Transcript Electric Charge

Electric Charge and Electric Field
AP Physics
Chapter 16
Electric Charge and Electric Field
16.1 Static Electricity
16.1 Static Electricity
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
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
Electric Charge and Electric Field
16.2 Electric Charge in the Atom
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
Electric Charge and Electric Field
16.3 Insulators and Conductors
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
16.3 Insulators and Conductors
Semiconductors – conduct electricity under
some circumstances, don’t under other
conditions
16.3
Electric Charge and Electric Field
16.4 Induced Charge; the Electroscope
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
Electric Charge and Electric Field
16.5 Coulomb’s Law
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
q1q2
F k 2
r
k  8.988 x10 Nm / C
9
2
2
16.5
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
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
e  1.602 x10
19
C
The constant k relates to the constant called
the permittivity of free space
 0  8.85 x10 C / Nm
12
2
2
16.5
16.5 Coulomb’s Law
AP tends to write k as
k
1
4 0
We will assume that charges are point
charges - size is negligible
16.5
Electric Charge and Electric Field
16.6 Solving Problems Involving Coulomb’s Laws
and Vectors
16.6 Coulomb’s Law and Vectors
Principle of Superposition – Electrostatic
forces are added just like any other force
As vectors
16.6
Electric Charge and Electric Field
16.7 The Electric Field
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
16.7 The Electric Field
The Electric field would then be
F
Or
E
kq
E 2
r
q
q is the test charge
We can also say that F  Eq
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
Electric Charge and Electric Field
16.8 Field Lines
16.8 Field Lines
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
16.8 Field Lines
For multiple charges, keep in mind
1. Field lines indicate the direction of the field
The actual field is tangent to the field lines
2. The magnitude of the field is relative to the
field line density
3. Fields start at positive and end at
negatives
Field Lines
16.8
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
Electric Charge and Electric Field
16.9 Electric Fields and Conductors
16.9 Electric Fields and Conductors
For a conductor
1. The electric field inside a
conductor is zero (static)
2. Any net charge is distributed on the surface of
a conductor
3. Electric field is always
perpendicular to the surface of a conductor
2. Charges concentrate at the
area of greatest
curvature
16.9