PRINCE ALFRED COLLEGE
THERMAL PHYSICS
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IB PHYSICS
Thermal Physics
This topic will look at matter from a macroscopic (bulk) and microscopic (atomic) viewpoint.
THERMAL CONCEPTS
1.
Temperature
At a macroscopic level temperature is the degree of __________ or ___________ of a body
as measured by a _________________.

Temperature is a property that determines the ______________ of thermal energy
transfer between two bodies in contact. Thermal equilibrium occurs when the
temperature of two bodies in ___________ is the same.
Nearly all matter expands when its temperature increases and conversely _______________
when temperature decreases. A thermometer uses the expansion and contraction of a liquid
(usually mercury or alcohol) in a glass _______________ tube with a scale to measure the
expansion or contraction. The upper fixed point is due to _____________________________
and the lower fixed point is ___________________________________________.
At a microscopic level :

Temperature is related to the random motion of the atoms or _______________ in a
substance. In an ideal gas (one that remains a gas no matter the temperature),
temperature is a _______________ of _______________ kinetic energy per molecule.
This is also essentially true for both liquids and solids.
Temperature is not a measure of the total _______________ energy of the atoms or molecules
in a substance. There is twice as much kinetic energy in 2 litres of boiling water than in 1 litre.
The temperature is however, the same in both containers as the _______________ kinetic of
the atoms or molecules is the same.
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2.
Internal Energy
The Internal (___________) energy of a body is the total energy associated with the
________ ___________ of the particles. It can comprise of both ___________ and
____________ energies associated with particle motion.
 Kinetic energy arises from the _____________ and rotational motions
 Potential energy arises from the ___________ between the molecules
3.
Heat
4.
Touch a hot saucepan; energy is transferred to your hand, as the saucepan is warmer than your
hand. If however you touch ice, energy is _______________ from your hand to the ice.
Thermal energy is always transferred from a _____________ substance to a cooler one. This
transfer only occurs when there is a temperature difference.
Most people tend to believe that all matter contains heat. All matter contains a number of
forms of energy but not _______________. Heat is the transfer of energy from a body with
higher temperature to one of _______________ temperature. Once the energy is transferred, it
ceases to be heat. The energy becomes _______________ energy.
Heat will not necessarily flow from a body with more _______ molecular kinetic energy to
one with less total molecular kinetic energy. A bowl of warm water has much more total
molecular kinetic energy than a red-hot bolt. If the red-hot bolt is immersed into the warm
water, heat will flow according to _______________ - average kinetic molecular energy difference.
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THERMAL ENERGY TRANSFER
1.
Conduction
Conduction can take place within materials and between different materials that are in
_______________ contact. It can be explained in terms of _______________ between atoms
or molecules and the actions of loosely bound electrons.
A flame applied to the end of an iron rod, causes the atoms in the heated end to
_______________more rapidly. They vibrate against neighbouring atoms, which then, in
turn, do the same. Free electrons that can _______________ through the metal also transfer
energy through collisions with other free electrons and atoms. As metals have
_______________ electrons, they are the best conductors of heat and electricity.
If you touch a piece of metal and a piece of wood, the metal feels cooler even though the
temperature of both the metal and wood is the same. The metal is not actually cooler but
appears so. This is because the metal is a better _______________ of heat. The heat moves
out of your warm hand and _______________ the cooler metal. As little heat moves from
your hand into the wood, as it is a _______________ conductor, your hand does not sense
that the wood is cooler.
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2.
Convection
This involves the transfer of energy from one _______________ to another without the
molecules moving. Another way in which convection works is for the hotter material to move
_______________ such as hot air. Hot air becomes less _______________ and rises. The
cooler air then moves to the _______________ to take its place.
Convection occurs in all _______________ whether they are liquids or gases. Hot air above a
stove becomes less _______________ and moves up toward the ceiling.
3.
Radiation
Convection and conduction cannot take place in a vacuum yet the sun’s energy reaches earth.
All energy, including heat, which is
transferred by radiation, is called
_______________ energy. This is in the form
of _______________ radiation, which
includes anything from radio waves,
microwaves, infrared, visible light, ultra
violet, X-rays and gamma rays. Higher
temperature objects emit shorter wavelengths
and vice - versa. When the radiant energy falls
on other objects, some is _______________
and some absorbed. The part that is absorbed
increases the internal energy of the object.
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THERMAL PROPERTIES OF GASES
A large volume of gas can be described by _______________ characteristics - pressure (p),
volume (V) & temperature (T). All three terms are inter-related.
The temperature of a gas is always measured in ______________.
The pressure of a gas is measured in ____________________.
