physics 123 16. electric charge & electric field 16.1 static electricity 16.2 electric charge in...
TRANSCRIPT
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.
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Coulomb’s Law
Like Charges repelUnlike Charges Attract
F = k Q q / r2
- +
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!