
of the nucleus is largely canceled by the
repulsive force of the inner electrons. The inner
electrons fell the full force of the nucleus, and a
large force is required to remove them.
Insulators
⇒ One or more of the outer electrons in a
conductor are free to wander within the metal.
⇒ Electrons are tightly bound to the atoms
in an insulator.
ex: copper, aluminum, silver, gold
ex: plastic, rubber, wood, pure water
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Exercise 22
Two equal charges exert equal forces on
each other. What if one of the charges has
twice the magnitude of the other. How do
the forces they exert on each other
compare?
⇒ The forces they exert on each other are
still the same! Newton’s third law applies
to electrical forces as well as all forces.
Charging by Friction
The three ways to charge an object
charging by friction ⇒ electrons are
transferred by friction when one material
rubs against another
charging by contact ⇒ electrons are
transferred from one material to another by
simply touching them together (no rubbing)
charging by induction ⇒ the process of
charging one object without touching it with
another charged object
Example of charging by friction
Objects can gain or
lose electrons by
being rubbed
together.
⇒ electron are rubbed off of the man’s shoes
and onto the rug (charge is conserved)
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Another example of charging by friction
⇒ electrons are rubbed off the man’s head
onto the balloon (charge is conserved)
Charging by induction
Charging by contact
⇒ Electrons are transferred from the rod to
the metal sphere by simply touching them
together.
Lightning
⇒ Charging by induction occurs during thunderstorms.
⇒ The negatively charged
bottom of clouds induce a
positive charge on the Earth’s
surface.
⇒ No contact was ever made with the charged
object!
⇒ Lightning is the electrical
discharge between a cloud and
the ground or between
oppositely charged parts of
clouds.
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Lightning (cloud to ground lightning)
Lightning (cloud to ground lightning)
⇒ If the negatively charged stepped leader meets a
positively charged upward streamer, a path is formed
and electrons rush from the cloud to the ground. This is
a lightning strike!
⇒ Before lightning strikes, negatively charged stepped
leaders come down from the cloud and positively
charged upward streamers rush upwards from objects on
the ground.
Lightning (cloud to ground)
⇒ In both of these picture, you can see a upward
streamer rushing up from the ground.
Lightning (cloud to cloud lightning)
⇒ Most lightning actually occurs between oppositely
charged parts of clouds.
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Lightning (ground to cloud lightning)
⇒ Lightning can also strike from the ground up to a
cloud.
Lightning (ball lightning)
⇒ There have also been reports of ball lightning, which
usually appears as a mysterious glowing sphere which
drifts horizontally through the air.
Why objects lose electric charge
Induced charge
⇒ water vapor and other molecules running
into a charged object can eventually remove all
excess charge from the object
induced charge: positive and negative charges in
the molecules of an insulating material become
slightly separated
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Since the negative charges are closer
to the (positively) charged comb than
the positive charges, the paper feels a
net attractive force (closeness wins).
⇒ The balloon sticks to the wall because the positive
charges in the wall (which exert an attractive force)
are closer to the balloon the negative charges in the
wall (which exert a repulsive force). Closeness wins!
Electric Field
⇒ Electric fields are represented by field lines.
⇒ The field lines point in
the direction of the electric
field.
⇒ The closer together the
field lines, the stronger the
electric field.
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Some Electric Fields
⇒ Excess
charge placed on a conductor
will move to the surface of the conductor
because the electrons mutually repel each
other.
⇒ Excess charge will arrange itself on the
surface of the conductor such that the electric
field inside the conductor is zero.
⇒ It will only be evenly distributed if the
conductor is spherical.
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The Milagro Gamma-Ray Detector
The Milagro Gamma-Ray Detector
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