chapter 12 thermal properties of materials
DESCRIPTION
A2 Thermal Properties of MaterialsTRANSCRIPT
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CAMBRIDGE A LEVEL
PHYSICS
MATERIALS
THERMAL
PROPERTIES OF
MATERIALS
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L EA R N I N G O U TC O M E SL EA R N I N G O U TC O M E SNo. LEARNING OUTCOME
i Use simple kinetic model to explain the structure of solids, liquids and gases.
ii Relate internal energy to the average potential energies and average kinetic
energies. Relate changes in temperature with changes in internal energy.
iii Use the simple kinetic model to explain why boiling and melting occurs
without any change in temperature and why liquids cool when evaporation
occurs.
iv Define specific latent heat (of fusion and vaporisation) and specific heat
capacity. Explain electrical methods to determine the specific latent heat and
specific heat capacity. Compare specific latent heats of fusion and
vaporisation.
v Relate the work done by/on a gas to its internal energy.
vi Relate changes of state with change in internal energy. Differentiate systems
from surroundings.
vii Define and use the first law of thermodynamics
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The kinetic model of matter assumes that all The kinetic model of matter assumes that allmatter is made up of molecules thatinteract with each other and are a state ofcontinuous , random motion.
Evidence: Brownian motion.
We will now compare the structure of 3phases of matter (solids, liquids and gases) interms of ordering, movement andintermolecular distances.
SIMPLE KINETIC MODEL
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P R O P E R T I E S O F PA R T I C L E S
PROPERTY SOLIDS LIQUIDS GASES
Ordering of
molecules
Regular structures
repeated
throughout (long
range order ).
Regular ordering
only in the
immediate
neighbourhood of a
few molecules (short
range order).
No ordering.
SIMPLE KINETIC MODEL
Figures 21.2 (a) , (b), and (c), page 328, Chapter 21: Thermal Physics; Cambridge
International AS and A Level Physics Coursebook, Sang, Jones, Chadha and Woodside,
2nd edition, Cambridge University Press, Cambridge, UK,2014.
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P R O P E R T I E S O F PA R T I C L E S
PROPERTY SOLIDS LIQUIDS GASES
Movement Free to vibrate
about a fixed
position
Can vibrate and
translate due to
some empty spaces
Free to undergo
translational
motion.
Distance
between
molecules
Least amount of
spacing of the
three phases.
Only slightly more
spaced that in solid
of the same
substance
Widely separated.
SIMPLE KINETIC MODEL
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INTERNAL ENERGY
Each molecule in a substance has a
certain amount of energy.
This energy is the sum of its kinetic
energy; due to its movement, and the
potential energy; due to the interaction
between the molecules.
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INTERNAL ENERGY
The amount of potential energy each
molecule has depends on the spacing
between the molecules.
The closer the molecules are, the more
negative (larger) the potential energy.
Potential energy is assigned a negative
sign.
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INTERNAL ENERGY
This is seen in the graph below.
Figures 21.5, page 329, Chapter 21: Thermal Physics; Cambridge International AS and
A Level Physics Coursebook, Sang, Jones, Chadha and Woodside, 2nd edition,
Cambridge University Press, Cambridge, UK,2014.
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INTERNAL ENERGYThis means that solid molecules have the This means that solid molecules have themost negative (largest) stored potentialenergy, followed by liquid molecules.
Gas molecules store an insignificant amountof potential energy.
However, the difference in stored potentialenergies between solid and liquid moleculesis lower compared to between liquid andgas molecules.
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INTERNAL ENERGY
All the molecules will have different
kinetic energies as some are moving
faster and some slower.
All the molecules also will have different
amount of potential energies the
separation between molecules change
continually.
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INTERNAL ENERGY
We can now say that the kinetic energies
and potential energies of all of the
molecules follow a random distribution.
When we add the kinetic energies and
potential energies of all the molecules,
we remove the random nature of the
energies.
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INTERNAL ENERGY
What we get is known as the internal
energy of the substance.
Definition: The internal energy of a
substance is the sum of the random
distribution of kinetic and potential energies
of all the molecules associated with the
system.
