Topic 12 Static Electricity. contents Electric charges Laws of Electrostatics Electrostatic charging Conductors and Insulators Electric fields

Topic 12

Topic 12

Static ElectricityStatic Electricity

contentscontents

Electric charges Laws of Electrostatics Electrostatic charging Conductors and Insulators Electric fields Applications of Electrostatics Chapter Review

Electric charges Laws of Electrostatics Electrostatic charging Conductors and Insulators Electric fields Applications of Electrostatics Chapter Review

Two types of charges Two types of charges

electric charges electric charges

negative charges (-Q) include electrons and negative ions, and positive charges (+Q) include protons and positive ions

Insulators are discharged by passing them quickly through a bunsen flame.

examples:

negative charges (-Q) include electrons and negative ions, and positive charges (+Q) include protons and positive ions

Insulators are discharged by passing them quickly through a bunsen flame.

examples:

Negative chargeNegative charge Positive chargePositive charge

polythene (rubbed with wool) perspex (rubbed with wool)

ebonite (rubbed with fur) glass (rubbed with silk)

electric charge electric charge

has a SI unit of coulomb (C)

charge carried by an electron is 1.6 x 10-19 C

has a SI unit of coulomb (C)

charge carried by an electron is 1.6 x 10-19 C

electric charges and the electric forces between them

electric charges and the electric forces between them

The The law of electrostatics law of electrostatics statesstates that that like charges repel and unlike charges attract.like charges repel and unlike charges attract.

+ +

++ --

+ +

+ -

strong repulsion

further weaker repulsion

strong attraction

further weaker attraction

Law of electrostaticsLaw of electrostatics

electrostatic chargingelectrostatic charging

simplified structure of neutral atom

+-ep

-

-

-

-

-

-

-

--

every atom has a massive, positively-charged nucleus

positive charge on nucleus = total negative charge on electrons

no net charge in the atom (neutral)

every atom has a massive, positively-charged nucleus

positive charge on nucleus = total negative charge on electrons

no net charge in the atom (neutral)

when one or more electrons are removed from an atom, atom is ionised and becomes a positive ion

atom with excess electrons becomes negatively charged ion

oppositely charged ions attract each other

when one or more electrons are removed from an atom, atom is ionised and becomes a positive ion

atom with excess electrons becomes negatively charged ion

oppositely charged ions attract each other

certain amount of negative (electrons) is transferred from the

wool to the polythene strip

When two different materials (especially insulating materials) are rubbed together, negative charges (electrons) will transfer from one object to another.

When two different materials (especially insulating materials) are rubbed together, negative charges (electrons) will transfer from one object to another.

before rubbing after rubbing

each material with equal number of positive and

negative charges

polythene strip

wool

A. Charging by rubbingA. Charging by rubbing

Polythene and perspex (cellulose acetate) can become charged with static electricity when rubbed with a dry woollen cloth.

Polythene and perspex (cellulose acetate) can become charged with static electricity when rubbed with a dry woollen cloth.

Two different types of charges can be produced by friction on the strips

polythene (-)

perspex (+)

attract

polythene (-)

polythene (-)

repel



• balloon, clothing & wall

• carpet, human body & door knob

Refer to resources from links at physics wikihttp://nyghsec3physics.pbworks.com

B. Charging by inductionB. Charging by induction1. Using 2 conducting spheres & a charged

rod: both given opposite charges (p. 320, Textbook)

2. Using 1 conducting sphere, a positively charged inducing specimen (a rod) & earthing: to give sphere negative charges

3. Using 1 conducting sphere, a negatively charged inducing specimen (a rod) & earthing: to give sphere positive charges

1. Using 2 conducting spheres & a charged rod: both given opposite charges (p. 320, Textbook)


3. Using 1 conducting sphere, a negatively charged inducing specimen (a rod) & earthing: to give sphere positive charges

Step 1 Step 2

Step 5 Step 4

Step 3


B. Charging by inductionB. Charging by induction

http://www.s-cool.co.uk/gcse/physics/static-and-current-electricity/revise-it/static-electricity


B. Charging by inductionB. Charging by induction No physical contact between the source of charge

(inducing specimen) and the conductors.

Can be repeated many times without loss of charge from the inducing specimen.

Only electrons (negative charge) can flow.

Earthing provides a path for electrons to flow:

Away from a negatively-charged conductor, or

Towards a positively-charged conductor

E.g. touching with hand or a wire

No physical contact between the source of charge (inducing specimen) and the conductors.

