1. Topic 12
Static Electricity
2012 Sec 4 12 Static electricity AJL

2. Laws of Electrostatics Electrostatic charging
contents
Electric charges
Laws of Electrostatics
Electrostatic charging
Conductors and Insulators
Electric fields
Applications of Electrostatics
Chapter Review
2012 Sec 4 12 Static electricity AJL

3. electric charges Two types of 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 charge
Positive charge
polythene (rubbed with wool)
perspex (rubbed with wool)
ebonite (rubbed with fur)
glass (rubbed with silk)

4. electric charges and the electric forces between them
has a SI unit of coulomb (C)
charge carried by an electron is 1.6 x C

5. Law of electrostatics + + + - + -
The law of electrostatics states that
like charges repel and unlike charges attract. +
strong repulsion +
further  weaker repulsion + -
strong attraction + -
further  weaker attraction

6. simplified structure of neutral atom
electrostatic charging + - e p
every atom has a massive, positively-charged nucleus
positive charge on nucleus = total negative charge on electrons
no net charge in the atom (neutral)
simplified structure of neutral atom
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

7. each material with equal number of positive and negative charges
A. Charging by rubbing
When two different materials (especially insulating materials) are rubbed together, negative charges (electrons) will transfer from one object to another.
before rubbing
after rubbing
polythene strip
wool
each material with equal number of positive and negative charges
certain amount of negative (electrons) is transferred from the wool to the polythene strip

8. A. Charging by rubbing
Polythene and perspex (cellulose acetate) can become charged with static electricity when rubbed with a dry woollen cloth.
polythene (-)
repel
polythene (-)
perspex (+)
attract
Two different types of charges can be produced by friction on the strips

9. Refer to resources from links at physics wiki
A. Charging by rubbing
balloon, clothing & wall
carpet, human body & door knob
Refer to resources from links at physics wiki

10. B. Charging by induction
1. 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

11. Step 1 Step 2 Step 3 Step 5 Step 4
2. Using 1 conducting sphere, a positively charged inducing specimen (a rod) & earthing: to give sphere negative charges
Step 5
Step 4

12. B. Charging by induction

14. B. 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

15. C. Charging by contact Between conductors Charging pith balls
Charged glass rod touches neutral pith balls
Charge transferred to pith balls of the same sign as on the rod  repulsion
Another charged rod brought near pith balls, attraction  opposite charge on rod compared to pith balls

16. 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. + -

17. Both spheres become electrically neutral (uncharged).
E.g. 1
(continued)
Draw on the diagram below to show the correct charge distribution in the two spheres after neutralization.
Both spheres become electrically neutral (uncharged).

18. A and B are identical metal-coated balloons.
E.g. 2
A and B are identical metal-coated balloons. A B –
nylon thread
(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.
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)

19. 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.
Objects of the same sign of charge repel
 A and B repel each other.

20. Van de Graff Generator
How the Van de Graff generator works

22. (a) Why do the girl’s hairs stand on-ends?
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.

23. (b) Why does she stand on plastic stool?
She is insulated from the ground when standing on a 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.

24. (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.

25. Removing electric charges
To neutralise an accidentally charged apparatus, remove these charges by discharging it.
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).

26. Removing electric charges

27. conductors and insulators
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

28. conductors and insulators
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

29. 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). 

30. Download, install and use Electric field.exe 2.01 from
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
Download, install and use Electric field.exe 2.01 from

31. electrostatic 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.

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

33. E.g. 4
A –ve charge is put at P. What is the direction of the electric force on the charge? P 
uniform electric field
A Towards the right.
B Towards the left.
C No electric force.

34. What are the electric field lines due to a –ve charge?
E.g. 5
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. –

35. And the denser the field lines, the __________ is the electric field.
E.g. 6
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

36. drum is charged as it rotates under the rod
applications of electrostatics
photocopier
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
charged metal rod
selenium coated drum
drum is charged as it rotates under the rod
drum is evenly charged

37. applications of electrostatics
photocopier
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
only the dark areas remain charged
toner particles are attracted to the charged part
toner

38. toner is transferred to the paper heat is supplied to ‘fix’ the toner
applications of electrostatics
photocopier
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
toner is transferred to the paper
heat is supplied to ‘fix’ the toner

39. applications of electrostatics
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.

40. E.g. 7 Why the paint droplets spread out as they leave the nozzle?
Why is the car body earthed during spraying?
State one advantage of using electrostatic paint spraying over normal spraying.
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.
The droplets would be attracted to the earthed car body.
This gives a more even coating, ensuring that the paint reaches even the most inaccessible parts.

41. ash and dust collects on plate waste gases carrying ash and dust
applications of electrostatics
electrostatic precipitator
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
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.

43. Unlike charges attract
Static Electricity
can be produced by
is used by
consists of
Method of charging
friction
induction
contact
Positive and
negative charges
Application
photocopier
spraying of paint
obey laws
are found in
form
beware of
Like charges repel
Unlike charges attract
Electric field
conductors
insulators
looks like
Potential hazards
sparks
fires + -