Exp SPA - Chp 16 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 are
1 C
1.6×10−19 C
= 𝟔. 𝟐𝟓 × 𝟏𝟎 𝟏𝟖 electrons 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 Conductors
Electrical 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 charged
Sphere 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 rod
Step 2 Negatively-charged rod used.
Electrons repelled away from the
rod.
Positively-charged rod used.
Electrons attracted to rod.

Charging
Charging by induction – 1 conductor
Step 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
Step 2:
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
Step 3:
Finger is removed. Finger is removed.
 Conductor left with negative
charge.
 Conductor left with positive charge.

Charging
Step 4:
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
for
excess electrons to flow away from the charged
conductor, or
electrons to flow to the charged conductor.

Neutralising
Practice Problem
The 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
Answer
a) 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, and
arrowheads 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 2q
Closer 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 average
due to lightning

1) 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 Electrostatics
Photocopier
Step Description Image
1 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)

Photocopiers
Photocopiers 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

Summary
1 Drum is (+ve)
reflection of light.
Areas without image neutralised.

Laser Printer
1 Drum is (+ve)
direct exposure to laser beam.
The laser draws on the drum.

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

Exp SPA - Chp 16 Static Electricity

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Exp SPA - Chp 16 Static Electricity