Can be repeated many times without loss of charge from the inducing specimen.

Only electrons (negative charge) can flow.

Earthing provides a path for electrons to flow:

Away from a negatively-charged conductor, or

Towards a positively-charged conductor

E.g. touching with hand or a wire

• Between conductors

• Charging pith balls1. Charged glass rod touches

neutral pith balls2. Charge transferred to pith balls

of the same sign as on the rod repulsion

3. Another charged rod brought near pith balls, attraction opposite charge on rod compared to pith balls

C. Charging by contactC. Charging by contact

E.g. 1

Two metallic spheres are suspended by insulating threads as shown in the diagram below. They have the same number of opposite charges and they are brought together until they touch. As a result, they neutralize each other electrostatically.

++ + +

- - - -

(continued)Draw on the diagram below to show the correct charge distribution in the two spheres after neutralization.

Both spheres become electrically neutral (uncharged).

E.g. 1

A and B are identical metal-coated balloons.

(a) A and B attract when A is rubbed (–ve charge). Why? What is the resultant charge on B?

+ve and –ve charges are induced on B as shown on the diagram.

A B

––

–+

++

––

–

Attraction between A and +ve induced charge greater than repulsion between A and –ve induced charge they attract.B is neutral (has no resultant charge)

nylon thread

E.g. 2

A and B are identical metal-coated balloons.

(b) If B is rubbed in the same way as A, what happens? Why?A B

––

––

––

When B is rubbed, it has the same sign of charge as A.

A B

––

–

–––

Objects of the same sign of charge repel A and B repel each other.

Van de Graff GeneratorVan de Graff Generator


• How the Van de Graff generator works

The hairs of a girl stand on-ends when she touches the dome of Van de Graaff generator.(a) Why do the girl’s

hairs stand on-ends?

The ends of the girl’s hair are given the same sign of charge. They repel each other and stand on-ends.

E.g. 3

She is insulated from the ground when standing on a plastic stool.

(b) Why does she stand on plastic stool?

(c) What if she stands on ground?

If she stands on the ground, she is earthed. The dome will be discharged through her body, giving her an electric shock. Her hairs will not stand on-ends.

(d) What if she stands on a plastic stool and touches a boy on ground?

Both the girl and the boy will get an electric shock. Her hairs will not stand on-ends.

jooliak

413/414 stopped here on 5 Feb

To neutralise an accidentally charged apparatus, remove these charges by discharging it.

To neutralise an accidentally charged apparatus, remove these charges by discharging it.

Removing electric charges Removing electric charges

for insulators: pass the apparatus quickly through a bunsen flame

for conductors: touch the apparatus to allow the excess charges to flow through our body (called earthing).

for insulators: pass the apparatus quickly through a bunsen flame

for conductors: touch the apparatus to allow the excess charges to flow through our body (called earthing).

Removing electric charges Removing electric charges

conductors and insulatorsconductors and insulators

conductorsconductors

materials that allow electric charges to flow through them easily

‘free electrons’ flow when an electrical force is applied to them

examples: metals, graphite (except diamond), solutions (acids, bases, salts), ionised gases and water

Uses: electrical circuits, lightning conductors

materials that allow electric charges to flow through them easily

‘free electrons’ flow when an electrical force is applied to them

examples: metals, graphite (except diamond), solutions (acids, bases, salts), ionised gases and water

Uses: electrical circuits, lightning conductors

conductors and insulatorsconductors and insulators

insulatorsinsulators

materials that do not allow electric charges to flow through them easily

electrons are tightly bound to atoms; not free to move

examples: rubber, glass, most plastics, dry wood and dry air

materials that do not allow electric charges to flow through them easily

electrons are tightly bound to atoms; not free to move

examples: rubber, glass, most plastics, dry wood and dry air

The triboelectric series

• A list that ranks various materials according to their tendency to gain or lose electrons.

• It usually lists materials in order of decreasing tendency to charge positively (lose electrons), and increasing tendency to charge negatively (gain electrons).

• http://www.siliconfareast.com/tribo_series.htm

Electric fieldElectric field An electric field is a region where an

electric charge experiences an electric force.

An electric field is a region where an electric charge experiences an electric force.

•Electric fields between 2 charges

•Directions of electric field and force

•Electric fields of all configurations

•Electric field of combination of charges

•Electric fields between 2 charges

•Directions of electric field and force

•Electric fields of all configurations

•Electric field of combination of charges

Download, install and use Electric field.exe 2.01 from http://www.physics-software.com/software.html

electrostatic hazardselectrostatic hazards

Sometimes objects get dangerously charged by accident.