The volume of a gas is measured in _____________________.
We use these properties to describe the behaviour of a gas. An ideal gas is a gas that remains a
gas regardless of the temperature.
There are a number of gas laws (they are summarised here but are not examinable).
1.
Boyle’s Law:
Variation of volume with
_______________
(const T).
Air was trapped in one end and varied the pressure on it by
raising the other end of the tube as shown below:
Taking a number of pressure and volume readings, the graph
looks like:
A more revealing graph is obtained by plotting p vs 1/V
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As a straight line is obtained that passes through the origin, we can say mathematically:
p  _______________
2.
Charles’ Law:
Variation of volume with _______________ (const p).
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The results obtained can be graphed as below:
Extrapolated to get origin:
This creates a new scale defined as zero degrees Kelvin (0K) = -273 oC.
A 1K rise = ____ oC rise.
Conversion equation:
T oC = T K + 273
3.
Guy-Lussac’s Law:
Variation of ____________ with temperature (const V).
(Law of pressures)
A bulb enclosing a fixed volume of gas is placed in a beaker of water and attached to a
mercury manometer as shown below:
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The graph of p vs T is shown below.
The graph does not pass through the origin
Ideal gas Equation
The three gas laws can be combined to give:
pV = nRT
where R = molar gas constant=8.31 J K-1 mol-1 (p has to be in atm), n= number of moles
MOLE
The mole is a quantity that allows us to compare the number of atoms of one substance with
another. The definition of a mole is : it is the amount of a substance that contains the same
___________ of elementary particles as _____g of carbon-12.
Experiments have shown that this number is
We refer to this number of particles as ____________________ _________________ and we
use the symbol NA with the unit ______ .
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MOLAR MASS
Not all substances contain just one element. Therefore we have to introduce the concept of a
molar mass. The molar mass is the mass of ____ mole of the substance. It is measured in
_______.
Example,
Molar mass of Oxygen gas is 32 x10-3 kg mol-1
If I have 20g of Oxygen, how many moles do I have and how many molecules?
20 x 10-3 kg / 32 x10-3 kg mol-1
 0.625 mol
 0.625 mol x 6.02 x 1023 molecules
 3.7625 x 1023 molecules
Questions
1.
2.
3.
4.
5.
6.
7.
8.
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9.
10.
11.
Now write your own questions and calculate the answers, using 5 more elements from
periodic table and 5 more compounds that you know.
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HEAT CAPACITY
Substances undergoing the ______ temperature change will release ________________ or
store different amounts of ___________. The temperature change that occurs when a
substance absorbs heat depends on the _______________ of the substance. To quantify heat
we must specify the amount of the substance. The heat capacity is a property of a
_______________ body. A common unit of heat is called the calorie. It is defined as:
The amount of heat required to _______________ the temperature of 1 g
of _______________ by 1oC.
A _______________e is the heat required to raise the temperature of 1 kg of water by 1oC.
The S.I. unit of heat is the same as all other forms of _______________ - the Joule (J). 1
calorie = _______________ J.
Heat capacity = ΔQ/ΔT
ΔQ …
ΔT …
Units for heat capacity _____ .
A body with a _____ heat capacity will take in thermal energy at a _________ rate than a
substance with a low heat capacity because it needs more _______ to absorb a _________
quantity of thermal energy
SPECIFIC HEAT CAPACITY
Why is it that you can touch the pastry on a McDonalds Apple pie but when you bite into it,
the Apple burns your tongue?
Why is it that a pizza can be just right but the pineapple is always much hotter?
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The answer lies in the fact that different substances have different capacities for storing
_______________. Put a litre of water in a saucepan and heat, it may take a few minutes to
boil. Put a metal knife on the same hotplate and it will reach the same _______________
much more quickly.
If we were given 1g of both iron and water, we would have a different number of molecules
of different type and mass in each sample. Water uses energy to increase the
_______________ of molecules, internal vibration and bond stretching. Iron atoms use the
energy to increase the translational _______________ energy. This means it takes 8 times the
amount of heat to raise 1g of water by 1 oC than it does for iron.
We say that water has a higher _______________ heat capacity.
The quantity of heat required to raise the _______________of a unit mass of a
substance by 1 degree is known as the _______________ heat capacity.
The specific heat capacity of one material is equal to the heat capacity divided by the mass of
the material. In equation form:
Heat capacity = mass * specific heat capacity
Using our definition of heat capacity this becomes:
specific heat capacity = Q/(mT)
Q …
T …
m…
we use the symbol c for specific heat capacity, so the equation becomes:
Q = _______________
The constant c is known as the specific heat of the material that is absorbing the energy and
has different values for different _______________.
