Chap 16 StaticE 12 S

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09/22/20

Static Electricity

a.

a. state that there are positive and negative charges and that charge state that there are positive and negative charges and that charge

is measured in coulombs is measured in coulombs

b.

b. state that unlike charges attract and like charges repelstate that unlike charges attract and like charges repel

c.

c. describe an electric field as a region in which an electric charge describe an electric field as a region in which an electric charge

experiences a force experiences a force

d.

d. draw the electric field of an isolated point charge and recall that draw the electric field of an isolated point charge and recall that

the direction of the field lines gives the direction of the force the direction of the field lines gives the direction of the force

acting on a positive test charge acting on a positive test charge

e.

e. draw the electric field pattern between 2 isolated point chargesdraw the electric field pattern between 2 isolated point charges

f.

f. show understanding that electrostatic charging by rubbing show understanding that electrostatic charging by rubbing

involves a transfer of electrons involves a transfer of electrons

g.

g. describe experiments to show electrostatic charging by inductiondescribe experiments to show electrostatic charging by induction

h.

h. describe examples where electrostatic charging may be a describe examples where electrostatic charging may be a

potential hazard potential hazard

i.

i. describe an example of the use of electrostatic charging e.g. describe an example of the use of electrostatic charging e.g.

photocopier and laser printer

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Law of Electrostatics

 _{Electric charge}_{Electric charge}

• When two different materials are rubbed When two different materials are rubbed together, negative charges (electrons) together, negative charges (electrons) will transfer from one object to the other. will transfer from one object to the other.

• One object will be positively charged One object will be positively charged while the other will be negatively

while the other will be negatively charged.

charged.

• Eg. When polythene strip is rubbed with _{Eg. When polythene strip is rubbed with} wool, electrons will move from wool to wool, electrons will move from wool to polythene strip. Which is the negatively polythene strip. Which is the negatively charged object?

charged object?

Acetate gains positive charge

Cellulose acetate with wool

Polythene strip gains negative charge

Polythene strip with wool (or cloth)

Ebonite gains negative charge

Ebonite rod rubbed with fur

Perspex gains positive charge

Perspex strip with soft cloth

Glass gains positive charge

Glass rod rubbed with silk

Result

Result

Action

• You may want to find out what is a gold leaf electroscope (optional)You may want to find out what is a gold leaf electroscope (optional)

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Law of Electrostatics



Structure of an atom

•

Positively charged nucleus and negatively charged

electrons.

•

When the atoms loses electrons, it becomes a positive

ion, or positively charged. Vice versa.

•

Force of attraction exist between positive and negative

charges



Electric charge

•

SI unit: coulomb (C)

•

The amount negative charge carried by an electron is

1.6 x 10

-19-19

C.

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Law of Electrostatics



Attraction and repulsion

•

Recall that polythene gains negative charge and Perspex

gains positive charge when rubbed with soft cloth.

•

When the two strips that are bought close together repel

each other, it shows that both have the same type of

charges. Vice versa.

•

The nearer the strips are, the stronger the forces

•

Law of electrostatic:

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Conductors and Insulators



Conductors

• Materials that allow electric charges to flow through them easily_{Materials that allow electric charges to flow through them easily}

• All points on the surface of conductors have same potential_{All points on the surface of conductors have same potential}

• Electrons are held loosely to their atoms_{Electrons are held loosely to their atoms}

• Existence of charged particles_{Existence of charged particles}

• Includes metals, some forms of carbon, solutions and ionized gas_{Includes metals, some forms of carbon, solutions and ionized gas}



Insulators

• Do not allow electric charges to flow through easily_{Do not allow electric charges to flow through easily}

• Electrons held tightly and not free to move, though they can be _{Electrons held tightly and not free to move, though they can be}

transferred by rubbing.

• Localised charging_{Localised charging}

• Includes rubber, glass, plastic. …_{Includes rubber, glass, plastic. …}



Earthing

• To _Toneutraliseneutralise a charged object by providing a path for negative a charged object by providing a path for negative charged object to flow to or away from the object

charged object to flow to or away from the object

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Principles of Electrostatics (P)



Charging by induction

1.

Two neutral insulated metal spheres

are in contact with each other

2.

A negative-charged polythene strip is

bought near to P, the negative charge

on P are repelled away from the strip

and move onto sphere Q.

3.

P has positive charge and Q has

negative charge.

4.