Trucks which transport petrol or other inflammable liquids usually have a metal chain or conductive strip at the rear end dangling from the metal body to the ground.

Sometimes objects get dangerously charged by accident.

Trucks which transport petrol or other inflammable liquids usually have a metal chain or conductive strip at the rear end dangling from the metal body to the ground.

electrostatic hazardselectrostatic hazards many synthetic fibres (like nylon and acrylic)

used in clothing are good insulators and are easily charged by rubbing

people may pick up charges as they walk on carpets made of synthetic fibres

many synthetic fibres (like nylon and acrylic) used in clothing are good insulators and are easily charged by rubbing

people may pick up charges as they walk on carpets made of synthetic fibres

in some situations like dry environment, sparks may be produced and the clothing may catch fire

in some situations like dry environment, sparks may be produced and the clothing may catch fire

A –ve charge is put at P. What is the direction of the electric force on the charge?A Towards the right.

B Towards the left.

C No electric force.

P

uniform electric field

E.g. 4

What are the electric field lines due to a –ve charge?

A B

C There is no electric field if only one type of charge is present.

– –

E.g. 5

How do the field lines show the direction and the strength of an electric field?

The arrows of the field lines show the direction of the force acting on a ________ charge at that point.

And the denser the field lines, the __________ is the electric field.

positive

stronger

E.g. 6

applications of electrostaticsapplications of electrostaticsphotocopierphotocopier

operates on the principles of electrostatics

makes uses of a metal selenium (a photoconductor) which conducts when it is in the light, and is an insulator when it is in the dark



drum is charged as it rotates

under the rod

charged metal rod

selenium coated drum

drum is evenly charged

applications of electrostaticsapplications of electrostatics

photocopierphotocopier





toner particles are attracted to

the charged part

only the dark areas remain

chargedtoner

applications of electrostaticsapplications of electrostaticsphotocopierphotocopier





heat is supplied to

‘fix’ the toner

toner is transferred to

the paper


electrostatic paint spraying electrostatic paint spraying

Paint droplets from an aerosol become charged by rubbing against the nozzle of the spray. The car body is earthed during spraying.

Paint droplets from an aerosol become charged by rubbing against the nozzle of the spray. The car body is earthed during spraying.

1. Why the paint droplets spread out as they leave the nozzle?

2. Why is the car body earthed during spraying? 3. State one advantage of using electrostatic

paint spraying over normal spraying.

E.g. 7

1. As paint droplets leave the aerosol, they become charged by rubbing against the nozzle of the spray. Like charges repel, so the droplets repel each other and spread out evenly.

2. The droplets would be attracted to the earthed car body.

3. This gives a more even coating, ensuring that the paint reaches even the most inaccessible parts.

1. As paint droplets leave the aerosol, they become charged by rubbing against the nozzle of the spray. Like charges repel, so the droplets repel each other and spread out evenly.

2. The droplets would be attracted to the earthed car body.

3. This gives a more even coating, ensuring that the paint reaches even the most inaccessible parts.


electrostatic precipitatorelectrostatic precipitator

The electrostatic precipitator is commonly used to clean the smoke coming out from industrial chimneys by removing fine ash and other dust from the waste gases.

chimney wall

ash and dust collects on plate

charged metal rod

earthed metal plate

near the charged rods, air is ionised;

the dust and ash pick up the ions

and are attracted to the earthed plates

waste gases carrying ash and

dust


Like charges repelLike charges repelUnlike charges attract Unlike charges attract

Potential hazardsPotential hazards• sparkssparks• firesfires

Method of chargingMethod of charging• frictionfriction• inductioninduction• contactcontact

ApplicationApplication• photocopierphotocopier• spraying of paintspraying of paint

looks likelooks like

are found inare found in

is used byis used by

obey lawsobey laws

beware beware ofof

• conductorsconductors• insulatorsinsulatorsElectric fieldElectric field

can be produced bycan be produced by

formform

consists ofconsists of

++++ ++

++++ ++

++++ --

-- --

---- --

----

Positive and Positive and negative chargesnegative charges

Static ElectricityStatic Electricity

Documents

Topic 12 Static Electricity. contents Electric charges Laws of Electrostatics Electrostatic charging Conductors and Insulators Electric fields