2. 21.1 Electric forces
Electric forces are created between all electric charges.
Because there are two kinds of charge (positive and negative) the electrical force between charges can attract or repel.

3. 21.1 Electric forces
The forces between the two kinds of charge can be observed with an electroscope.
An electroscope contains two very thin leaves of metal that can swing from a central rod connected to a metal ball. Charges can flow freely between the ball and the leaves.
Suppose a positively charged rod touches the metal ball of an electroscope.
Some negative electrons are attracted to the rod.
The metal ball and leaves of the electroscope are left with a net positive charge.
Since both leaves have the same positive charge, the leaves repel each other and spread apart.
Once an electroscope is charged, it can be used to test other charged objects.
The leaves spread farther apart if another positively charged rod is brought near the metal ball.
This happens because the positive rod attracts some negative electrons from the leaves toward the ball, increasing the positive charge on the leaves.

4. 21.1 Electric forces Charge can be transferred by conduction.
If a negatively charged rod touches the ball, the opposite effect occurs.
A negatively charged rod repels negative electrons from the ball into the leaves
where they neutralize some of the positive charge. The positive charge on the
leaves is reduced and the leaves reduce their repulsion.

5. 21.1 Electric current
The direction of current was historically defined as the direction that positive charges move.
Both positive and negative charges can carry current.
In conductive liquids (salt water) both positive and negative charges carry current.
In solid metal conductors, only the electrons can move, so current is carried by the flow of negative electrons.

6. 21.1 Electric current
Current is the movement of electric charge through a substance.
Charge that flows
(coulombs)
Current
(amps)
I = q t
Time (sec)

7. 21.1 Calculate current
Two coulombs of charge pass through a wire in five seconds.
Calculate the current in the wire.
1) You are asked to find the current.
2) You are given the charge and the time.
3) Use the equation I = q/t.
4) Solve: I = (2 C) ÷ (5 sec) = 0.4 C/sec or 0.4 A

8. 21.1 Conductors and insulators
All materials contain electrons.
The electrons are what carry the current in a conductor.
The electrons in insulators are not free to move—they are tightly bound inside atoms.

9. 21.1 Conductors and insulators
A semiconductor has a few free electrons and atoms with bound electrons that act as insulators.

10. 21.1 Conductors and insulators
When two neutral objects are rubbed together, charge is transferred from one to the other and the objects become oppositely charged.
This is called charging by friction.
Objects charged by this method will attract each other.
A charged balloon will stick to a (neutral) wall or other insulating surface.
When a negatively charged balloon is near a wall, electrons inside atoms in the wall are repelled.
Since the wall is made of insulating material, the repelled electrons are not free to travel between atoms.
The electrons can move within each atom, so they spend more time on the side of the atom that is farthest from the balloon.

11. The atoms become polarized; one end is positive and the other is negative.
Atoms in a material only become polarized if the material is an insulator.
In a conductor, electrons are free to move from atom to atom so the entire object becomes polarized.
This is why a balloon sticks to a wood door but not a metal doorknob.

12. Attraction/Repulsion
Nothing
or Attraction (electrons move) + - + - + + -+ - - +-

13. 21.2 Coulomb's Law
Coulomb’s law relates the force between two single charges separated by a distance.
Constant
9 x109 N.m2/C2
Force
(N)
F = K q1 q2 r2
Charges (C)
Distance (m)

15. 21.2 Coulomb's Law
The force between two charges gets stronger as the charges move closer together.
The force also gets stronger if the amount of charge becomes larger.

16. 21.2 Coulomb's Law
The force between two charges is directed along the line connecting their centers.
Electric forces always occur in pairs according to Newton’s third law, like all forces.

17. 21.1 Coulomb's Law
The force between charges is directly proportional to the magnitude, or amount, of each charge.
Doubling one charge doubles the force.
Doubling both charges quadruples the force.

18. 21.1 Coulomb's Law
The force between charges is inversely proportional to the square of the distance between them.
Doubling the distance reduces the force by a factor of 22 = (4), decreasing the force to one-fourth its original value (1/4).
This relationship is called an inverse square law because force and distance follow an inverse square relationship.

19. 21.2 Calculating force
Two balls are each given a static electric charge of one ten-thousandth (0.0001) of a coulomb.
Calculate the force between the charges when they are separated by one-tenth (0.1) of a meter.
Compare the force with the weight of an average 70 kg person.
1) You are asked to calculate the force and compare it to a person’s weight.
2) You are given the charges and separation, and the mass of the person.
3) Use Coulomb’s law, F= -Kq1q2/R2, for the electric force and F=mg for the weight.
4) Solve:
F = (9×109 N•m2/C2)(0.0001C)(.0001C) ÷ (0.1 m)2 = 9,000 N
The weight of a 70 kg person: F = mg = (70 kg)(9.8 N/kg) = 686 N
The force between the charges is 13.1 times the weight of an average person (9,000 ÷ 686).

20. 21.2 Fields and forces
Gravitational forces are far weaker than electric forces.

22. 21.2 Drawing the electric field

23. 21.2 Electric fields and electric force
On the Earth’s surface, the gravitational field creates 9.8 N of force on each kilogram of mass.
With gravity, the strength of the field is in newtons per kilogram (N/kg) because the field describes the amount of force per kilogram of mass.