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1
Electric Forces and Fields
Charge
Coulomb's Law
Electric Fields
Conductors & Insulators
Parallel plates
Dipoles
2
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
3
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
5
Insulators
• Electrons cannot move through the material
• Electrons can be removed or added by friction
• Examples – Glass, Plastic
7
Charge Induction
• Separation of charge by the influence of an electric field
• Action over a distance
• Charges can be induced on insulators and conductors
8
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
9
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
10
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)
11
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
12
Coulomb's Law
• The force between two charged bodies
F12
is the force between two charges, Q1
and Q2,
separated by a distance r12. K is the Electrostatic Constant (9.0x109 Nm2/C2)
2
12
2112
r
QQK=F
13
cf. Newton's Law of Universal
Gravitation
• The force between two massive bodies
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)
2
12
2112
r
MMG=F
14
What is an Electric Field ?
It is a concept used to describe how electric forces
will act on a charged particle in space.
15
More on Electric Field lines
Lines follow the path of a freely moving positive charge
Originate at positive charges
Terminate at negative charges
16
Even more on electric fields
• Higher density of lines means higher field
• Field lines cannot cross
18
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)
22
1
r
QK=
r
QqK
q=
q
F=E
2r
QKE
EF q
19
Electrostatic fields are vectors
• The overall field on qC can be calculated using vector addtition:
qA
qB
qC
+ +
-
Field due to qA
Field due to qB Total field on qC
20
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
21
Electric field between “Infinite” parallel plates in a vacuum
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
Aε
Q=
A
Q=E
0
K4
22
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
23
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
24
Are charges spread evenly on the surface of a conductor?
Not necessarily
Depends on the shape
Tend to accumulate near the “pointy” ends.
25
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
26
Example Faraday cages
• Microwave ovens
• Antistatic bags for computer memory
• Shields on co-axial cable
27
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