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C H A N G E S I N I N T E R N A L E N E R G Y C H A N G E S I N I N T E R N A L E N E R G Y
A N D T E M P E R AT U R E
How do temperature and internal energy of
a substance are related?
When the internal energy of a substance
changes, in certain cases, the temperature
of the substance also changes.
However, if the temperature of a substance
changes, the internal energy of that
substance will change.
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For example, when we heat a liquid from 20 For example, when we heat a liquid from 20C to 80 C, the temperature increasesbecause the supplied thermal energyincreases the average kinetic energy of themolecules of the liquid (but not thepotential energy).
When cooling, say from 80 C to 20 C,thermal energy is dissipated since theaverage kinetic energy of the moleculesdecreases.
C H A N G E S I N I N T E R N A L E N E R G Y C H A N G E S I N I N T E R N A L E N E R G Y
A N D T E M P E R AT U R E
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However, when water boils, the supplied
thermal energy increases the potential
energy of the water molecules, but not
the average kinetic energy (no increase
in temperature).
C H A N G E S I N I N T E R N A L E N E R G Y C H A N G E S I N I N T E R N A L E N E R G Y
A N D T E M P E R AT U R E
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In conclusion, only changes in average
kinetic energy of the molecules will change
the temperature of the substance. This
happens when matter is heated.
During change of phase, no change in
temperature occurs as the added thermal
energy is used to change the potential
energy stored in the molecules.
C H A N G E S I N I N T E R N A L E N E R G Y C H A N G E S I N I N T E R N A L E N E R G Y
A N D T E M P E R AT U R E
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I N T E R N A L E N E R G Y A N D
I D E A L G A S E S
I N T E R N A L E N E R G Y A N D
I D E A L G A S E S
molecules.
Recall that for ideal gases, there are no
forces that exist between the gas
molecules. This means that ideal gas
molecules do not store potential energy.
Hence, the internal energy of an ideal
gas equals it sum of the random
distribution of kinetic energies of all its
molecules.
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M E LT I N G , B O I L I N G A N D
E VA P O R AT I O N
M E LT I N G , B O I L I N G A N D
E VA P O R AT I O N
We will now answer these two
questions:
1. Why melting and boiling occur at a
constant temperature for a specific
substance?
2. Why liquids that undergo evaporation
cool?
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M E LT I N G , B O I L I N G A N D
E VA P O R AT I O N
M E LT I N G , B O I L I N G A N D
E VA P O R AT I O N
Question 1:
Melting and boiling involve change of phase.
During melting, the phase of a substance
changes from solid into liquid at a constant
temperature.
During boiling, the phase change that occurs
is from liquid into gas, also without any
change in temperature.
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M E LT I N G , B O I L I N G A N D
E VA P O R AT I O N
M E LT I N G , B O I L I N G A N D
E VA P O R AT I O N
In solids, the stored potential energy in In solids, the stored potential energy inmolecules is more negative (greater inmagnitude) as compared to that in liquidsbecause the separation between solidmolecules is lesser than that between liquidmolecules.
In order to liquefy, the molecules need tobe separated more. (i.e. the potential energymade more positive).
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M E LT I N G , B O I L I N G A N D
E VA P O R AT I O N
M E LT I N G , B O I L I N G A N D
E VA P O R AT I O N
During melting, the thermal energy
supplied is used to increase the
separation between molecules without
increasing the average kinetic energy of
the molecules. Hence, solids melt
without any increase in temperature.
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M E LT I N G , B O I L I N G A N D
E VA P O R AT I O N
M E LT I N G , B O I L I N G A N D
E VA P O R AT I O N
In liquids, the stored potential energy in In liquids, the stored potential energy inthe molecules is more negative (greaterin magnitude) as compared to that ingases due to the lesser separationbetween molecules in liquids.
In order to boil, the liquid moleculesneed to separated further (i.e. thepotential energy made more positive).
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M E LT I N G , B O I L I N G A N D
E VA P O R AT I O N
M E LT I N G , B O I L I N G A N D
E VA P O R AT I O N
During boiling, the thermal energy
supplied is used to increase the
separation between molecules without
increasing the average kinetic energy of
the molecules. Hence, liquids boil
without any increase in temperature.