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Water has a specific heat of 4200 J kg-1 oC-1. This means it takes 4200 J of energy to raise the
_______________ of 1kg of water by 1oC.
DIRECT METHOD - liquids
Mechanical systems are difficult to achieve accurate results, as there is a loss of energy due to
friction that cannot easily be accounted for. The most accurate values of the specific heat of
water have been made with _______________ systems.
The work done by an electric circuit is given by the equation:
W = _______________
Where V = voltage,
I = _______________
and t = the time the current was flowing.
This means that in this system, the work done by the electric circuit is equal to the
_______________ gained by the water:
VIt = _______________
To be very precise you must account for the energy gained by the __________ and the
_________________. This leads to
VIt =
Another quantity that is often used when dealing with electric circuits is power (W):
P = VI
DIRECT METHOD - solids
To measure the specific heat capacity of a solid you need a specially prepared block.
The block is cylindrical and has 2 holes drilled in it
•one for the ______________ and one for the heater
•Heater hole in the centre, so the heat spreads ________ through the block
•Thermometer hole, ½ way between the heater and the outside of the block, so that it gets the
_________ temperature of the block.
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Electrical energy input =
Energy gained by solid =
Overall:
INDIRECT METHOD
It can be difficult to measure the specific heat capacity of composite or non-destructible
materials. In such cases the specific heat capacity is found using the
‘______________________________’.
If an object such as a cube of brass whose mass and temperature is known, is put into a beaker
of boiling water, the brass gains heat in a fixed time.
It is then placed in a _______ mass liquid in a known ______ calorimeter,
The temperature difference can be recorded
diagram
Equations:
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Questions
1. If c for water is 4200 J kg-1 K-1 how much heat energy is required to raise the
temperature of 0.5 kg of water by 20 K?
2. If a 2 kg solid is heated and it takes 64000J of heat energy to raise its temperature by 5
K, what is the specific heat capacity of the solid?
3. If a 4 kg solid is heated and it takes 360 x 103J of heat energy to raise its temperature
by 15 K, what is the specific heat capacity of the solid?
4. a) An aluminium kettle has a mass of 1.5 kg when empty. The specific heat capacity
of aluminium is 900 J KG-1 K-1. What is the heat capacity of the kettle? (unit j k-1)
b) If the kettle is filled with 2.0kg of water how much energy is required to heat the whole
caboodle by from 20oC to 100oC?
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5. Estimate the heat energy required to heat a room full of air from 0oC to 20oC (for air: c =
993 J/kgK, density = 1.3 kg/m3)
6. A saucepan has a heat capacity of 800J/K and contains 3kg of water at 25 oC. How
much thermal energy is needed to raise the temperature of the saucepan and water to
75 oC (for water c=4200 J/kgK)
7. A 2 kg piece of hot iron is quenched by dropping it into 5 kg of water. If the
temperature of the water increases from 20oC to 42 oC, what was the temperature of
the iron to start with (water c=42000 J/kgK, iron c= 450 J/kgK)
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THE ATOMIC MODEL OF MATTER
There are three common states of matter on earth. A fourth state of matter, _________ is more
common. Stars are in this state of matter.
Macroscopic Characteristics:
Characteristics
Solid
Shape
Volume
Definite
Compressibility
Almost
Incompressible
Liquid
Gas
Variable
Variable
Definite
Diffusion
Highly
Compressible
Slow
Comparative Density High
Fast
Low
Microscopic characteristics:
Characteristics
Solid
High
1.
Liquid
Gas
Vibrational
Rotational
Translational
Rotational
Vibrational
Higher
Solids
Before the turn of the previous century, it was thought that the content of a solid determined
its _______________. It was what made diamonds hard, lead heavy and iron magnetic.
We now know that the characteristics of a solid are due to its _______________ - the
arrangement of atoms within the material.
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In samples of rock minerals such as quartz or galena, we see many smooth, flat surfaces. Each
mineral sample is made of crystals with regular geometric shapes. The atoms in a crystal are
in an orderly arrangement as shown below.
To keep the atoms in the regular pattern (_______________), there are forces
(_______________ in nature), which bind them together. If however, the atoms get too close,
the force becomes _______________ (between electrons in their outer shells). At the same
time, each atom has _______________ energy. This energy allows the atoms to move. This
intermolecular _______________ energy opposes the thermal motion and tries to hold them
in position. In solids, the thermal energy is very much smaller than the intermolecular binding
energy and so solids have specific _______________ properties.
