Transcript Unit 15 Static Electricity

Static Electricity
Learning objectives
In this section, you’ll be able to:
• State that there are positive and negative
electric charges and charges are measured in
coulombs (C).
• State that like charges repel, unlike charges
attract
Electric charge
 Electric charge is measured in coulombs (C)
For info:
• A proton carries a charge of 1.6×10-19 C
• An electron carries a charge of -1.6×10-19 C
 E
Examples of positive
charges
Positive ions
Examples of negative
charges
Negative ions
Electrons (more common)
Electric charge
Like charges repel
+ +
- -
Unlike charges attract
+
-
Attraction between charged objects
and neutral objects
• A charged object can also attract a neutral
conducting object.
• This is because electrons in a neutral
object can be attracted or repelled by a
charged object and move to the side closer
to the charged object.
• Charging by induction works on this
principle, which you will learn in later part
of this topic.
Attraction between charged objects
and neutral objects
The object on the insulating stand is neutral, with equal number of
positive and negative charges. A charged rod is brought near it.
negatively
Electrons are attracted to the
side nearest to the positively
charged rod.
Electrons are repelled to the side
furthest from the negatively
charged rod.
The unlike charges attract.
The unlike charges attract.
Example
M, N, O and P are small spheres which behave as follows:
M repels N;
M attracts O and
O repels P.
If P is positively charged, _____________.
A
N must be positively charged
B
N must be negatively charged
C
N must be positively charged or neutral
D
N must be negatively charged or neutral
Lesson from the example:
• A
charged
object
can
uncharged/neutral object
• But a charged object
uncharged/neutral object
attract
cannot
repel
an
an
Learning objectives
In this section, you’ll be able to:
• Describe an electric field as a region in which
an electric charge experiences a force
• Draw the electric field of an isolated point
charge
• Draw the electric field pattern between two
isolated point charges
Electric Field
• The attractive or repulsive forces between
charges is known as an electric force.
• An electric force is an example of a noncontact force.
• An electric field is a region where electric
charge experiences an electric force.
Representing an electric field
• An electric field can be represented by lines
with arrows
• The direction of the field lines gives the
direction of the force that acts on a small
positive charge
Things to note when drawing field
lines
• Lines are to originate from the charge
• Lines not to cross each other
• Arrows are to be drawn on centre of each line
• Arrows are to point away from a positive
charge, and towards a negative charge
• The lines should be close to each other when
near the charge, and get more apart as the lines
get further from the charge. (see next slide)
Strength of electric Field
The strength of an electric field is indicated by
how close the field lines are to each other.
Closer field lines indicate
a stronger electric field
Field lines further apart
indicate a weaker electric field
Electric
point
field
between
2
Eg. Between two unlike charges
isolated
charges
Between two unlike charges
Between two positive charges
Between two positive charges
X
Note region X where an electric charge will not experience
any electric force if placed there.
Between two negative charges
Practice Task 16a
• Task 1: GLM Pg 270 Qn 2(a) and 2(d)
• Task 2: GLM Pg 281 Qn 5
• Task 3: GLM Pg 270 Qn 2(c), 3(a) – 3(c)
Summary
• An electric field is a region in which an electric
charge experiences a force
• You must be able to draw the electric field of
a point charge and between two point
charges.
Quiz 16a
See attached
Assignment 16a
• TYS Topic 16
• Paper 1: Qn 5, 6
• Supplementary worksheet on drawing
of electric field
Learning objectives
In this section, you’ll be able to:
• show understanding that electrostatic
charging by rubbing/friction involves a
transfer of electrons
Charges in objects
 All matter are made of atoms, and electrons are
present in the region of space outside the nucleus.
 Electrons can be delocalised from the atom.
Charges in objects
Which of the following statements is/are true?
a)A positively charged object has lost electrons
b)A positively charged object has gained protons
c)A negatively charged object has lost protons
d)A negatively charged object has gained electrons and lost
protons
 When an electron is removed from an atom, we say that
the atom is positively charged.
 When an electron is added to an atom, we say that the
atom is negatively charged.
Note: Only electrons can move or be transferred from one object to another.
Electrostatic Charging
• Electrostatic charging means to give an object a net
charge
• There are two ways:
o Electrostatic charging by rubbing – for insulators
o Electrostatic charging by induction – for conductors
What are insulators and conductors?
Insulators
• Insulators are materials where electrons are not
free to move about inside the material.
E.g. glass, silk, perspex and wool
• Insulators do not conduct electricity and are
charged by rubbing.
The charge remains
at the region where
it was transferred.
Conductors
• Conductors are materials that allow electrons
to move freely within them.
E.g. metals like copper, iron or steel
• They are able to conduct electricity and are
charged by induction.
When electrons are
gained or lost in a
conductor, the
electrons will be
redistributed.
Videos
https://www.youtube.com/wat
ch?v=aO-phqmyqdY
Demonstration
Comb and
paper pieces
Examples of static around us…
Electrostatic charging by rubbing/ friction
• Rubbing transfers electrons from one object to
another.
• When a glass rod and a piece of silk are rubbed
together, some electrons from the surface atoms of
glass is transferred to the silk.