With the polythene strip still in place, P

and Q are separated.

5.

The strip is then removed.

6.

P and Q now possess equal amount of

opposite charges.

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Principles of Electrostatics (P)



Charging by Earthing (-ve charge)

1.

Positively charged acetate strip brought near

to neutral conductor.

2.

Negative charges attracted to the strip and

end P, and a positive charges forms at end Q.

3.

Conductor is earthed by touching Q with a

finger.

4.

Negative charges flow from the earth to

neutralize the positive charge at Q.

5.

The finger is removed, followed by the acetate

strip.

6.

Conductor is left with negative charge.

7.

Charge induced is opposite that on the

charged strip.

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Discussion (P)



How would obtain a positively charged object using

the Earthing process?



Additional questions from pg. 316

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Principles of Electrostatics



Electric field

•

A region where an electric charge

experiences an electric force.

•

Direction of the field is defined as

the direction of the force on a small

positive charge

•

Strongest near the charged object

producing it.

•

The closer the line of force, the

stronger the electric field.

•

When field lines are parallel and

evenly spaced, field is uniform.

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Electrostatic Hazard (P)



Lightning

• Base of cloud is negatively charged_{Base of cloud is negatively charged}

• Induced positive charges to form on the _{Induced positive charges to form on the} ground or buildings

ground or buildings

• ions_ions forms in the air, may become a good forms in the air, may become a good conductor

conductor

• If attraction is strong enough, the cloud _{If attraction is strong enough, the cloud} may discharge, and charges are pull may discharge, and charges are pull through the air

through the air



Use of lighting conductor

• Sharp spikes allows concentration of _{Sharp spikes allows concentration of} positive charges

positive charges

• Provide steady path for negative charges _{Provide steady path for negative charges} to move to the earth

to move to the earth

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Straits Times, 1 Jan 07

COMPUTER analyst Pink Thomas, 38, was concerned that a missing COMPUTER analyst Pink Thomas, 38, was concerned that a missing

lightning conductor at her Housing Board block in Choa Chu Kang lightning conductor at her Housing Board block in Choa Chu Kang could put lives at risk.

could put lives at risk.

She said: "I returned from an overseas posting a month ago and She said: "I returned from an overseas posting a month ago and

noticed that 2m of the copper strip that link the lighting rod to the noticed that 2m of the copper strip that link the lighting rod to the ground had been removed."

ground had been removed."

She called the Hong Kah Town Council, which oversees the estate. It She called the Hong Kah Town Council, which oversees the estate. It said it has reported the theft to the police; A spokesman said the said it has reported the theft to the police; A spokesman said the council replaces missing conductors promptly but declined to council replaces missing conductors promptly but declined to provide further details.

provide further details.

Miss Thomas said: "There have been frequent thunderstorms the past Miss Thomas said: "There have been frequent thunderstorms the past

few days and I'm afraid that the lightning could pose a danger to few days and I'm afraid that the lightning could pose a danger to the residents since the lightning conductors would not carry the the residents since the lightning conductors would not carry the electricity into the ground."

electricity into the ground."

Professor Liew Ah Choy of the electrical and computer engineering department at the National University of

Singapore described it as a serious safety problem.

He explained that when lightning energy is attracted to the

building's roof, it must be conveyed into the ground where it

can be dispersed safely. "

can be dispersed safely. "If there is a gap in the conductor, If there is a gap in the conductor, the energy will try to jump across and the flashover could the energy will try to jump across and the flashover could result in fire. It might jump to a person nearby with fatal result in fire. It might jump to a person nearby with fatal results,"

results," said Prof Liew, who is also chairman of the said Prof Liew, who is also chairman of the

Singapore Code of Practice for Lightning Protection, which

sets standards for lightning safety here.

The Building and Construction Authority said that if the "integrity of the system is compromised because of missing

conductors; it is the responsibility of the building owner to rectify the problem quickly". "We understand that the

Hong Kah Town Council is replacing the copper lightning conductor today," it said last Saturday.

Safety issues are irrelevant to thieves who steal the copper for sale as scrap metal. These thieves also stole more

than 550 dry riser fittings, valued at about $19,600, on Dec 11 from 30 HDB blocks in Jurong West and Hong

Kah.

Without these metal fittings, firelighters cannot draw water to douse high-rise fires and the Singapore Civil Defence

Force had to devise a special clamp to connect their fire hoses to dry risers with missing fittings.

