Transcript Introduction to Energy Concepts

Electrostatics
Physics
Electric Charge
• Term “Charge” has new meaning from when it was
first defined.
• Charge Carriers are…
• Electrons, protons, ions
• Charge is quantized
• Indivisible discreet amounts of charge carried by electrons /
protons.
• Units
• Coulomb; 1 Coulomb = 6.25 x 1018 electrons
• Charge of electron is 1.6 x 10-19 C
• Law of Conservation of Charge:
• Charge cannot be destroyed nor created, only transferred
from one object to another.
Transfer of Charge
• Conductors: Atoms / Molecules with several loosely bound
electrons in outer energy levels
• Ex’s: Gold, Silver, Copper, Aluminum, …
• Insulators: Atoms / Molecules whose electrons are more
“tightly” bound to the nucleus.
• Ex’s: Hydrocarbons (plastic, acrylic, rubber, ..), Air, Wood
• Semiconductors: Atoms / Molecules that have one loosely
bound electron for every few atoms
• Transistors: Act as switches; by applying a small electric field –
more or less electrons enter the material making it either a
conductor (on) or insulator (off).
Semiconductor doping
3 ways objects get “Charged”
1. Charging by Contact / Friction:
•
•
•
Electrons are “scraped” off one material and adhere to
another (happens with insulators)
Poor John Travolta
Tape repel
2. Charging by Induction:
•
A charged object is used to separate the charges in a
nearby conductor. Then the opposite side of the
conductor is “grounded”
3. Charge Polarization
•
A charged object is held near another object and
“polarizes” the atoms / molecules in the other object.
GROSS.
• How do cockroaches climb walls?
What is the purpose of hair
conditioner?
Electric Force: Coulomb’s Law
• Fe = k (q1 q2 ) / r2
• k = 9 x 109 N m2/C2
• Ex:
• Magnitude of electric force compared to gravity.
• Calculate the electric force between an electron and
proton in a Hydrogen atom, r = 5 x 10-12 m. Electron
mass = 9.11 x 10-31 kg, Proton mass = 1.67 x 10-27 kg.
• Fe =
• Fg =
Reminder: G=6.67e-11
Tinsel Levitation
• Determine the gravitational force pulling down on a strand
of tinsel.
• By using the induction method, charge a pie tin positively
and DROP a loop of tinsel onto the pie tin and observe
what happens.
• Why does the tinsel attract to the pie tin and then repel?
• Approximate the height above the pie tine that the tinsel
levitates.
• What is the net force on the tinsel?
• What is the Electric Force supporting the tinsel?
• By assuming the charge on the tinsel and the charge on the
pie tine to be the same, what the amount of charge on
each?
Electric Field
• Definition: an area of “space” in which if a
charged object were placed in, it would experience
a force.
• Magnitude:
• E = Fe / qtest
• (Honors) E = k q1 / r2
• Direction: the direction of the field is taken to be
the direction of the force felt by a positive test
charge.
• Shielding: Inside of a conductor, the electric field
is always zero!
• Ex’s:
“Anatomy of a Lightning
Bolt”
• Rising Air, Water Droplets, and / or Ice crystals gain or loose
electrons leaving the bottom of the cloud typically (-).
• The bottom of the cloud induces a charge on the surface of the
ground below.
• “Leaders” branch out from the top of the cloud, the base of the cloud
and from the ground.
• When the “leaders” connect, the air ionizes and becomes a good
conductor.
• The Ionized air then allows for an avalanche of electrons to flow
from (-) to (+).
• Tall, pointed objects on the ground prevent charge from building up
• If a lightning is produced it typically strikes tall, pointed objects
(trees, poles, buildings,..)
•To be safe, stay away from tall objects, crouch down into a “ball”
Electric Potential Energy
• PEe = q E d
• Similar to gravitational PE (mgh)
• Assumes Electric field is constant through the
distance, d.
• (Honors) If distance d is considerable as
compared to the size of the object that is
establishing the field, then use other PE
equation!
• (Honors) PEe = k (q1 q2) / r
Electric Potential
• Electric Potential = Electric Potential Energy per
Coulomb of charge
• V = PEe / q
• Honors: V = k q / r
– NOTE: PE and V are scalars not vectors!
• Ex: 120 Volts means that there is 120 Joules of
energy for every Coulomb of charge present.
• Ex’s
Capacitance
• Capacitors store PEe by separating positive and
negative charge by a small distance.
• The smaller the separation distance the more energy they can
store per charge.
• The larger the surface area the more charge they are able to
separate.
• Capacitance is used as a description of how much
charge a “capacitor” can separate per Volt applied to
it.
• C = Q / V (Honors)
• Ex’s
• Net Charge of a capacitor is always ZERO – They
store SEPARATED charge (equal amounts of + and -)
Capacitors come in many sizes
Warm-up
1. Consider two objects with the following initial
conditions. The charge on each is +q, the distance
between them is R, and the electric force acting
between them is F. If the distance between the
objects were halved, the force between them would
be what?
2.If the charge on both objects were doubled and the
distance between the objects were quartered, the
force between them would be what?
3. What is a capacitor?
4. What is voltage? What does voltage measure?
Batteries
• Act as an “electric pump” – They do not
store charge!!!
• Because of the different affinity for
electrons between different metals, charge
will move from one metal to another.
• This movement is of electrons and ions – so
there needs to be a medium in between the
metals that allows for the movement on ions
(electrolytes, acids, etc..)
• https://www.khanacademy.org/partnercontent/mit-k12/mit-k12-physics/v/theinvention-of-the-battery