Exp SPA - Chp 16 Static Electricity

Static Electricity

Learning Objectives

Laws of Electrostatics

a) state that there are positive and negative charges and that charge is measured in coulombs

b) state that unlike charges attract and like charges repel


Matter is made up of atoms. Atoms are made of neutrons, protons and electrons. If electrons are

removed, the atom becomes positively charged.

If electrons are added, the atom becomes negatively charged.

If the numbers of electrons and protons are equal, the atom is in the neutral state.


How do positive and negative charges interact?

++ + –

Like charges repel Unlike charges attract


Measuring Electric Charges SI Unit: coulomb (C)

The amount of charge carried by an electron is 1.6 × 10-19 C

In other words, there areelectrons in 1 C of charge!

Learning Objectives

Electrostatic Charging

c) show understanding that electrostatic charging by rubbing involves a transfer of electrons

d) describe experiments to show electrostatic charging by induction

Charging

We can charge various objects by different methods.

When we charge an object, do we lose the proton or the electron?

Electron!Why? Mass of a proton is 1.67 × 10-27 kg. Mass of electron is 9.109 × 10-31 kg. Electrons are less massive (10000 times smaller) than

protons and thus easier to move!

Charging

Electrical Insulators and ConductorsElectrical Insulators Electrical Conductors

Motion of charged particles

Ability to conduct electricity

Method of charging

Examples

Charged particles are not free to move about

Charged particles are free to move about

No Yes

By friction (rubbing) By induction

Glass, perspex, silk, wool Copper, steel, fluids with mobile charged particles

Charging

Charging by friction – insulators Before rubbing, both are neutral.

During rubbing, some electrons from the fur separate from atoms and transfer to the rod.

Rod – negatively charged due to extra electrons

Fur – positively charged due to unpaired protons

Charging

Charging by friction – insulators The electrons in the rod remain at the

surface where it was rubbed.

This is because electrons do not move freely in insulators!

Charging

Rubbing

Glass rod

Fur

Perspex rod

Wool

Hair

Silk

Ebonite rod

Wool

Polythene

Rubber balloon

with

Direction of electron transfer

Negatively chargedPositively charged

Charging

Charging by induction – conductors

Induction is a method of charging in which a conductor is charged without contact with the charging body.

Conductors cannot be charged by friction because mobile electrons can be easily transferred to and away from conductors.

Charging

Charging by induction – 2 conductors

Step 1:2 metal spheres on insulating stands are placed touching each other.

Insulating stands do not allow charges to flow away

Charging


Step 2:A negatively-charged rod is brought near, but not touching, sphere P.

Electrons in both spheres P and Q are repelled to the far end of sphere B.

Sphere P – excess positive charges, Sphere Q – excess negative charges.

NOTE!If you draw 4 negative charges on the rod, there should be the same number of positive and negative charges on the sphere!

Charging


Step 3:Sphere Q is moved away so that it no longer touches sphere P.

The induced charges remains.

Charging


Step 4:The charged rod is removed.

Sphere P – positively chargedSphere Q – negatively charged.

Spheres P and Q have an equal number of opposite charges.

Charges will distribute equally on each sphere.

Charging

What if a positively-charged rod is used? Difference in step 2!

Negatively-charged rod Positively charged rodStep 2 Negatively-charged rod used.

Electrons repelled away from the rod.

Positively-charged rod used.

Electrons attracted to rod.

Charging

Charging by induction – 1 conductorStep 1:

To obtain negative charge To obtain positive charge

Positively charged rod brought near to but not touching the neutral, insulated conductor.

Negatively charged rod brought near to but not touching the neutral, insulated conductor.

Electrons attracted towards rod at end P.

Positive charge at the other side of conductor.

Electrons repelled away from end P, leaving positive charges behind.

Negative charge at the other side of conductor.

Charging



Conductor earthed by touching it with finger.

Conductor earthed by touching it with finger.

Closed path for electrons to flow from Earth to neutralise positive charge at Q.

Closed path for electrons to flow from conductor to Earth.

Charging



Finger is removed. Finger is removed.

Conductor left with negative charge.

Conductor left with positive charge.

Charging



Positively charged rod is removed Negatively charged rod is removed The negative charge is redistributed

evenly on the conductor. The positive charge is redistributed

evenly on the conductor.

Charging

Important SUMMARY!

Only electrons move!! NOT the protons.Either they get repelled or attracted.

For insulators, the electrons remain at the same place it was transferred.

For conductors, the electrons would distribute itself evenly in it.

Neutralising

All charged objects can be neutralised by discharging the excess charges.

Insulators can be discharged through heating or by providing humid conditions.

The heat from the flame ionises (becomes charged) the surrounding air particles.

The ions neutralise the excess charges on the glass rod.

Neutralising

Conductors can be discharged by earthing.

When we earth a charged conductor, we provide a path forexcess electrons to flow away from the charged

conductor, orelectrons to flow to the charged conductor.

Neutralising

Practice ProblemThe figure below shows a negatively-charged conductor suspended from a string. When it is brought near the metal plate connected to the ammeter, a quick deflection is seen on the ammeter. a) Why does the charged conductor have to be suspended on a

string?

b) Explain why a deflection is seen.

A

− − −−−

−−−

− − metal plate

Neutralising

Answera) To ensure that no charges are lost through contact with

other surfaces.

b) When the negatively-charged conductor is brought near the metal plate, the electrons on the plate are repelled.

Since the plate is connected to earth, the negative charges are transferred to the ground.

