Chapter 21 New

2006 Physics 2112 Fundamentals of Physics Chapter 21 1

Fundamentals of Physics

Chapter 21 Electric Charge

1. Electromagnetism

2. Electric Charge

+-

3. Conductors & Insulators4. Coulomb s Law5. Charge is Quantized

e6. Charge is Conserved

Review & SummaryQuestionsExercises & Problems


Atoms Have No Net Charge

electrons

protons

neutrons


Electric Charge

Two types of charge:

Un-likes attract

Likes repel


Electric Charge

A small fraction of the electrons in the fur stick to the rod.


Electric Charge Experimentation


Conductors & Insulators

Neutral Atomsnegative electronspositive nucleus

protonsNeutrons

ConductorOutermost electrons are free to move

Conduction electronsInsulator

All electrons stick with a nucleus

Semi-conductors


Static Electricity


Electroscope


Checkpoint 1


Coulomb s Law

221

r

qqkF


Coulomb s Law

221

02

21

4

1

r

qq

r

qqkF

22120

229

/1085.8

/1099.8

mNC

CmNk

space free ofty Permittivi

ConstantsCoulomb'

0

k


Electrostatic Force >> Gravitational Force

221

r

qqkF

229 /1099.8 CmNk

2211 /1067.6 kgmNG

Principle of superposition applies to both forces:

nnet FFFFF ,14,13,12,1,1

221

r

mmGF


Definition of Electric Charge

The SI unit of charge is the CoulombOne coulomb of charge is the current transferred through the cross section of a wire in 1 second when there is a current of 1 ampere in the wire

dq = i dt


Checkpoint 2


q1 = 1.60 x 10-19 Cq2 = 3.20 x 10-19 CR = 0.0200 mWhat is magnitude and direction of F12

Sample Problem

Add particle three to configuration 1q3 = -3.20 x 10-19 Cdistance ¾ RWhat is F1, net

Add particle four to configuration 1q4 = -3.20 x 10-19 Cdistance ¾ R at 600

What is F1, net


Checkpoint 3


Sample Problem

q1 = +8q at originq2 = -2q at distance L

Where can a proton be placed such that is in equilibrium (net force = 0)Is the equilibrium stable or unstable?


Sample Problem

Initially q = 0 on Bq = +Q on A

A and B are connected for a short time by a wire.Electrostatic force between spheres?

A is now momentarily grounded.Electrostatic force between spheres?


22-4 Charge on a Conducting Sphere

Like charges will repel each other and spread uniformly over a metal sphere.

A shell of uniform charge attracts or repels a charged particle that is outside the shell as if all of the shell s charge were concentrated at its center.

If a charged particle is located inside a shell of uniform charge, there is no net electrostatic force on the particle from the shell.


Checkpoint 4


Charging by Induction

Touching metal spheres close to a charged object

Separate the metal spheres while close to the charged object

Remove the charged object

Oppositely charged spheres


Charging by Induction

A negatively charged sphere


Charge is Quantized

Electron charge

Proton charge

Neutron has no net charge.

-1.60217733 x 10-19C

+1.60217733 x 10-19C


Charge is Conserved

electrons

positrons

aka anti-electrons

e+ + e-


Charge QuantizationThe charge on the electron:

e- = - 1.60 x 10-19CAll observed charges are multiples of |e-|.

Charge ConservationCharges are only created in equal positive and negative amounts.Net charge is conserved in the universe.

Conductors & InsulatorsAtomic Structure: nucleus + a cloud of electrons

Conductors - outermost electrons are free to moveInsulators - all electrons are bound to atoms

Charge Characteristics

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