Solids maintain a fixed shape and a fixed size. Even if a force is applied to it, it does not
readily change its _______________ or volume. The result is that the atoms vibrate about a
_______________ position.
2.
Liquids
In a liquid, the thermal energy (due to an increase in _______________) is greater allowing
the atoms to move farther apart. This means the _______________ forces are less and the
atoms are able to roll over each other. This gives rise to the _______________ properties of
liquids.
Liquids do not maintain a _______________ shape - it takes the shape of the container - but
like a solid, it is not readily _______________ and only a very large force can significantly
change its volume.
3.
Gases
The forces of attraction are so _______________ and the _______________ energy is so high
(due to another increase in temperature), the atoms do not even stay close together. They
move very rapidly in a _______________ manner filling the container and occasionally
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colliding with one another. The speed at which the atoms are moving is so fast that when they
do collide, the force of _______________ is not strong enough to keep them together and
they fly off in a new direction.
A gas has neither a fixed shape nor a fixed _______________ - it will expand to fill its
container.
4.
Plasma
At extremely high temperatures such as those found in stars, atoms are _______________.
The result is a collection of nuclei (ions) and electrons referred to as _______________. The
most common form of plasma is found in candle flames and fluorescent lights.
PHASES (states) OF MATTER AND LATENT HEAT
Add energy to ice and it turns to water. Add energy to water and it turns to steam. The state of
matter depends upon its temperature and the _______________ that is exerted upon it. To
change state, a transfer of _______________ is required.
Draw a flowchart showing how a change of state occurs
Heating Curve:
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1.
Evaporation/Condensation
A change of state from liquid to gas that takes place at the _______________ of the liquid.
The temperature of any body is related to the _______________ kinetic energy of its
molecules. As the molecules move in a random manner, some molecules may collide. Some
may lose _______________ energy and some may collide and increase kinetic energy,
enough to _________ the attractive forces of their _____________________ molecules.
As the higher kinetic energy molecules have escaped, the average kinetic energy of the liquid
has been _______________. This means the liquid left behind has been cooled and there is a
corresponding temperature _______________.
This is the principle used by evaporative air conditioners and humans _______________.
Condensation is the opposite process to _______________. This is the cooling of a gas to a
liquid. When water vapour molecules collide with a cold can of Coke, giving up so much
kinetic energy, they condense into a _______________.
Condensation is a _______________ process. The kinetic energy lost by the gas molecules
warms the surface that they strike. A steam burn is more dangerous than a boiling water burn
at the same _______________. Steam gives up energy when it condenses to the liquid that
wets the skin.
Factors that affect rate of evaporation:
1.
2.
3.
2.
Freezing/Melting
When energy is continually withdrawn from a liquid, molecular motion slows until the forces
of _______________ becomes enough to cause them to fuse. The molecules then vibrate
about _______________ positions and form a solid. This is called freezing.
If foreign material such as sugar or salt is added to water, the _______________temperature
is lowered. The foreign ions get in the way of the water molecules and stop them forming ice
_______________. Antifreeze is often used for this purpose.
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Freezing is a _______________ process, the average kinetic energy is reduced and so the
temperature is _______________.
The opposite of freezing is _______________. This occurs when energy is added. The
average kinetic energy is _______________ and so the temperature is increased.
3.
Vaporisation
Evaporation takes place at the _______________ of a liquid. A change of state from liquid to
gas can also take place within the liquid . The gas that forms beneath the surface occurs as
_______________, which moves up and _______________ into the surrounding air. This is
also called boiling.
The pressure of the _______________ within the bubble must be great enough to resist the
pressure of the _______________ water. If the temperature is below the boiling point, the
vapour _______________ is not great enough and so bubbles do not form.
4.
Sublimation
This is the process whereby a solid changes directly into a vapour without passing through the
_______________ phase. Carbon dioxide will do this at atmospheric pressure.
LATENT HEAT
During a change of state, there is no change in _______________ until all of the substance
has changed _______________. If we study boiling water and steam that are both at 100oC,
they both have the same average _______________ energy. The molecules in steam however,
have much more _______________ energy, as they are free to move and are not held
together. When water turns to steam, no temperature rise is observed as the energy absorbed
goes into increasing the _______________ energy.
Let us look at what happens when 1.0 kg of water is heated from -20oC where it is ice, until it
has become steam at 100oC at 1 atm pressure (1.03 x 105Pa). As heat is added, its temperature
_______________ at the rate of about 1oC for every 2 kJ of heat added. When the temperature
reaches 0 oC, the temperature stops rising even though heat is still added. When 340kJ have
been added, all the ice has turned to water and the temperature is still _______________ oC.