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M E LT I N G , B O I L I N G A N D
E VA P O R AT I O N
M E LT I N G , B O I L I N G A N D
E VA P O R AT I O N
kinetic energy of the liquid.
Question 2:
During evaporation, the fastest moving
molecules , i.e. the molecules with the
greatest kinetic energies on the surface
of the liquid undergo a change in phase
(into gas).
This causes a reduction in the average
kinetic energy of the liquid.
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M E LT I N G , B O I L I N G A N D
E VA P O R AT I O N
M E LT I N G , B O I L I N G A N D
E VA P O R AT I O N
A reduction in the average kinetic energy
would reduce the temperature of the
liquid because the average kinetic energy
is proportional to the temperature of the
liquid.
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S P EC I F I C H EAT C A PAC I T Y
Definition: Specific heat capacity is the
amount of thermal energy needed to cause a
unit temperature change per unit mass of a
substance without any change in phase.
How do we determine the specific heat
capacity of a substance? Refer to the next 3
slides.
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S P EC I F I C H EAT C A PAC I T Y
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S P EC I F I C H EAT C A PAC I T Y
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S P EC I F I C H EAT C A PAC I T Y
Box 21.1, pages 337, Chapter 21:
Thermal Physics; Cambridge
International AS and A Level
Physics Coursebook, Sang, Jones,
Chadha and Woodside, 2nd edition,
Cambridge University Press,
Cambridge, UK,2014.
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EXAMPLEEXAMPLEWorked Example and Figure
21.14, page 337, Chapter 21:
Thermal Physics; Cambridge
International AS and A Level
Physics Coursebook, Sang,
Jones, Chadha and Woodside,
2nd edition, Cambridge
University Press, Cambridge,
UK,2014.
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SPECIF IC LATENT HEAT
Definition: Specific latent heat is the amount
of thermal energy needed to cause phase
change to occur in per unit mass of a
substance without any change in
temperature.
Note: Specific refers to per unit of mass.
How do we determine the specific latent heat
of a substance? Refer to the next 2 slides.
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SPECIF IC LATENT HEAT
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SPECIF IC LATENT HEAT
Box 21.2, pages 339 and 340, Chapter 21: Thermal Physics; Cambridge International
AS and A Level Physics Coursebook, Sang, Jones, Chadha and Woodside, 2nd edition,
Cambridge University Press, Cambridge, UK,2014.
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T Y P E S O F L AT E N T H EAT
There are two kinds of latent heat; latent heat
of fusion and latent heat of vaporisation.
Fusion is the phase change that occurs when a
liquid undergoes phase change to become
solid. Fusion is the opposite of melting.
Vaporisation is the phase change that occurs
when a liquid undergoes phase change into
gas. Vaporisation is opposite to condensation.
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E X A M P L E S
Worked Example 3, page 339, Chapter 21: Thermal Physics; Cambridge
International AS and A Level Physics Coursebook, Sang, Jones, Chadha and
Woodside, 2nd edition, Cambridge University Press, Cambridge, UK,2014.
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The data in the table compares the
specific latent heats of fusion and
vaporisation. You will observe that the
specific latent heats of vaporisation will
be higher than the specific latent heats
of fusion for all the substances given?
Why is this so?
C O M PA R I N G L AT E N T H E AT S O F
F U S I O N W I T H VA P O R I S AT I O N
C O M PA R I N G L AT E N T H E AT S O F
F U S I O N W I T H VA P O R I S AT I O N
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C O M PA R I N G L AT E N T H E AT S O F
F U S I O N W I T H VA P O R I S AT I O N
C O M PA R I N G L AT E N T H E AT S O F
F U S I O N W I T H VA P O R I S AT I O N
Source: http://www.kshitij-school.com/Study-Material/Class-11/Physics/Heat-and-
first-law-of-thermodynamics/Latent-heat/2.jpg
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the same substance.
When a unit mass of a substance changes
phase from a liquid to gas, it undergoes a
greater increase in volume as compared to
when a solid changes into a liquid.
This means that the required amount of
change of average potential energy per unit
mass during the vaporisation process is much
greater than that for the fusion process for
the same substance.