Electrostatic charging between some common
materials
Practice Task 16b
• Task 1: GLM Pg 278 Qn 1 (a),(b) and (c)
Learning objectives
In this section, you’ll be able to:
• Describe experiments to show
electrostatic charging by induction
Electrostatic charging by induction
• Induction is the process of charging without
any contact with the charging body
• Insulators cannot be charged by induction.
Attraction between charged objects and
neutral objects
• Recall an earlier example
• The conducting object is
neutral and on an
insulating stand.
• Electrons are attracted to
the side nearest to the
positively charged rod.
• We say that a negative
charge is induced by the
rod.
Animation
Charging a conductor by induction
Step 1:
• Bring a positively charged glass rod near the metal
conductor on an insulating stand.
• The free electrons in the metal will be drawn
towards the side nearer the positively charged
glass rod.
Charging a conductor by induction
Step 2:
•Without removing the glass rod, earth the positively
charged side of the metal conductor by touching it with
your hand. (Earthing will be explained in next section)
•Electrons will flow from the ground into the conductor.
conductor
Charging a conductor by induction
Step 3: With the glass rod still in place, remove your
hand from the conductor.
Charging a conductor by induction
Step 4:
• Remove the glass rod
• The conductor is now negatively charged
Discharging charged
conductors
(not specifically in syllabus
but good to know)
Discharging a charged conductor
 A charged conductor can be neutralised by
earthing it.
 To earth a charged conductor is to provide a
path for the excess electrons to flow away or
flow to the conductor.
Discharging a charged conductor
Earthing
Practice Task 16c
• Task 1: GLM Pg 281 Qn 9
• Task 2: GLM Pg 283 Qn 4
• Task 3: GLM Pg 278 Qn 2(a),(b) and (c)
(Note: 2(d) shows electric field between 2 plates,
which is not specifically in the syllabus)
Summary
 There are two ways to charge an object:
o Rubbing/friction – for insulators
o induction – for conductors
 In both ways, the net charge is always due to
movement of electrons.
 Charging through rubbing/friction involves
transfer of electrons through contact, whereas
for induction, no contact is needed.
Quiz 16b
See attached
Assignment 16b
• TYS Topic 16
• Paper 1: Q1, 3, 4, 7, 9
• Paper 2 section A: Q1, Q4(a) and (b)
Learning objectives
In this section, you’ll be able to:
• describe examples where electrostatic
charging may be a potential hazard
Hazards of electrostatics
1. Lightning
• Thunderclouds are
charged by friction
between the water
molecules in the
thunderclouds and
air molecules.
• It then ionises the air and the ionised air
provides a conducting path for electric charge to be
discharged to the nearest or sharpest object on the
ground.
Hazards of electrostatics
 If a negatively charged cloud
passes overhead, it induces a
positive charge at the top of
the lightning conductor.
 The point then repels positive ions
to the cloud to neutralise it,so it is
likely to produce a lightning flash
less
 The electrons that are attracted to the conductor
travel down it to the earth. An electric current flows
through the conductor.
Hazards of electrostatics
2. Electrostatic discharge
•
Excessive charges may build up due to friction
•
Electronic equipment, such as computer boards
and hard drive, can be easily damaged.
•
Such equipments are usually packed in antistatic
packaging materials.
Video
https://www.youtube.c
om/watch?v=T6VKx
mUPb3g
Learning objectives
In this section, you’ll be able to:
• describe the use of electrostatic charging in a
photocopier, and apply the use of electrostatic
charging to new situations.
Practical applications of electrostatics
1. Photocopier (Compulsory to know)
2. Laser printer (Refer to textbook)
3. Spray painting
4. Electrostatic precipitator
Photocopier
https://www.youtube.com/wat
ch?v=MJ5ghlTdF9k
How the Photocopier works
• A photocopier uses electrostatic charge to
produce a copy.
• The original (the page you want copied) is
placed onto a sheet of glass. An image of this
page is projected onto a positively charged
drum.
• The drum has a coating that conducts
electricity when light falls on it. The parts of
the drum which are lit by the projected image
lose their electrostatic charge when they start
to conduct. These are the parts where there
are no image.
How the Photocopier works
• A black powder (called toner) is negatively
charged/neutral. The toner is attracted to the
positively charged parts of the drum.
• The drum rotates and rolls against a piece of
copier paper. The toner is transferred from
the drum to the paper making a black and
white image of the original.
• Finally, the paper is heated which makes the
toner stick to it.
Spray painting
• As the spray leaves the nozzle, the particles of paint
become charged by friction.
• The paint particles contain like charges thus they
spread out when they are sprayed on a car body.
• The charged paint particles will be attracted to the
metallic car body.
Electrostatic precipitator
• Coal-burning power stations
produce huge amounts of smoke
pollution. The smoke is a cloud of
small dust particles or ash.
• Some thin wires are stretched
across the centre of the chimney.
These wires are charged and
cause the gas around them to be
ionised.
• Because of this, the smoke
particles become negatively
charged. They are repelled by the
wire towards the earthed metal
plates, where the dust sticks.
Summary
There are hazards and applications for
electrostatic charging:
Hazards
o Lightning, Electrostatic discharge etc.
Applications
o Photocopier (compulsory), spray painting,
precipitator etc.
Quiz 16c
See attached
Assignment 16c
• TYS Topic 16
• Paper 2 Section A: Q3(b), Q5(c)
• Paper 2 Section B: Q1(b)