The energy required to change 1 kg of a substance from the solid to liquid state is called the
______________________________ (Lf). This also refers to the amount of heat released
when a liquid is turned to solid. For water, the Lf = 3.34 x 105 J.
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The water will now increase in temperature at the rate of 1 oC for every 4 kJ of heat added.
When the temperature reaches 100 oC, the temperature again remains _______________ until
all of the water is turned to steam. The 1kg of water we started with requires 2260 kJ to
change the state from liquid to gas.
The energy required to change 1 kg of a substance from the liquid to gaseous state is called
the ______________________________ (Lv). This also refers to the amount of heat released
when a gas is turned to liquid. For water, the Lv = _______________ J.
o
Temperature ( C)
The cooling curve for water is shown below:
120
100
80
60
40
20
0
-20
4
38
80
306
Heat added (kJ x 104)
The heat required to change the state of a substance can also be expressed mathematically.
Q = _______________
Where L is the _______________ heat of the particular process and substance, m is the mass
of the substance and Q is the heat required or given off.
To find the latent heat of a substance you use similar methods to the specific heat capacity.
PRESSURE
Pressure of a gas is due to the particles bombarding the walls of the container. Each collision
with the wall causes a _______________ change (as there is a change in direction). The force
on the wall by one molecule = average rate of change of _______________.
If the temperature of a gas increases, the average _______________ of the particles increases.
The increase in ________ of the particles leads to a greater __________ of collisions and
hence the pressure of the gas ____________ as the collisions with the side have increased
Also the change in momentum is greater, therefore greater ________.
Eg.When a force is applied to a piston in a cylinder containing a volume of gas
The particles take up a smaller volume
Smaller _________ to collide with
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And hence collisions are more __________ with the sides leading to an increase in pressure
Also, as the piston is being moved in
It gives the particles colliding with it more __________
Therefore they have more __________________
Therefore the __________________ of the gas rises.
POSTULATES OF THE KINETIC THEORY OF GASES
1.
A gas consists of a large number of ___________ particles in continual random motion.
Evidence
a)
1g of hydrogen atoms contain 6.023 x 1023 particles.
b)
Smoke particles move in a _______________ path which are bombarded by gas
molecules which in turn have velocities that are ______ in magnitude and direction.
2.
c)
Gas fills a _______________
d)
Only _______________ holds the atmosphere in.
Collisions between particles and the wall of the container are _______________.
Evidence
A gas left at constant p, V & T does not change _______________
3.
The particles in a gas are considered to be point masses with no _______________ Ek.
This simplification allows us to make straightforward calculations.
4.
Volume occupied by the gas particles themselves is assumed to be _______________
compared with the volume of the container as you can compress a gas.
5.
There are no forces acting between the particles themselves or between the particles and
the wall of the container except during _______________. There are large distances
between particles  _______________ is negligible.
6. Time between collisions is _______________ compared with the time for collisions.
There are no _______________ between particles  particles travel in straight lines
and constant v and then collide _______________ and then changes v. Forces act on
particles only during collision  collision time is _______________.
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Questions
1. If an aluminium kettle of mass of 1.5 kg when empty and specific heat capacity of 900 J
kg-1 K-1 is filled with 2kg of ice at 0oC. What is the total energy required to boil the water
dry? (Specific latent heat of ice is 336 000 J K-1).
2. How much energy must be removed from 5kg of water to freeze it?
3. If a fridge has to do 3J of work for every J of energy it removes from its contents, how
much work will the fridge do to achieve Q2?
4. If I have 4 moles of a gas at a pressure of 5.6 atm and a volume of 12 liters, what is the
temperature?
5. If I have an unknown quantity of gas at a pressure of 1.2 atm, a volume of 31 liters, and a
temperature of 87 0C, how many moles of gas do I have?
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THERMODYNAMICS
Thermodynamics is the study of heat and its transformation into ______________ energy.
The word is derived from the Greek meaning ‘_____________________’. It was developed
in the mid 1800’s and before atomic and molecular theory was developed.
Work is defined as the quantity of ________________ transferred from one system to another
by ordinary mechanical processes. From this we can see that thermodynamics describes the
relationship between ______________ and ________________.
The foundation of this area of study is the law of conservation of energy and the fact that heat
flows from ___________ to _______________.
Consider a hot gas separated from a cold gas by a glass wall. In macroscopic terms, we know
that the hot gas gets _________________ and the cool gas gets _______________. The
molecules in the hot gas hit the glass and set those molecules in faster motion. This then sets
in train a set of _____________ which sees the energy being transferred to the cold gas. If we
were to observe a single collision, we could analyse the energy transfer using the laws of
mechanics. We could say that one molecule has transferred energy by doing ______________
on another. Heat is therefore the work done on a molecular level.