C O M PA R I N G L AT E N T H E AT S O F
F U S I O N W I T H VA P O R I S AT I O N
C O M PA R I N G L AT E N T H E AT S O F
F U S I O N W I T H VA P O R I S AT I O N
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Hence, the required increase in internal
energy per unit mass would be larger in
vaporisation (or condensation) as compared
to melting (or fusion) for the same
substance.
The added thermal energy changes only the
average potential energy of the molecules.
C O M PA R I N G L AT E N T H E AT S O F
F U S I O N W I T H VA P O R I S AT I O N
C O M PA R I N G L AT E N T H E AT S O F
F U S I O N W I T H VA P O R I S AT I O N
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E X A M P L E S
Oct/Nov 2008 Paper 4, Question 2.
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E X A M P L E S
Oct/Nov 2008 Paper 4, Question 2 (contd).
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E X A M P L E S
Oct/Nov 2008 Paper 4, Question 2 (contd).
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E X A M P L E S
Oct/Nov 2008 Paper 4, Question 2 (contd).
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E X A M P L E S
Oct/Nov 2008 Paper 4, Question 2 (contd).
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W O R K D O N E O N A G A S
Assume we have a frictionless moving piston
that attached to a container that contains gas.
We apply a compressive force as shown below.Figure 21.9 b, page 332, Chapter 21: Thermal
Physics; Cambridge International AS and A Level
Physics Coursebook, Sang, Jones, Chadha and
Woodside, 2nd edition, Cambridge University
Press, Cambridge, UK,2014.
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W O R K D O N E O N A G A S
The gas is pushed inwards; i.e. undergoes
compression under constant pressure
conditions as seen below.Figure 19.4(b), page 626: Chapter 19:
THE FIRST LAW OF
THERMODYNAMICS; SEARS AND
ZEMANSKYS UNIVERSITY PHYSICS
WITH MODERN PHYSICS; Young,
Hugh D. and Freedman, Roger A.,
Addison Wesley, San Francisco, 2012.
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W O R K D O N E O N A G A S
We are pushing the against the gas; i.e. doing
positive work on the gas.
The molecules that collide with the moving
piston will bounce off faster, thus increasing
the average kinetic energy of the molecules.
This causes the internal energy to increase,
hence the temperature of the gas increases.
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W O R K D O N E O N A G A S
How do we calculate this work done on the
gas? Use
o work done on the gas, in J;
o the constant pressure, in Pa;
o = change in volume (amount of compression),
in m3
o = final volume, in m3
o = initial volume, in m3
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W O R K D O N E BY A G A S
What happens if the gas pushes outwards; i.e.
undergoes expansion under constant pressure
conditions as seen below?Figure 19.4(a), page 626: Chapter 19:
THE FIRST LAW OF
THERMODYNAMICS; SEARS AND
ZEMANSKYS UNIVERSITY PHYSICS
WITH MODERN PHYSICS; Young,
Hugh D. and Freedman, Roger A.,
Addison Wesley, San Francisco, 2012.
-
W O R K D O N E BY A G A S
Now, the gas does work on the surroundings;
i.e. work done by gas is negative.
This is because the molecules bounce off
slower, thus reducing the average kinetic
energy of the molecules.
This reduces the internal energy of the
molecules and the temperature of the gas.
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W O R K D O N E BY A G A S
How do we calculate this work done by the
gas? We may use
o where work done by the gas, in J;
o the constant pressure, in Pa;
o = change in volume (amount of expansion), in
m3
o = final volume, in m3
o = initial volume, in m3
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C H A N G E S I N I N T E R N A L E N E R G Y C H A N G E S I N I N T E R N A L E N E R G Y
A N D C H A N G E S I N S TAT E
Recall the state variables: P, V and T.
A change in any one of the state
variables will cause a change in state;
the system moves from one state to
another.
A change in state may cause the internal
energy of the system to change.
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C H A N G E S I N I N T E R N A L E N E R G Y C H A N G E S I N I N T E R N A L E N E R G Y
A N D C H A N G E S I N S TAT E
Changes in state are path
independent. This means that if we
move from state A to state D directly, or
move from state A to B to C then to D ,
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Before we look at the first law of
thermodynamics, it is a good idea
first to understand what is meant by
the term system and its
surroundings.
