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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

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What is charge ?

• Basic property of matter

• Carried by Electrons (-ve) and Protons (+ve)

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Insulators

• Electrons cannot move through the material

• Electrons can be removed or added by friction

• Examples – Glass, Plastic

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Conductors

Electrons are free to

move through the

material

Example – metals,

graphite

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Charge Induction

• Separation of charge by the influence of an electric field

• Action over a distance

• Charges can be induced on insulators and conductors

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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

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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

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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)

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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

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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

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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

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What is an Electric Field ?

It is a concept used to describe how electric forces

will act on a charged particle in space.

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More on Electric Field lines

Lines follow the path of a freely moving positive charge

Originate at positive charges

Terminate at negative charges

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Even more on electric fields

• Higher density of lines means higher field

• Field lines cannot cross

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Pictures of electric fields

Unlike charges

attract

Like charges repel

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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)

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1

r

QK=

r

QqK

q=

q

F=E

2r

QKE

EF q

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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

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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

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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

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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

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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

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Are charges spread evenly on the surface of a conductor?

Not necessarily

Depends on the shape

Tend to accumulate near the “pointy” ends.

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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

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Example Faraday cages

• Microwave ovens

• Antistatic bags for computer memory

• Shields on co-axial cable

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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

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Homework

• Knight PROBLEMS page 681

• 52, 53, 56, 57, 58, 60, 63, 64.