This is, however, not the complete story. Although the cool gas contains, on average,
________________ molecules than in the hot gas, it does contain some fast moving
molecules. Likewise, the hot gas contains ___________________ moving molecules.
From above, it should be possible to for the cold gas to transfer ______________ to the hot
gas and so the cool gas would get cooler and the hot gas hotter. This does not disobey any
classical theory of mechanics. We do know however that this cannot occur.
To explain this, we cannot look at this the effects of single _______________ or even a few
molecules. We must, when discussing heat, look at the overall effects of a large number of
molecules and the average energies and _______________ of energies and velocities. This is
what is meant by a system of particles in thermodynamics. A system could be any group of
atoms, molecule or particles we wish to deal with. It may be the steam in a steam engine, the
earth’s atmosphere or the body of a living creature.
The operation of changing the system from its initial state to a final state is called
_______________________________. During this process, heat may be transferred into or
out of the system and work may be done on or by the system. We assume all processes are
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carried out very slowly so that the system remains in ______________________________at
all stages.
ISOTHERMAL AND ADIABATIC PROCESSES
Previously, we discussed the relationship between pressure and volume and found that:
P  _____________
We also stated that this was true, only if we held the temperature constant. A graph of P vs V
is shown below.
The volume has increased from Vi to Vf while the pressure has ______________. The solid
line is an _______________, that is, a curve giving the relationship between V and P at a
constant ___________________. This is known as isothermic expansion.
The process of compression or expansion of a gas so that no heat enters or leaves the system
is said to be _________________. This comes from the Greek which means ‘impassable’.
Adiabatic changes of volume can be achieved by performing the process so rapidly that
________________ has little time to enter or leave the system (like a bicycle pump) or by
thermally insulating a system from its surroundings (with Styrofoam).
A common example of a near adiabatic system is the ______________________ and
expansion of gases in the cylinders of a car engine. Compression and _____________ occur
too rapidly for heat to leave the system.
When _______ is done on a gas by adiabatically compressing it, the gas gains internal energy
and becomes warmer. When the gas adiabatically ________________, it does work on the
surroundings and gives up its internal energy and becomes cooler.
Adiabatic processes occur in the __________________ in large masses of air. Due to their
large size, mixing of different ________________ and temperatures only occur at the edges
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of these large masses and do little to change the composition of these air masses. As it flows
up the side of a mountain, its pressure ______________ allowing it to expand and cool. The
reduced pressure results in a reduced ____________. It has been shown that dry air will drop
by ____oC for every km it rises. Air can flow over high mountains or rise in thunderstorms or
cyclones many kilometres. If a mass was 25oC at sea level and was lifted 6 kilometres, its
temperature would become -____oC while an air mass that was -20OC at 6 km would be
______oC at sea level.
An example - cold air is blown over the Mt Lofty Ranges. Warm moist air is cooled as it rises
over the ranges and so starts to _____. On the other side, the air begins to warm as it flows
down the other side causing a warm wind. As the Mt Lofty ranges are not very high, the
change in temperature is not as great as if you were to compare it to the Rocky Mountains in
the USA.
P - V DIAGRAMS
Thermodynamic processes can be represented by pressure - volume graphs.
In the above diagram, an ideal gas is expanding ______________, absorbing heat Q and
doing ___________ W. In this case, the system has not been restored to its original state at
the end of the process.
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This diagram is from a reversible heat ____________. Process 1-2 takes place at a constant
volume, process 2-3 is ______________ while process 3-1 is at a constant
____________________.
In the next case, the volume of an ideal gas is decreased by adding weight to the piston. The
process is ___________________ (Q = 0).
The process is plotted on a graph as shown below.
In the next case, the temperature of an ideal gas is raised from T to T + T by a constant
__________ process. Heat is added and _________ is done in lifting the loaded piston.
The process is shown below on a P-V diagram
The work, PV, is the shaded area under the line connecting the initial and final states.
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FIRST LAW OF THERMODYNAMICS
Previously, heat was thought to be an invisible fluid called a _____________ which flowed
like water from hot objects to cold objects. Caloric was conserved in its interactions which led
to the discovery of the conservation of _________________.
Within any system, the less heat energy it has, the more ordered is the ___________ of its
molecules. This can be seen in solids where the molecules all vibrate about a
________________ position. As heat is added, the more disorderly the motion until in a gas
we can say that all molecules move in _______________ motion. In a sense then, heat is the
disordered energy of molecules. There can be _______ heat in a single molecule. Heat is a
statistical concept that applies only to a large number of molecules as it is only when we have
a great number of molecules does the concept of ____________________ or disorderly
movement have meaning.
