Download Electrostatics (aka “Static Electricity”)

Electrostatics
(aka
“Static Electricity”)
What is Electrostatics?
• Electrostatics is
the study of the
behavior of
stationary
charged objects.
It’s all about the electrons (in constant,
random motion around the nucleus) and
protons (stuck in the nucleus)
Negative
Electron
Nucleus:
positive
protons &
neutral
neutrons
The Atom & its particles…
• Electrons are negatively charged particles
moving around the nucleus (in orbitals)
• electron = e• Protons are positively charged particles
located inside the nucleus (w/the neutral
neutrons)
• proton = p+
• Electrons are able to be stripped off an
atom (friction: fur & pvc pipe)
• Protons are NOT able to leave an atom
because they’re stuck in the nucleus!
Types of Materials
1. Conductor: a material that
transfers charge easily (ex.
Metals: gold, silver, copper).
2. Insulator: a material that does
not transfer charge easily (ex.
Plastics, glass, cork)
3.
4.
5.
Semiconductors: somewhere between 1
& 2 (ex. Silicon, carbon, germanium).
Superconductors: some metals become
perfect conductors below certain
temperatures
Train Conductor: a person who drives a
train. 
Law of Conservation
of Electrical Charge
• Electrons may be
transferred from one
object to another, but
they will never disappear
or appear from nowhere.
• Electron abundant
= negative charge
• Electron deficient =
positive charge
Electrically charged objects...
• Exert a force on each other
• Unlike charges attract
• Like charges repel
There are
four methods
to charge an
object:
1. Charging by Friction
• using friction to remove electrons from one
object and placing them on the other object.
Result: two objects with opposite charge
• Walk across carpeted floor with wool socks and you can build up an excess
of charge (either on you or the carpet) and therefore, become charged.
• Fur becomes positively charged. Why?
• Rod becomes negatively charged. Why?
• Can use friction to remove electrons from fur and deposit onto rod.
2. Charging by Contact (or
Conduction)
• The process of giving
one object a net
electric charge by
placing it in contact
w/another object that is
already charged is
known as charging by
contact.
• Result: two objects with
same charge
3. Temporarily Charging by Induction
• An object may become
polarized (opposite ends)
while a charged object is
brought near. But, the
overall charge on this piece
of plastic (for example) is
neutral.
• Result: two objects with
opposite charge temporarily
• When the charged rod goes
away, the electrons
rearrange and disperse
evenly. Object is still
neutral.
4. Permanently Charging by Induction
• The process of giving one object a net electric
charge without touching the object to a second
charged object is called charging by induction.
Permanently Charging
by Induction
Polarization
5. Credit Card Charging:
• You may use Visa, Master Card, or
American Express
• Result: Debt from high interest rates!
Check out these animations!
• http://phet.colorado.edu/simulations/sims.p
hp?sim=John_Travoltage
• http://phet.colorado.edu/simulations/sims.p
hp?sim=Balloons_and_Static_Electricity
Van de Graaf
Generator
Electric Fields
• What is an electric field?
• A region of space where a charge exerts a force on
other charged objects. This field extends outward
and permeates all of space.
• Direction of Electric Field = Direction of Force the
field produces on a positive charge in the field.
• In comparison: What is a gravitational field?
• How to measure an electric field: Using a SMALL
POSITIVE test charge, move it around a charged
object (that is creating an Electric Field) and the
magnitude and direction of force acting on the small
positive test charge is measured.
• The strength of the field is measured based on the
force on the test charge.
F ..on..q
E
q
F
E
q
• F = The force felt by (acting on) the test charge, q, due
to the point charge’s electric field. (N)
• q = the charge of the test charge. (C)
• E = Electric Field Strength. (N/C) (Direction is based on
imagining a small positive charge in the field.)
• The Electric Field (E) at a given spot can exert a variety
of forces, depending on the magnitude of the test charge
placed there.
F/q
• E = F/q = F/q =
• E is constant for that particular point charge at a
given location (the Point Charge is the charge
whose field is being tested… the point charge is
creating the Electric Field. )
Compare
• Compare the electric field equation to the one we
used to measure Earth’s gravitational field
strength:
F
F
a
m
E
q
• So, Electric Fields are measured in N/C and are
a measure of the force felt by a small positive
test charge. (The test charge must be small in
charge so that we can ignore its electric field.)
Electric fields: Man holding point charge, girl
holding test charge.
Must click on 2nd link!
• http://phet.colorado.edu/simulations/sims.p
hp?sim=Electric_Field_Hockey
http://phet.colorado.edu/simulations/sims.ph
p?sim=Charges_and_Fields
http://phet.colorado.edu/simulations/sims.ph
p?sim=Electric_Field_of_Dreams