It is up to us to define the system
(and surroundings).
SYS T E M / S U R R O U N D I N G S
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SYS T E M / S U R R O U N D I N G S
For example, if we have cylinder that is
fitted with a piston that contains an ideal
gas, then:
I. the ideal gas alone could be the system, then
the cylinder and piston and everything else
would be the surroundings;
II. the cylinder, piston and ideal gas can be the
system. In this case everything else will be the
surroundings.
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If we have an electrical heater that is
placed in a beaker containing a liquid, the
system could either be:
I. the heater only , in this case the liquid
and the beaker and everything else is the
surroundings; or
II. the heater, the liquid and the beaker; i.e.
everything else is the surrounding.
SYS T E M / S U R R O U N D I N G S
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F I R S T L AW O F
T H E R M O D Y N A M I C S
F I R S T L AW O F
T H E R M O D Y N A M I C S
Definition: The first law of thermodynamics
states that the change in the amount of
internal energy of a system is equal to the
sum of the amount of the work done on the
system and the amount of thermal energy
added to the system.
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F I R S T L AW O F
T H E R M O D Y N A M I C S
F I R S T L AW O F
T H E R M O D Y N A M I C S
Mathematically, ; where
the increase in internal energy
of the system;
the thermal energy added to the
system; and
the work done on the system
(by the surroundings).
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F I R S T L AW O F
T H E R M O D Y N A M I C S
F I R S T L AW O F
T H E R M O D Y N A M I C S
What this means is we can change the internal
energy of a system either by:
I. adding to ( or removing from (
thermal energy in a system;
II. The surrounding doing work on a system (,
or the system doing work on the surroundings
();
III. Both I and II above
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EXAMPLESEXAMPLESQuestion 3, page
332, Chapter 21:
Thermal Physics;
Cambridge
International AS
and A Level Physics
Coursebook, Sang,
Jones, Chadha and
Woodside, 2nd
edition, Cambridge
University Press,
Cambridge,
UK,2014.
-
CYCLIC PROCESSES
A cyclic process is a process which returns the
state of a system to its initial state.
Since the system returns to its initial state, the
increase in internal energy of the system is
zero; or .
Thus total thermal energy added to the
system plus the work done on the system; or
.
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EXAMPLESEXAMPLES
Example 19.4, page 633: Chapter 19: THE FIRST LAW OF THERMODYNAMICS; SEARS AND
ZEMANSKYS UNIVERSITY PHYSICS WITH MODERN PHYSICS; Young, Hugh D. and Freedman,
Roger A., Addison Wesley, San Francisco, 2012.
-
EXAMPLESEXAMPLESFigure 19.13, example 19.4, page 633:
Chapter 19: THE FIRST LAW OF
THERMODYNAMICS; SEARS AND
ZEMANSKYS UNIVERSITY PHYSICS WITH
MODERN PHYSICS; Young, Hugh D. and
Freedman, Roger A., Addison Wesley,
San Francisco, 2012.
-
EXAMPLESEXAMPLESOct/Nov 2010, Paper 41, question 2.
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EXAMPLESEXAMPLESOct/Nov 2010, Paper 41, question 2 (contd).
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EXAMPLESEXAMPLESOct/Nov 2010, Paper 41, question 2 (contd).
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EXAMPLESEXAMPLESOct/Nov 2010, Paper 41, question 2 (contd).
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EXAMPLESEXAMPLESOct/Nov 2010, Paper 41, question 2 (contd).
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H O M E W O R K
1. Question 3, Paper 4, Summer 2009.1. Question 3, Paper 4, Summer 2009.
2. Question 3, Paper 41, Winter 2009.
3. Question 2, Paper 42, Winter 2009.
4. Question 2, Paper 41, Summer 2010.
5. Question 3, Paper 42, Summer 2010.
6. Question 2, Paper 41, Winter 2011.
7. Question 2, Paper 43, Winter 2011.
8. Question 3, Paper 41, Summer 2012.
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H O M E W O R K
9. Question 3, Paper 42, Summer 2012.
10.Question 2, Paper 41, Winter 2012.
11.Question 3, Paper 41, Winter 2012.