The discussion of heat, _____________ energy and temperature has given rise to the law of
conservation of _____________, and when applied to thermal systems is often referred to as
the ____________________________________. In a general form it is:
Whenever heat is added to a system, it transforms to an ______________ amount
of some other form of ____________________.
The added energy does one or both of two things to the system:
1.
It increases the _____________ energy of the system if it remains in the system.
2.
It does __________________________ work if it leaves the system.
Heat added = __________________________________________________________
It can also be described mathematically:
Q = ________________
Where Q = ____________ energy
U = __________________ energy
W = __________________
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This can apply to a number of cases:
1.
Adiabatic Processes.
In this case, no heat enters or leaves the system, ie Q = 0. Substituting this into the 1st
Law;
0 = ______________ or,
U = ________________
This means that if work is done, there must be a decrease in the internal energy of the
system.
2.
Constant Volume Processes (Isochoric)
If the volume of a system is held constant, the system can do no ____________, ie W
= _________________________. Substituting this into the 1st Law;
Q = ___________________
If heat is added to the system, Q is +ive, the internal energy of the system increases.
The converse is also true.
3.
Cyclical Processes
There are processes in which, after certain interchanges of heat and work, the system is
returned to its initial state. In this case, ______ property of the system can change,
including the ____________________ energy, ie U =0. Substituting this into the 1st
Law;
Q = _________________
The net work done must exactly __________ the net amount of heat transferred.
4.
Free Expansion
This is an adiabatic process in which no ________ is done on or by the system, ie Q =
____ = _____. Substituting this into the 1st Law;
U =_______.
An example of this is when a gas, confined in an insulated container is released into
another container that originally was a vacuum and then waiting until an equilibrium is
established as shown below.
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No _____________ is transferred because of the insulation and no ____________ is
done because the expanding gas rushes into an evacuated space, its motion
________________________ by any counteracting pressure.
A summary of these processes can be seen in the table below:
Process
Restriction
Consequence
Adiabatic
Q = _____
U = _____
Const V
W = _____
U = _____
Closed Cycle
U = _____
Q = _____
Free Expansion
Q = _____ = _____
U = _____
THERMODYNAMIC CYCLES
A thermodynamic cycle is one where _________ may be transferred into (or out of) a system
or _______ may be done on or by the system. It is assumed that all transfers are done very
slowly so that the system remains essentially in
______________________________________ at all stages.
An _____________ is a device that changes heat into mechanical work. Example of this
includes the steam engine (______________ combustion) and the petrol & diesel
(_______________ combustion) engines. Although we will discuss it in more detail later, it is
enough to say at this point, it is impossible to convert all the heat energy into mechanical
work.
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Consider the internal combustion engine. Once the fuel is injected into the cylinder, the piston
moves up and compresses the gas (Q = _______), the spark plug fires and the temperature
increases. ______________________ expansion pushes the piston down and the burnt gases
are pushed out.
A heat engine is a device that changes ___________________ energy into
____________________ work. Examples include a steam engine and an internal combustion
engine and a jet engine. The mechanical work can only be obtained when heat flows from a
high to low temperature and only some of the heat is transferred into work.
Every heat engine will:
1.
absorb _____________________ energy from a reservoir of higher temperature
2.
convert some of this energy into ____________________ work expel the remaining
energy to some lower temperature reservoir (often called a _______).
In 1924, a French engineer called Sadi Carnot analysed the compression and expansion of in a
heat engine and made a fascinating discovery. The upper fraction of heat that can be
converted to useful work, even under ideal conditions, depends on the temperature
____________________ between the hot reservoir and the cold sink. His equation gives the
ideal or Carnot efficiency of a heat engine.
Efficiency = _____________
Where ______ = temperature of the hot reservoir
and ______ = temperature of the cold sink.
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ENTROPY
The idea of ordered energy (concentrated energy available in a way that can be used) tending
to disordered energy (unusable energy that is unavailable and lost) is evident in the concept of
___________________.
When petrol burns in a car engine, some of it does useful work, some heats the engine and
some goes out of the exhaust. Another example is the ordered energy of electricity going into
a light bulb in the house and being lost to ________________.
The measure of disorder is called entropy. If disorder __________________, entropy
increases. Gases escaping from a bottle move from relative order to disorder. In any physical
system, if the energy is allowed to distribute its energy freely, it will always do so in a way
that allows entropy to ________________________.
