94271774 c22 electrostatics
TRANSCRIPT
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Apr 7, 2023 Physics 1 (Garcia) SJSU
Chapter 22Electrostatics
Part V: Electricity & Magnetism
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Apr 7, 2023 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.
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Apr 7, 2023 Physics 1 (Garcia) SJSU
Electrons, Protons, & Atoms
Helium atom
Electrons
Protons
Electrons carry negative charge.
Protons carry positive charge.
Electrons are loosely bound to atoms
Protons are tightly locked
within the nucleus.
Objects are usually neutral but can easily acquire or lose electrons to become charged.
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Apr 7, 2023 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.
Electrons taken off of fur onto rubber rod
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Apr 7, 2023 Physics 1 (Garcia) SJSU
Demo: Electroscope
The “legs” of an electroscope separate when the electroscope is charged due to the repulsion of like charge on the two legs.
Charged ebonite rod deposits negative charge (electrons) onto electroscope.
“Legs”
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Apr 7, 2023 Physics 1 (Garcia) SJSU
Demo: Dial Electroscope
Alternative design for electroscope
Charged
Neutral
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Apr 7, 2023 Physics 1 (Garcia) SJSU
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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Apr 7, 2023 Physics 1 (Garcia) SJSU
Conservation of Charge
+++ ++
Rod & FurNeutral
Electrons are rubbed off the hairs of a piece of fur, collecting and charging a plastic rod.
Is the fur also charged?
Positive or negative.
Charge cannot be created nor destroyed.Objects become charged by transfer of charges.
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Apr 7, 2023 Physics 1 (Garcia) SJSU
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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Apr 7, 2023 Physics 1 (Garcia) SJSU
Plastic Insulators
Plastic insulation on step of van de Graff and charge wand keep charge from leaving the metal balls.
Plastic insulation on wire prevents charge to leave wire except at the ends, where the conducting metal is exposed.
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Apr 7, 2023 Physics 1 (Garcia) SJSU
Demo: Tesla Coil
Air is an insulator but at high voltages it can conduct electricity as lightning.
PortableTesla coil
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Apr 7, 2023 Physics 1 (Garcia) SJSU
Tesla Coils
Very high voltages achieved by large Tesla coils.
Homemade, backyard model Fry’s Superstore, Fremont CA
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Apr 7, 2023 Physics 1 (Garcia) SJSU
Coulomb’s Law
Electric force has magnitude given by
(Electric Force) = (k) x
ObjectA
ObjectB
( Charge of Object A ) x ( Charge of Object B)
( Distance ) x ( Distance )
DISTANCE
Force Force
Equal and opposite forces(Newton’s Third law)
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Apr 7, 2023 Physics 1 (Garcia) SJSU
Demo: Induce the CanCharges separate in the can. Force of attraction is
stronger since opposite charges are closer.
MetalCan
(Neutral)
Charged
Object
++
+++
+
AttractionForce
RepulsionForce
Can Rolls
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Apr 7, 2023 Physics 1 (Garcia) SJSU
Charging by Induction
Can separate charges by inducing them to opposite sides of a conducting object.
StartNeutral
Induce +’sto one side
Allow –’sto Escape
Finish withnet + charge
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Apr 7, 2023 Physics 1 (Garcia) SJSU
Demo: Electrophorus
The electrophorus consists of a plastic plate and a metal disk with an insulated handle.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
- - - - - - - - - - -+ + + + + + + + + +
+ + + + + + + + + +
- - - - - - - - - - Charge plastic plateby rubbing it with fur
Bring metal disk to plate; separate charge
Remove electrons; plate positively charged
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Apr 7, 2023 Physics 1 (Garcia) SJSU
Demo: Electroscope & Induction
The legs of the electroscope separate when charged rod brought near the electroscope.
Charge induction pushes electrons into the legs.
Negatively charged rod repels electrons so they move as far away as possible.
“Legs”
++++
++
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Apr 7, 2023 Physics 1 (Garcia) SJSU
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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Apr 7, 2023 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.
Discharge
LightningRods
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Apr 7, 2023 Physics 1 (Garcia) SJSU
Demo: Electric Pinwheel
Pinwheel turns as charge leaves the Van de Graff from the points at the tips of the spokes.
Van deGraff
ChargedAir molecules
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Apr 7, 2023 Physics 1 (Garcia) SJSU
Charge Polarization
Charge polarization occurs when the positions of the electrons and protons in an atom are skewed.
Neutral Polarized
ChargedRod
PolarizedAtoms
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Apr 7, 2023 Physics 1 (Garcia) SJSU
Demo: Static “Cling”
Charged object attracts a neutral insulator by inducing charge polarization in the neutral object.
ChargedBalloon
NeutralWall
Po
lari
zatio
n
Polarization
ChargedComb Neutral
Paper
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Apr 7, 2023 Physics 1 (Garcia) SJSU
Check Yourself
What causes a stream of water to be deflected when you bring a charged object next to it?
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Apr 7, 2023 Physics 1 (Garcia) SJSU
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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Apr 7, 2023 Physics 1 (Garcia) SJSU
Demo: Streamers
Paper streamers attached to the van de Graff show the direction of the electric field.
Similar to hair-raising demo
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Apr 7, 2023 Physics 1 (Garcia) SJSU
Lab: Electric Field Lines
In lab you map out electric field lines and electric potential
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Apr 7, 2023 Physics 1 (Garcia) SJSU
Electric Potential Energy
Potential energy associated with electric force, similar to potential energy associated with gravitational force.
GravityForce
ElectricForce
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Apr 7, 2023 Physics 1 (Garcia) SJSU
Electric Potential
Electric potential is potential energy per charge, like temperature is internal energy per molecule.
5000 °F ?
Both situations are safe because the total amount of energy is small
Check Yourself:Is this safe?
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Apr 7, 2023 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.
Industrial capacitor
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Apr 7, 2023 Physics 1 (Garcia) SJSU
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!