1. Static electricity

2. Static electricity
Benjamin Franklin was America’s first famous scientist.
He and many others were studying the properties of static electricity even in the 1700’s.
The study of electric charges at rest is called electrostatics.
Static electricity results from a tiny excess of positive or negative electric charge on an object.
Franklin’s portrait appears on the $100 bill.

3. An example Rub a balloon against a wool sweater.
This transfers some negative charges from the sweater to the balloon.
The sweater now has a net positive charge and the balloon has a net negative charge.
They will attract and stick together.

4. Electric charge
There are two kinds of electric charge: positive (+) and negative (–).
In electrically neutral objects, positive (+) charges are balanced by an equal number of negative (–) charges.

5. Opposites attract As with magnetic poles, unlike charges attract:
Like electric charges repel:

6. Electrostatic induction
It is also possible to charge an object without touching it.
Here, a negatively-charged rod repels negative charges and attracts positive charges. The sphere is now “charged”.
This process of separating charge is called electrostatic induction.

7. Demo: the electroscope
An electroscope uses induction to detect electric charge.

8. Summary: charging an object
There are three ways that objects typically become charged.
conduction: A charged object contacts an uncharged object and transfers charges to it.
friction: Different materials have different affinities for charge. Rubbing these materials together causes charges to transfer between the materials.
electrostatic induction: A charged object is brought close to a neutral object, resulting in charge separation in the neutral object.

9. Coulomb’s law

10. Where does charge come from?
All ordinary matter is made of atoms, and all atoms contain charged particles.
The atomic nucleus contains positively-charged protons.
Outside of the nucleus are tiny fast-moving, negatively charged particles called electrons.
Review

11. Where does charge come from?
The charge of an electron and
proton are exactly equal and
opposite.
A complete atom has equal numbers of protons and electrons.
Therefore, a complete atom has no net electric charge.
The unit of electric charge is the
coulomb (C)
Review

12. How big is a Coulomb? A coulomb is a very large amount of charge.
One coulomb equals the charge of 6 × 1018 electrons or protons.
Ordinary static electricity results from net charges of less than a millionth of a coulomb.
Review

13. Coulomb’s law Fe = electrostatic force (N)
Coulomb’s law quantifies the electric force:
where:
Fe = electrostatic force (N)
ke = Coulomb constant = 9.0×109 Nm2/C2
q1 = electric charge of object 1 (C)
q2 = electric charge of object 2 (C)
r = distance between the two objects (m)

14. Fe increases with charge
The electric force increases with increasing charge:

15. Fe increases with charge
The electric force increases with increasing charge:
Doubling one charge doubles the force:

16. Fe increases with charge
The electric force increases with increasing charge:
Doubling one charge doubles the force:
Doubling both charges quadruples the force:

17. Fe obeys an inverse square law
The electric force varies inversely with the
square of the distance between charges.
DOUBLING the distance reduces the force by a factor of ¼.
HALVING the distance increases the force by a factor of 4.

18. Test your knowledge
Two +1.0 μC charges are separated by a distance of 1.0 cm. What is the magnitude and direction of their mutual electric force?
Asked:
Given: r
Relationships:
Solution:

19. Test your knowledge
Two +1.0 μC charges are separated by a distance of 1.0 cm. What is the magnitude and direction of their mutual electric force?
Asked: force
Given: charge q1 and q2, distance r
Relationships:
Solution:
The force is repulsive.

20. Comparing forces: Fe and Fg
The electric forces between elementary particles are much much greater than their gravitational attraction.
The force of gravity is so much smaller in these situations that we say it is negligible—meaning you can ignore it.

21. Assessment
Two charged particles are located 1.0 m apart. Describe the direction for the electric force between them if the two charges are:
+1 μC and +1 μC
+1 μC and -1 μC
-1 μC and -1 μC

22. Assessment
Two charged particles are located 1.0 m apart. Describe the direction for the electric force between them if the two charges are:
+1 μC and +1 μC repulsive
+1 μC and -1 μC attractive
-1 μC and -1 μC repulsive

23. Assessment Two charged particles are located 1.0 m apart.
Calculate the magnitude of the electric force between them if the two charges are +1.0 μC and +1.0 μC.

24. Assessment Two charged particles are located 1.0 m apart.
Calculate the magnitude of the electric force between them if the two charges are +1.0 μC and +1.0 μC.
What would be the magnitude of the force if the charges were 50 centimeters apart?

25. Assessment Two charged particles are located 1.0 m apart.
Calculate the magnitude of the electric force between them if the two charges are +1.0 μC and +1.0 μC.
What would be the magnitude of the force if the charges were 50 centimeters apart?
Four times as strong: N