ConcepTest 16.1a Electric Charge I
Two charged balls are
repelling each other as
they hang from the ceiling.
What can you say about
their charges?
1) one is positive, the other
is negative
2) both are positive
3) both are negative
4) both are positive or both
are negative
ConcepTest 16.1a Electric Charge I
Two charged balls are
repelling each other as
they hang from the ceiling.
What can you say about
their charges?
1) one is positive, the other
is negative
2) both are positive
3) both are negative
4) both are positive or both
are negative
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?
Topic 9
Static Electricity. Electric Charge
and Its Conservation
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
Static electricityConservation
Electric
Charge
and
Its
• 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
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
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.
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
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.