Electric Forces and Fields Charge Coulomb's Law Electric Fields - - PowerPoint PPT Presentation

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Electric Forces and Fields

 Charge  Coulomb's Law  Electric Fields  Conductors & Insulators  Parallel plates  Dipoles

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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

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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 –

• pposite 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
• f 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 2 1 12

r Q Q K = 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 2 1 12

r M M G = 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)

2 2

1 r Q K = r Qq K q = q F = E

2

r Q K E 

E F 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

A ε Q = A Q = E K 4

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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

• bject, and spread out.

Charges move so that the field lines are always perpendicular to the surface

• f 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.