physics 123 16. electric charge & electric field 16.1 static electricity 16.2 electric charge in...

Physics 123

16. Electric Charge & Electric Field

16.1 Static Electricity

16.2 Electric Charge in the Atom

16.3 Insulators and conductors

16.4 Induced Charge -- Electroscope

16.5 Coulomb’s Law

16.6 Problem Solving

16.7 Electric Field

16.8 Electric Field Lines

Example 16.1 . . . Static

When one rubs a plastic comb with a cloth

A. The comb and the cloth acquire opposite charges

B. The comb and the cloth acquire the same kind of charge

C. The comb is charged but the cloth is not

D. The cloth is charged but the comb is not

Solution 16.1 . . . Static

Plastic is negatively charged

Cloth is positively charged

- - - - - - - -

+ +

+ + +

+ +

Glass and Silk

Glass is positively charged

Silk is negatively charged

+ + + + + + +

- - -

- - -

- -

Conservation of Charge

Charge is transferred not created!

In other words …

Total Charge in the Universe is Constant!

Example 16.2 . . . Material World

Plastic and glass are examples of

A. ConductorsB. SemiconductorsC. Insulators (non-conductors)D. Metals

Solution 16.2 . . . Material World

Plastic and glass are examples of

A. ConductorsB. SemiconductorsC. Insulators (non-conductors)D. Metals

Charges move freely in conductorsCharges do not move in insulators

Example 16.3 . . . Polarization

A charged comb will attract small pieces of paper. This is because

A. The comb has a net positive charge and the paper has a net negative charge

B. The comb has a net negative charge and the paper has a net positive charge

C. The comb has a net charge and the paper has a net negative charge.

D. The comb has a net charge and the paper has no net charge.

Solution 16.3 . . . Polarization

A charged comb will attract small pieces of paper. This is because an opposite charge is induced by polarization in the uncharged (neutral ) piece of paper!

D. The comb has a net charge and the paper has no net charge.

- - - - - - - -

+ + - -

+ + - -

+ + - -

Electroscope

Gold leaves separate to indicate the presence of charge.

- - - - - - - -

Coulomb’s Law

Like Charges repelUnlike Charges Attract

F = k Q q / r2

- +

Qq

r

Example 16.4 . . . Coulomb’s Law

The positive charge is located at a distance of 1 cm from the - 1 C charge and 2 cm from the + 4 C charge. What is the net force experienced by

the positive charge?

q1 = - 4 C q2 = - 1 Cq3 = + 1 C

Solution 16.4 . . . Coulomb’s Law

F = k Q q / r2

F31 = 9x10 9 x 4x10-6 x 1x10-6 / (2x10-2)2

F31 = 90 N

F32 = 9x10 9 x 1x10-6 x 1x10-6 / (1x10-2)2

F32 = 90 NNet Force is zippo … rhymes with hippo!

q1 = - 4 C q2 = - 1 Cq3 = + 1 C

Electric Field

E = F /q

Electric Field at a point in space is the force per unit positive charge

A charge Q will create an electric field around it.

F = k Q q / r2

E = k Q / r2

Problem 16.5 . . . Electric Field

Where in the world is E = 0?

q1 = - 4 C q2 = - 1 C

Solution 16.5 . . . Electric Field

E = 0 where the is twice as far from the bigger charge!

q1 = - 4 C q2 = - 1 C

Problem 16.6 . . . Electric Field Lines

Which Electric Field Pattern is incorrect?

+ -

+ +

Solution 16.6 . . . Electric Field Lines

Correct Electric Field Pattern

+ +

That’s all folks!