1. Electric Charge and Electric Field
AP Physics
Chapter 16

2. Electric Charge and Electric Field
16.1 Static Electricity

3. Static electricity – not moving Two types of charge
positive (+) when electrons are lost
negative (-) when electrons are gained
Objects can gain charges by rubbing
16.1

4. Unlike charges attract
16.1 Static Electricity
Like charges repel
Unlike charges attract
Law of Conservation of electric charge – the net amount of electric charge produced in a process is zero
16.1

5. Electric Charge and Electric Field
16.2 Electric Charge in the Atom

6. 16.2 Electric Charge in the Atom
Atoms are made of
positive protons
negative electrons
Electrons move from
one object to another when the objects
are rubbed
Eventually charges bleed off
objects because water
molecules are polar
16.2

7. Electric Charge and Electric Field
16.3 Insulators and Conductors

8. 16.3 Insulators and Conductors
Conductors – outer
electrons of atoms
are free to move
through the
material
Insulator – electrons
tightly held, do
not move
16.3

9. 16.3 Insulators and Conductors
Semiconductors – conduct electricity under some circumstances, don’t under other conditions
16.3

10. Electric Charge and Electric Field
16.4 Induced Charge; the Electroscope

11. 16.4 Induced Charge; the Electroscope
Induction – charging without
contact
Object is brought near a
charged object
Electrons move
Object is grounded
An electroscope measures if
an object has a charge on it
16.4

12. Electric Charge and Electric Field
16.5 Coulomb’s Law

13. Electric charges apply forces to each other From experiments
16.5 Coulomb’s Law
Electric charges apply forces to each other
From experiments
Force is proportional
to charge
Inversely proportional
to square of distance
16.5

14. Equation – gives magnitude of force
16.5 Coulomb’s Law
Equation – gives magnitude of force
Opposite charges – force directed toward
each other
Like charges – force directed away from each
other
Charge is measured in Coulombs
16.5

15. Charges are quantized – that is they come in discrete values
16.5 Coulomb’s Law
1 Coulomb is the amount of charge, that if placed 1 m apart would result in a force of 9x109 N
Charges are quantized – that is they come in discrete values
The constant k relates to the constant called the permittivity of free space
16.5

16. We will assume that charges are point charges - size is negligible
16.5 Coulomb’s Law
AP tends to write k as
We will assume that charges are point charges - size is negligible
16.5

17. Electric Charge and Electric Field
16.6 Solving Problems Involving Coulomb’s Laws and Vectors

18. 16.6 Coulomb’s Law and Vectors
Principle of Superposition – Electrostatic forces are added just like any other force
As vectors
16.6

19. Electric Charge and Electric Field
16.7 The Electric Field

20. Electrical forces act over distances Field forces, like gravity
16.7 The Electric Field
Electrical forces act over distances
Field forces, like gravity
Michael Faraday
electric field – extends
outward from every charge
and permeates all of space
The field is defined by the force
it applies to a test charge
placed in the field
16.7

21. The Electric field would then be Or q is the test charge
We can also say that
Remember that E is independent of the test charge.
The electric field is also a vector (free body diagrams are probably a good idea)
16.7

22. Electric Charge and Electric Field
16.8 Field Lines

23. To visualize electric fields Draw electric field lines
Direction of the lines is the
direction of force on a
positive test charge
The density of the lines
indicates relative
strength of the field
Note: the field density increase
as you get closer
16.8

24. For multiple charges, keep in mind
16.8 Field Lines
For multiple charges, keep in mind
Field lines indicate the direction of the field
The actual field is tangent to the field lines
The magnitude of the field is relative to the field line density
Fields start at positive and end at negatives
Field Lines
16.8

25. If the field is produced by two closely spaced parallel plates
16.8 Field Lines
If the field is produced by two closely spaced parallel plates
The field density is constant
So the electric field is
constant
16.8

26. Electric Charge and Electric Field
16.9 Electric Fields and Conductors

27. 16.9 Electric Fields and Conductors
For a conductor
The electric field inside a
conductor is zero (static)
Any net charge is distributed on the surface of a conductor
Electric field is always
perpendicular to the surface of a conductor
Charges concentrate at the
area of greatest
curvature
16.9