Prices of several metals are at all-time high to feed the growing demand throughout Asia, particularly China. For

stealing public property, like lightning conductors or dry riser fittings from a town council area, offenders are

charged under the Vandalism Act, and will receive at least three strokes of the cane and a fine of up to $2,000

or jail of up to three years.

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Electrostatic Hazard (P)



Vehicles carrying fuel

• Constant rubbing of the tyres_{Constant rubbing of the tyres}

• Tires acquires negative charges_{Tires acquires negative charges}

• Parts of metal body (of the truck) _{Parts of metal body (of the truck)}

near tyres becomes positively charged. near tyres becomes positively charged.

• Discharges (sparks) may occurs._{Discharges (sparks) may occurs.}

• May ignite flammable loads (or the gas)_{May ignite flammable loads (or the gas)}



Earthing

• A conductive strip (metal chain) that connects the metal body to the _{A conductive strip (metal chain) that connects the metal body to the} ground.

ground.

• Conducts electrons from ground to neutralise the positive charge_{Conducts electrons from ground to neutralise the positive charge} 

Other examples

• In factories manufacturing electronic goods_{In factories manufacturing electronic goods}

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Applications of Electrostatics (P)



Photocopier

– Light sensitive drum (conducts when there is light) is chargedLight sensitive drum (conducts when there is light) is charged – Image of original projected onto plate. Bright area lose charge_{Image of original projected onto plate. Bright area lose charge} – Toner attracted to charged area_{Toner attracted to charged area}

– Blank paper pressed against plate_{Blank paper pressed against plate}

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Applications of Electrostatics (P)



Laser printer (pg. 325)

– Light sensitive drum (conducts when there is light) is charged _{Light sensitive drum (conducts when there is light) is charged} positively

positively

– Laser beam shines across drum. Bright area lose charge_{Laser beam shines across drum}. Bright area lose charge – Positively charged toner attracted to uncharged area_{Positively charged toner attracted to uncharged area}

– Blank paper pressed against plate_{Blank paper pressed against plate}

– Heated to melt toner and fix it onto paper_{Heated to melt toner and fix it onto paper}

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Applications of Electrostatics (P)



Electrostatic precipitator (pg. 326)

–

Remove ash and dust from waste

gas in industries

–

Two earthed, flat metal plate

–

Negatively charged rods

–

Electric field forms resulting in

ionised air molecules

–

Dust and ash picks up negative

charges as it moves through

charged rod / wire grid

–

moves towards the metal plates

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

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Applications of Electrostatics (P)



Electrostatic paint spraying

–

Paint droplet becomes charged

when rubbed against the

nozzle of the spray

–

Car body earthed

–

Paint droplets attracted onto

the metal body

–

All droplets have same charge,

so they repels each other. This

results in even coating

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Interesting Observations



Van de Graaff generator

• Can reach over 100KV_{Can reach over 100KV}

• Current flowing through about 0.02m_{Current flowing through about 0.02m}AA

• Repulsion of charges_{Repulsion of charges}



Action of paper

• Perspex with positive charge_{Perspex with positive charge}

• When held over paper, negative charge is induced_{When held over paper, negative charge is induced}

• Unlike charges attract, thus paper moves towards the Prespex_{Unlike charges attract, thus paper moves towards the Prespex}

• When in contact, paper loses negative charges and becomes _{When in contact, paper loses negative charges and becomes} positively charged.

positively charged.

• Like charges repel, paper moves _{Like charges repel, paper moves} away from the Perspex away from the Perspex



Fluorescent lamp

• you find out why_{you find out why}

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Summary

Like charges repel

Unlike charges attract

Potential hazards

Potential hazards • sparkssparks

• firesfires

Method of charging

Method of charging • frictionfriction

• inductioninduction

• contactcontact

Application

Application • photocopierphotocopier

• spraying of paintspraying of paint

looks like

are found in

is used by

obey laws

beware

of

of Electric fieldElectric field •_• conductors_insulatorsconductors_insulators

can be produced by

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Points to note

a.

Under humid conditions, charged objects could

discharge faster



more water molecules in the air. Water is a better conductor

than air. Therefore discharge is faster.

b.

When a person stands on a insulating platform, charges

will not flow and he will not receive an electric shock.

c.

When a person touch a Van de Graaff generator, their

hair will rise because all the hair strands contain like

charges and will repel each other