Electric Forces and Fields

Charge

Coulomb's Law

Electric Fields

Conductors & Insulators

Parallel plates

Dipoles
1
Friction causes these effects

Pollen sticks to bees

Dust sticks to TV




Static cling of
clothes
Shocks touching
metal door handles
Sparks stroking cats
Friction is producing
some type of force
2
Experimental evidence has lead to
the Electric Charge Model
•
Friction between objects can cause charge to be
added or lost
•
Charge has two kinds - Positive and Negative
•
Charges exert force
–
like charges repel
–
opposite attract
•
The force acts over a distance (non-contact)
•
Neutral objects have an equal mixture of +ve and
-ve charges
3
What is charge ?
•
Basic property of
matter
•
Carried by
Electrons (-ve)
and Protons (+ve)
4
Insulators
•
Electrons cannot move through the material
•
Electrons can be removed or added by
friction
•
Examples – Glass, Plastic
5
Conductors


Electrons are free to
move through the
material
Example – metals,
graphite
6
Charge Induction
•
Separation of charge by the influence of an
electric field
•
Action over a distance
•
Charges can be induced on insulators and
conductors
7
Charge Induction in Conductors



A charged object can
induce a charge in a
conductor
The separation of
charges is called
charge polarization
Causes an attractive
polarization force
8
Charge Induction in Insulators
•
An external charge
displaces the
electron cloud
around an atom
•
Causes a net
attractive force
•
Creates an electric
dipole - equal
charges separated
by small distance
9
Electric Charge Model (contd.)
•
Two types of material
–
Conductors – charge moves easily through
–
Insulators – charges are stuck
•
Charge is conserved – just like Energy and
Momentum
•
Charge is quantized – it comes in multiples
of small units (-1.6x10-19 Coulombs)
10
What is the Electric Force ?

One of the fundamental forces of nature (like
gravity)

Like charges repel, unlike charges attract

Proportional to amount of electric charge

Decreases with distance between charges

Strength and direction are quantified by
Coulomb's Law
11
Coulomb's Law
•
The force between two charged bodies
Q1Q2
F12 = K 2
r12
F12 is the force between two charges, Q1 and Q2,
separated by a distance r12. K is the Electrostatic
Constant (9.0x109 Nm2/C2)
12
cf. Newton's Law of Universal
Gravitation
•
The force between two massive bodies
M 1M 2
F12 = G 2
r12
F12 is the force between two masses, M1 and M2,
separated by a distance r12. G is the Gravitational
Constant (6.67x10-11 Nm2/kg2)
13
What is an Electric Field ?

It is a concept used to describe how electric forces
will act on a charged particle in space.
14
More on Electric Field lines

Lines follow the path of a freely moving
positive charge

Originate at positive charges

Terminate at negative charges
15
Even more on electric fields
•
Higher density of lines means higher field
•
Field lines cannot cross
16
Pictures of electric fields
Unlike charges
attract
Like charges repel
17
Electric Field, E, Strength and
Direction


Defined as the force on a positive unit
charge, or force per unit charge.
Units are Newtons per Coulomb (N/C)
F 1 Qq
Q
E= = K 2 = K 2
q q r
r
F  qE
Q
EK 2
r
18
Electrostatic fields are vectors
• The overall field on qC can be calculated using
vector addtition:
qC
+
Field due to qA
+
qA
Field due to qB
Total field on qC
-
qB
19
Electric field between “Infinite”
parallel plates in a vacuum

Very large plates each with charges +Q and Q, with a small gap relative to the area of the
plates
20
Electric field between “Infinite”
parallel plates in a vacuum
Q
Q
E = 4K =
A ε0 A
Epsilon Ɛ0 is the permittivity
constant 8.85x10-12 Nm2/C2 for a
vacuum
Note: The Electric field is
independent of the distance
between the plates
21
Conductors and Electric Fields



Charge moves freely within conductors
The excess charges will repel each other to
reach a stable equilibrium
The charges collect at the surface of the
object, and spread out.
Charges move so that the
field lines are always
perpendicular to the surface
of the conductor
22
Electric Field inside conductors


Any charge inside a
conductor would move to
cancel out any electric field
The electric field inside a
conductor is therefore
always ZERO after it has
reached electrostatic
equilibrium
23
Are charges spread evenly on the
surface of a conductor?



Not necessarily
Depends on the
shape
Tend to accumulate
near the “pointy”
ends.
24
Conductors inside Electric Fields


The charges inside a
conductor in an
electric field will flow
to reach static
equilibrium – until the
field inside the
conductor is ZERO.
Called a Faraday
cage
25
Example Faraday cages
•
Microwave ovens
•
Antistatic bags for computer memory
•
Shields on co-axial cable
26
Summary




Charge is a fundamental property of matter
Charges exert forces described by
Coulomb's law
Electric Fields are used to describe the
forces on a unit electric charge in space
Charge flows to cancel out the field inside
conductors
27
Homework
•
Knight PROBLEMS page 681
•
52, 53, 56, 57, 58, 60, 63, 64.
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