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Chapter 22Electrostatics

Part V: Electricity & Magnetism

Lecture 23

Electricity• Electricity is the name given to phenomena

involving electric charge, such as– lightning– electric power– the force holding atoms together– electronic devices

• Electrostatics involves electric charges at rest – the forces between them, – the “aura” that surrounds them– their behavior in materials.

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Electric Charge (q)Fundamental property of elementary particles

There are two types of charge: positive charge and negative charge

SI Unit of charge: Coulomb (C)– 1 C is a HUGE charge; usually have µC or nC

Conservation of Charge: Net charge cannot be created or destroyed

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Electrons, Protons, & Atoms

Helium atom

Electrons

Protons

Protons carry positive charge.Electrons carry negative charge.

Protons are tightly locked together with neutrons in the atomic nucleus.

Electrons occupy “shells” around the nucleus:

• Inner shell electrons—attracted very strongly to oppositely charged atomic nucleus

• Outer shell electrons - attracted loosely; can sometimes be dislodged

Atoms are usually neutral but can acquire or lose electrons to become charged.

Electric Charge (q)

• Charge “quantum”: e = 1.602 x 10-19 C– Proton charge = e– Electron charge = -e

• Can transfer electrons from one material to another by rubbing, etc., to get net charge

• Electrons can move through some materials– Conductors: Some electrons move easily– Insulator: Limited or no electron motion

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Charging by Friction/ContactOuter shell electrons can be

transferred from one object to another, so an object can be charged by rubbing electrons onto or 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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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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Electric ForceAny two objects with net electric charge

exert electric force on each other.The electric force can be attractive or

repulsive.

REPEL REPEL ATTRACT

Like charges repel; opposite charges attract.

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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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Demo: Van de Graff Generator

The 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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Coulomb’s LawCoulomb’s Law describes the electric force

between two charged objects.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)

Coulomb’s law (continued)• In equation form:

where k = 9 x 109 Nm2/C2 is the electric constant, q1 is charge on first particle, q2 is charge on second particle, d is distance between particles

• Unit of charge is coulomb, C• Force Direction: If the charges are alike in sign,

the force is repelling; if the charges are opposite in sign, the force is attractive.

• Similar to Newton’s law of gravitation for masses• Underlies the bonding forces between atomic

electrons and atomic nuclei

F = kq1q2

d2

According to Coulomb’s law, a pair of charged particles that are placed twice as far apart will experience electric forces that areA. half as strong.B. one-quarter as strong.C. twice as strong.D. 4 times as strong.

Coulomb’s LawCHECK YOURSELF

According to Coulomb’s law, if the amount of charge on one of the particles is doubled, the electric forces will be

A. half as strong.B. one-quarter as strong.C. twice as strong.D. 4 times as strong.

According to Coulomb’s law, a pair of charged particles that are placed twice as far apart will experience electric forces that areA. half as strong.B. one-quarter as strong.C. twice as strong.D. 4 times as strong.

Coulomb’s LawCHECK YOURSELF

According to Coulomb’s law, if the amount of charge on one of the particles is doubled, the electric force will be

A. half as strong.B. one-quarter as strong.C. twice as strong.D. 4 times as strong.

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Conductors & InsulatorsMaterials such as metals, in which some

electrons can 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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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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Demo: Tesla Coil

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

PortableTesla coil

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Tesla CoilsVery high voltages achieved by large Tesla coils.

Homemade, backyard modelFry’s Superstore, Fremont CA

When you buy a water pipe in a hardware store, the water isn’t included. When you buy copper wire, electrons

A. must be supplied by you, just as water must be supplied for a water pipe.

B. are already in the wire.C. may fall out, which is why wires are insulated.D. None of the above.

Conductors and InsulatorsCHECK YOURSELF

When you buy a water pipe in a hardware store, the water isn’t included. When you buy copper wire, electrons

A. must be supplied by you, just as water must be supplied for a water pipe.

B. are already in the wire.C. may fall out, which is why wires are insulated.D. None of the above.

Conductors and InsulatorsCHECK YOURSELF

Methods of Charging• Charging by friction and contact.

Example:Stroking cats fur, combing your hair, rubbing your shoes on a carpet

– Electrons transfer from one material to another by simply touching.

• Charging by induction – If you bring a charged object near a conducting

material, electrons are made to move in the conductor, even without physical contact.

– If electrons move and are then trapped in the new location, the conductor gets a net charge

ChargingInduction: Consider two insulated metal spheres A and B.a. They touch each other, so in effect they form a single

uncharged conductor. b. When a negatively charged rod is brought near A, electrons in

the metal, being free to move, are repelled as far as possible until their mutual repulsion is big enough to balance the influence of the rod. The charge is redistributed.

c. If A and B are separated while the rod is still present, each will be equal and oppositely charged.

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Charging by Induction & “Grounding”Can separate charges by inducing electrons to one side of

a conducting object and then allowing them to escape into your finger. When finger is removed, positive charge is trapped on conducting object.

StartNeutral

Induce electronsto one side

Allow electronsto Escape

Finish withnet + charge

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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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Lightning StormsCharge 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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Demo: Lightning Rod

Charge flows readily to and from sharp metal points.

Purpose of a lightning rod is to discharge a building beforelightning occurs.

Discharge

LightningRods

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Demo: Electric PinwheelPinwheel turns as charge leaves the Van de Graff

from the points at the tips of the spokes.

Van deGraff

ChargedAir molecules

Electric Polarization of Atoms• If one side of an atom or molecule is induced to become

more negative (or positive) than the opposite side, the atom or molecule is said to be electrically polarized.

• The electrons surrounding the atomic nucleus form an electron “cloud”.

a. The center of the negative cloud normally coincides with the center of the positive nucleus in an atom

b. When an external negative charge is brought nearby (to the right), the electron cloud is distorted so that the centers of negative and positive charge no longer coincide. The atom is now electrically polarized

Polarization• If the charged rod is negative,

then the electron cloud of each atom or molecule in the blue object is pushed in a direction away from the rod.

• The object becomes electrically polarized.

Attraction by Polarization• When a charged comb is

brought nearby, molecules in the paper are polarized.

• The sign of charge closest to the comb is opposite to the comb’s charge.

• Charges of the same sign are slightly more distant. Closeness wins, and the bits of paper experience a net attraction to the comb.

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Demo: Static “Cling”Charged objects attract neutral objects by inducing

electric polarization in the neutral object.

ChargedBalloon

NeutralWall

Pol

ariz

atio

nPolarization

ChargedComb Neutral

Paper

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Check Yourself

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

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Demo: Polarize the CanCharges separate in the can. Force of attraction is

stronger since opposite charges are closer.

MetalCan

(Neutral)Charged

Object

+ ++++

+Attraction

Force

RepulsionForce

Can Rolls

Key Points of Lecture 23Key Points of Lecture 23

Before Friday, read Hewitt Chap. 22 (second half).

Homework #16 due by 11:00 PM Friday Oct. 22.

• Electric charge• Electric force & Coulomb’s Law• Separation & Conservation of Charge• Conductors & Insulators• Charging by Contact and by Induction• Electric Polarization