94271774 C22 Electrostatics

Apr 7, 2023 Physics 1 (Garcia) SJSU

Chapter 22Electrostatics

Part V: Electricity & Magnetism


Electric Charges and Forces

There are two types of charge:

positive charges and negative charges

REPEL REPEL ATTRACT

Like charges repel, opposite charges attract.


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.


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


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”


Demo: Dial Electroscope

Alternative design for electroscope

Charged

Neutral


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.


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.


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.


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.


Demo: Tesla Coil

Air is an insulator but at high voltages it can conduct electricity as lightning.

PortableTesla coil


Tesla Coils

Very high voltages achieved by large Tesla coils.

Homemade, backyard model Fry’s Superstore, Fremont CA


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)


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


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


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


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”

++++

++


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.


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


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


Charge Polarization

Charge polarization occurs when the positions of the electrons and protons in an atom are skewed.

Neutral Polarized

ChargedRod

PolarizedAtoms


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


Check Yourself

What causes a stream of water to be deflected when you bring a charged object next to it?


Electric Field Lines

Electric field lines indicate the direction of electric force if a positive charge was placed in the electric field.


Demo: Streamers

Paper streamers attached to the van de Graff show the direction of the electric field.

Similar to hair-raising demo


Lab: Electric Field Lines

In lab you map out electric field lines and electric potential


Electric Potential Energy

Potential energy associated with electric force, similar to potential energy associated with gravitational force.

GravityForce

ElectricForce


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?


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


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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94271774 C22 Electrostatics