Charges moving through ammeter would result in a deflection.

Learning Objectives

e) draw the electric field of an isolated point charge and recall that the direction of the field lines gives the direction of the force acting on a positive test charge

f) draw the electric field pattern between two isolated point charges

Electric Field

Recall Attractive forces between 2 objects with unlike charges.

Repulsive forces between 2 objects with like charges

The force experienced by the charges is an electric force.

The force is experienced without the charges being in contact with each other.

An electric force is the attractive or repulsive force that electric charges exert on one another.

Electric Field

An electric field can be illustrated with electric field lines, andarrowheads to indicate the direction of the electric

field.

An electric field is a region in which an electric charge experiences an electric force.

The direction of an electric field is the direction of the force that would act on a small positive charge.

Electric Field

If you place small positive test charges around a positive charge, what would happen?

F

F

++

F

+ F

+

+

Electric field lines of a positive charge

+

Electric Field

If you place small positive test charges around a negative charge, what would happen?

F

F

-+

F

+

F

+

+

Electric field lines of a negative charge

–

Electric Field

The closer the electric field lines are, the stronger they are. Can you draw the electric field lines of the 2nd charge?

The strength of an electric field is indicated by how close the field lines are to one another.

q 2qCloser field lines, stronger electric field

Further field lines, weaker electric field

Twice the number of field lines!

Electric Field

Can you draw the field lines for 2 charges together?Answers are on the next slide.

Both positive Both negative Opposite charges

+ + – – + –

Electric Field

Answer

Both positive Both negative Opposite charges

Reverse the arrow directions from both

positive.

Electric Field

What happens if one charge is greater?

The number of lines leaving charge +2q is twice the number of lines entering -q

Electric Field

Electric field line of parallel plates

The electric field between parallel oppositely charged plates is uniform at the central region.

(a) Electric field lines between parallel

oppositely charged plates

(b) Electric field lines are closer to each other in a

stronger electric field

Learning Objectives

g) describe examples where electrostatic charging may be a potential hazard

h) describe the use of electrostatic charging in a photocopier, and apply the use of electrostatic charging to new situations

Hazards of Electrostatics

Lightning Singapore has one of the highest occurrences of

lightning activity in the world.

On average, Singapore experiences 168 thunderstorm days per year.

0.35 lightning deaths per million people each year on averagedue to lightning

Hazards of Electrostatics1) Thunderclouds are charged by friction

between water molecules in the thunderclouds and air molecules in the atmosphere.

2) Negative charges accumulate at the bottom of the clouds.

3) Negative charges in cloud repel the electrons near the surface of the Earth, causing the surface of the Earth to be positively charged.

4) When the accumulation of charges is large, the air particles nearby are ionised.

The ionised air particles provide a conducting path for the electrons in the clouds to reach the Earth.

5) When the electrons travel down the conducting path to the Earth, lightning forms.


Electrostatic discharge Excessive charges may build up on objects due to friction.

Electronic equipment, such as computer boards and hard drives, can be easily damaged by electrostatic discharge. Such equipment are usually packed in antistatic packaging.

For clothing, sparks can be produced and it may catch fire.


Fuel tank Tyres gain negative charge due to friction

with road.

Body of metal truck becomes positively charged, and may produce sparks.

Sparks may ignite the fuel.

Conductive strip conducts electrons from the ground to neutralise the positive charge on the metal body to prevent sparks.

Applications of ElectrostaticsPhotocopier

Step Description Image1 Drum is (+ve)

2 Image of document form on drum by reflection of light.

Areas without image neutralised.3 Toner (-ve) applied

4 Toner (-ve) is transferred to paper (+ve)

Applications of Electrostatics

PhotocopiersPhotocopiers use static electricity to produce copies of documents.

The metal drum inside the photocopier is coated with selenium.

Selenium is a photoconductor — it only conducts electricity in the presence of light.

The drum’s surface is charged positively by a charged wire.

1


The original image to be photocopied is placed on a sheet of clear glass above the drum.

An intense light beam is shone onto the image.

2

The darker areas of the image reflect less light to the drum That part of the drum remains non-conductive The drum remain positively charged.

The lighter areas of the image reflect more light to the drum That part of the drum becomes conductive Electrons neutralises the positive charge.


Negatively-charged toner powder is attracted to the positively-charged drum.

3

A positively-charged sheet of paper is passed over the drum’s surface.

The paper attracts the negatively-charged toner and the image is formed on the paper.

The paper is heated and pressed to fuse the toner powder to the paper permanently.

4

Applications of ElectrostaticsSummary


2 Image of document form on drum by reflection of light.

Areas without image neutralised.3 Toner (-ve) applied


Applications of ElectrostaticsLaser Printer


2 Image of document form on drum by direct exposure to laser beam.

The laser draws on the drum.3 Toner (-ve) applied



Electrostatic Spray Painting Neutral car body Paint droplets becomes charged by rubbing against nozzle

of spray and attracts to car body They repel each other due to like charges, spreading out

evenly throughout the sprayed surface.

Advantages Even coat of paint Reaches even the most inaccessible part


Electrostatic Precipitator Plates earthed

Rods strongly negative charged

Strong electric field between rods and plates Air molecules becomes ionised.

Positive ions attracted by rods.

Dust particles pick up negative ions and collected by metal plates

QUESTIONS?

Exp SPA - Chp 16 Static Electricity

Education

Transcript of Exp SPA - Chp 16 Static Electricity