Individual examples can be cited that tend to break this law. In a human being, energy must
be transferred to it so that life can be supported. When it is not, the person soon dies and then
starts to tend to disorder. This means there can be pockets of order within the total system of
disorder.
You may like to suggest to your parents that your room is only obeying entropy!
SECOND LAW OF THERMODYNAMICS
A coin, when put flat on a table, cannot spontaneously rise into the air, suddenly get too hot to
touch or flatten out to something twice its diameter. These phenomena can easily be
explained.
Each of these situations requires energy to be ________________ to the system and so violate
the conservation of energy.
We also know that coffee in a cup cannot spontaneously cool down and start to swirl around,
one end of a spoon gets hot while the other end cools down and the molecules of air in the
room do not move to one corner of the room and stay there. These events however do obey
the conservation of ______________________________ and the first law of
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________________________. The coffee could get its energy from the cooling process, the
hot end of the spoon could get its energy from the cool end and the molecules of air do not
need to change their kinetic energy, only their position.
These events, however, do not happen although the reverse does happen. There are many
other cases where an event will happen in one direction but not the other. The direction in
which natural events happen is determined by the
_________________________________________________________.
It can be described on a macroscopic and microscopic base:
It is not possible to change heat completely into ______________, with no other
_________________ taking place.
It is not possible for heat to flow from one body to another body at a
____________________ temperature, with no other change taking place.
The example above that obeyed the first law but could not happen, violate the 2nd law of
thermodynamics.
REFRIGERATORS AND HEAT PUMPS
Heat flows from the inside of warm houses in winter to the cold outside. The reverse can
happen, but only by imposing external effort as do ________________________________.
These are used by air conditioners or refrigerators.
The second form of the 2nd law of thermodynamics states:
It is not possible for heat to flow from one body to another body at a higher temperature, with
no other change taking place.
A device that causes heat to move from a cold place to a hot place is called a
____________________________.
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In the diagram on the left, heat Qc is extracted from a ________ temperature reservoir (the
food storage area) and some ______ W is done on the system by an external agent. The heat
and work are combined and discharged as heat QH to a ____________ temperature reservoir
(the kitchen). The work shows up on the quarterly electricity bill and is done by the motor that
drives the unit.
The diagram on the right shows a perfect refrigerator where no work is required. This fridge is
yet to be built.
In an air-conditioner, the low temperature reservoir is the __________ to be cooled and the
high temperature _________________ is the outside air where the condenser coils are
located. Again, the motor does the work.
Both the fridge and the air-conditioner are rated by the amount of work they have to do. The
ratings are by the _______________________________________ K.
This is defined from:
K = _______________________________________
Design engineers want the performance of a fridge to be high as possible. A value of 5 is
typical for a household fridge, while a room air-conditioner is in the range 2-3. If there was a
perfect fridge, the value of K = .
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THE SECOND LAW AND TECHNOLOGY
The second law indicates limits for technology. Heat engines and refrigerators cannot be
perfect. It is not possible for heat to flow from one body to another at a higher
______________________________, with no other changes.
As the world is full of low-grade thermal energy from concept of entropy why can’t we
concentrate and harvest that energy? Why not lower the temperature of the oceans by 1 oC
and use that enormous amount of energy? It can be done but it requires that
_______________________ be put into the system which requires energy to drive a fridge
like machine. This energy input would make it unfeasible from an energy perspective.
As mentioned previously, every living creature from bacteria to higher life forms such as
_______________________ extract energy from their surroundings to increase their own
organisation. This tends to indicate that all life (including the above), plus their waste
products have a net increase in ___________________. The 1st law is a universal
______________ for which no exceptions have been observed. The 2nd law is, however, a
_________________________________. Given enough time, even the most improbable
states could exist. The 2nd law tells us the most probable event, not the only possible event.
The laws of thermodynamics are often put this way:
You can’t ____________ (because you can’t get more energy out of a system than you put
in), you can’t ___________________ (because you can’t even get as much energy as you put
in), you can’t ___________________________ (entropy in the universe is always increasing).
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Questions
1. Two ideal gases are kept at the same temperature in two containers separated by a
valve. One container is at 12atm and holds 6L of gas, the other is at 6atm and holds 3L
of the same gas. What will the pressure be when the valve is opened.
2. A gas is compressed isothermally so that an amount of work equal to 6500J is done on
it. How much heat is taken out or given to the gas?
3. In an adiabatic expansion of an ideal gas initially at 280K, 2.2L, 4.8atm, an amount of
work of 350J is done on the gas. Find the final temperature of the gas.
4. Explain in terms of entropy why the following processes are irreversible:
a) two different liquids mix
b) an ice cube melts in a warm room
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