1. ConcepTest 16.1a Electric Charge I
1) one is positive, the other is negative
2) both are positive
3) both are negative
4) both are positive or both are negative
Two charged balls are repelling each other as they hang from the ceiling. What can you say about their charges?

2. ConcepTest 16.1a Electric Charge I
1) one is positive, the other is negative
2) both are positive
3) both are negative
4) both are positive or both are negative
Two charged balls are repelling each other as they hang from the ceiling. What can you say about their charges?
The fact that the balls repel each other only can tell you that they have the same charge, but you do not know the sign. So they can be either both positive or both negative.
Follow-up: What does the picture look like if the two balls are oppositely charged? What about if both balls are neutral?

3. Topic 9 Static Electricity. Electric Charge and Its Conservation
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4. Contents of Topic 9
Static Electricity; Electric Charge and Its Conservation
Insulators and Conductors
Induced Charge; the Electroscope
Coulomb’s Law
Solving Problems Involving Coulomb’s Law and Vectors
The Electric Field- Electric Field Lines
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5. Static electricity- Electric Charge and Its Conservation
The word electricity comes from the Greek word elektron, which means “amber.”
Amber is petrified tree resin, and the ancients knew that if you rub a piece of amber with a cloth, the amber attracts small pieces of leaves or dust.
Static electricity: “amber effect”
Objects can be charged by rubbing
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6. Static Electricity; Electric Charge and Its Conservation
An object becomes “charged” as a result of rubbing, and is said to possess a net electric charge.
All charged objects fall into one of two categories.
Charge comes in two types, positive and negative; like charges repel and opposite charges attract
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7. Static Electricity; Electric Charge and Its Conservation
The two types of electric charge were referred to as positive and negative by the American statesman, philosopher, and scientist Benjamin Franklin (1706–1790).
Franklin argued that whenever a certain amount of charge is produced on one object, an equal amount of the opposite type of charge is produced on another object.
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8. Static Electricity; Electric Charge and Its Conservation
During any process, the net change in the amount of charge produced is zero.
When a plastic ruler is rubbed with a paper towel, the plastic acquires a negative charge and the towel acquires an equal amount of positive charge.
The charges are separated, but the sum of the two is zero.
This is an example of the law of conservation of electric charge: The net amount of electric charge produced in any process is zero
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9. Electric Charge in the Atom
Atom is electrically neutral.
Rubbing charges objects by moving electrons from one to the other.
When objects are charged by rubbing, they hold their charge only for a limited time and eventually return to the neutral state. Where does the charge go?
Usually the excess charge “leaks off” onto water molecules in the air.
This is because water molecules are polar—that is, their charge is not distributed uniformly.
© 2014 Pearson Education, Inc.