Download Electrostatics (Chap. 22)

Part V: Electricity & Magnetism
Chapter 22
Electrostatics
5-May-17
Physics 1 (Garcia) SJSU
Electric Charges and Forces
There are two types of charge:
positive charges and negative charges
REPEL
REPEL
ATTRACT
Like charges repel, opposite charges attract.
5-May-17
Physics 1 (Garcia) SJSU
Electrons, Protons, & Atoms
Electrons carry negative charge.
Protons carry positive charge.
Electrons are loosely bound to atoms
Protons are tightly locked
within the nucleus.
Helium atom
Electrons
Protons
Objects are usually neutral but can
easily acquire or lose electrons to
become charged.
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Physics 1 (Garcia) SJSU
Charging by Friction/Contact
Electrons move easily
so an object can
become charged by
rubbing electrons off
the object’s surface.
• Brush your hair with a plastic
comb.
• Walk across a carpet with
plastic-soled shoes.
5-May-17
Physics 1 (Garcia) SJSU
Electrons
taken off of
fur onto
rubber rod
Demo: Electroscope
The “legs” of an
electroscope
separate when
the electroscope
is charged due to
the repulsion of
like charge on the
two legs.
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Physics 1 (Garcia) SJSU
Charged ebonite
rod deposits
negative charge
(electrons) onto
electroscope.
“Legs”
Demo: Dial Electroscope
Alternative design for electroscope
Neutral
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Charged
Demo: Van de Graff Generator
Van de Graff deposits
large quantities of
excess charge on its
globe.
A person with long hair
can become a human
electroscope.
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Conservation of Charge
Charge cannot be created nor destroyed.
Objects become charged by transfer of charges.
Electrons are rubbed off the hairs of
a piece of fur, collecting and
charging a plastic rod.
Is the fur also charged?
Rod & Fur
Neutral
Positive or negative.
+
++
++
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Conductors & Insulators
Materials, such as metals, in which electrons
flow freely are called electrical conductors.
Materials, such as plastic and wood, in
which electrons do not move easily are
called electrical insulators.
Materials, such as silicon, that can act as
conductors or insulators under different
conditions are called semiconductors.
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Physics 1 (Garcia) SJSU
Plastic Insulators
Plastic insulation on step of van de
Graff and charge wand keep charge
from leaving the metal balls.
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Plastic insulation on wire prevents
charge to leave wire except at the ends,
whereSJSU
the conducting metal is exposed.
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Demo: Tesla Coil
Air is an insulator but at high voltages it
can conduct electricity as lightning.
Portable
Tesla coil
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Tesla Coils
Very high voltages achieved by large Tesla coils.
Homemade, backyard model
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Fry’s Superstore, Fremont CA
Coulomb’s Law
Electric force has magnitude given by
(Electric Force) = (k) x
( Charge of Object A ) x ( Charge of Object B)
( Distance ) x ( Distance )
DISTANCE
Object
A
Force
Force
Equal and opposite forces
(Newton’s Third law)
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Physics 1 (Garcia) SJSU
Object
B
Demo: Induce the Can
Charges separate in the can. Force of attraction is
stronger since opposite charges are closer.
Can Rolls
+
Repulsion
Force
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Metal
Can
(Neutral)
+
+
+
+
+
Attraction
Force
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Charging by Induction
Can separate charges by inducing them to
opposite sides of a conducting object.
Start
Neutral
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Induce +’s
to one side
Allow –’s
to Escape
Physics 1 (Garcia) SJSU
Finish with
net + charge
Demo: Electrophorus
The electrophorus consists of
a plastic plate and a metal
disk with an insulated handle.
Bring metal disk to
plate; separate charge
Charge plastic plate
by rubbing it with fur
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++++++++++
------------Remove electrons;
plate positively charged
Demo: Electroscope & Induction
The legs of the
electroscope
separate when
charged rod
brought near the
electroscope.
Charge induction
pushes electrons
into the legs.
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Physics 1 (Garcia) SJSU
++
++
+
+
Negatively charged
rod repels electrons
so they move as far
away as possible.
“Legs”
Lightning Storms
Charge separates inside of
thunderclouds.
The lower part of the cloud
is negatively charged.
This induces a positive
charge on the ground.
When the voltage difference
is high enough, a
lightning bolt can occur.
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Physics 1 (Garcia) SJSU
Demo: Lightning Rod
Charge flows
readily to and
from sharp
metal points.
Purpose of a
lightning rod is
to discharge a
building before
lightning occurs.
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Discharge
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Lightning
Rods
Demo: Electric Pinwheel
Pinwheel turns as charge leaves the Van de Graff
from the points at the tips of the spokes.
Charged
Air molecules
Van de
Graff
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Physics 1 (Garcia) SJSU
Charge Polarization
Charge polarization occurs when the
positions of the electrons and
protons in an atom are skewed.
Neutral
Polarized
Charged
Rod
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Polarized
Atoms
Demo: Static “Cling”
Charged
Balloon
Polarization
Charged object attracts a neutral insulator by
inducing charge polarization in the neutral object.
Neutral
Wall
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Charged
Comb
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Neutral
Paper
Check Yourself
What causes a stream
of water to be
deflected when you
bring a charged
object next to it?
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Electric Field Lines
Electric field lines
indicate the
direction of
electric force if a
positive charge
was placed in the
electric field.
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Demo: Streamers
Paper streamers attached to the van de Graff
show the direction of the electric field.
Similar to hairraising demo
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Lab: Electric Field Lines
In lab you map out electric field lines and electric potential
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Electric Potential Energy
Potential energy associated with electric force, similar to
potential energy associated with gravitational force.
Gravity
Force
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Electric
Force
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Electric Potential
Electric potential is potential energy per charge,
like temperature is internal energy per molecule.
5000 °F ?
Check Yourself:
Is this safe?
Both situations are safe because the total amount of energy is small
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Physics 1 (Garcia) SJSU
Capacitors
Capacitors are used to store separated
positive and negative electric charges.
Basic design
is pair of
conducting
plates
insulated
from each
other.
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Industrial
capacitor
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Demo: Discharging a Capacitor
Charged capacitor holds enough charge to
light a light bulb when discharged.
Low voltage (low electric potential) but
LARGE amount of charge – Dangerous!
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Physics 1 (Garcia) SJSU