IB Physics
Topic 10
Thermal Physics
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 carried out very slowly so that the system remains in
______________________________at all stages.
ISOTHERMAL AND ADIABATIC PROCESSES
Previously is the SSC section of heat, 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 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.
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.
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 = __________________
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
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.
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.
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.
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 ________________________. 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
____________________________.
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 = .
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).