Thermodynamics is the branch of physics that deals with heat and other forms of energy. The first law of thermodynamics states that the total energy of a system remains constant, such that any increase in one form of energy (such as heat) results in an equal decrease in another form (such as work). The second law states that heat cannot spontaneously flow from a colder body to a hotter body without an input of work. The third law states that the entropy of a perfect crystal approaches zero as the temperature approaches absolute zero.

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2. WHAT IS
THERMODY
NAMICS?
 Thermodynamics is the branch of
physics that deals with the
relationships between heat and other
forms of energy.
 Thermal energy is the energy a
substance or system has due to its
temperature, i.e., the energy of moving
or vibrating molecules, Units of Heat.
 Thermodynamics involves measuring
this energy.

3. Thermal conductivity (k) is the rate at which heat passes
through a specified material, expressed as the amount of
heat that flows per unit time through a unit area with a
temperature gradient of one degree per unit distance.
The unit for k is watts (W) per meter (m) per kelvin (K).
Values of k for metals such as copper and silver are
relatively high at 401 and 428 W/m·K, respectively.
Other materials are useful because they are extremely poor
conductors of heat; this property is referred to as thermal
resistance, or R-value, which describes the rate at which
heat is transmitted through the material.
R-value is given in units of square feet times degrees
Fahrenheit times hours per British thermal unit
(ft2·°F·h/Btu) for a 1-inch-thick slab.

4.  In 1701, Sir Isaac Newton first stated his Law of
Cooling in a short article titled ‘Scala graduum
Caloris’ (‘A Scale of the Degrees of Heat’) in the
Philosophical Transactions of the Royal Society.
Newton's statement of the law translates from the
original Latin as, ‘the excess of the degrees of the
heat ... were in geometrical progression when the
times are in an arithmetical progression.’
Worcester Polytechnic Institute gives a more
modern version of the law as ‘the rate of change of
temperature is proportional to the difference
between the temperature of the object and that of
the surrounding environment.’

5. THE CARNOT
CYCLE
 In 1824, Nicolas Léonard Sadi Carnot
proposed a model for a heat engine based
on what has come to be known as the
Carnot cycle. The cycle exploits the
relationships among pressure, volume
and temperature of gasses and how an
input of energy can change form and do
work outside the system.

6.  All thermodynamic systems generate
waste heat. This waste results in an
increase in entropy, which for a closed
system is a quantitative measure of the
amount of thermal energy not available to
do work.
 Entropy is also defined as a measure of
the disorder or randomness in a closed
system, which also inexorably increases.

7.  Thermodynamic system: A thermodynamic system refers to a
definite quantity of matter, which is considered unique and
separated from everything else, which can influence it. Every system
is enclosed by an arbitrary surface, which is called its boundary.
 (a) Open System: It is a system, which can exchange mass and
energy with the surroundings. A water heater is an open system.
 (b) Closed system: It is a system, which can exchange energy but not
mass with the surroundings. A gas enclosed in a cylinder fitted with
a piston is a closed system.
 (c) Isolated system: It is a system, which can exchange neither mass
nor energy with the surrounding. A filled thermos flank is an ideal
example of an isolated system.

8. THERMODYNA
MIC TERMS
 Thermodynamic Variables or Coordinates: To
describe a thermodynamic system, we use its physical
properties such on temperature (T), pressure (P), and
volume (V). These are called thermodynamic
variables.
 Indicator diagram: To study a thermodynamic system,
we use a pressure-volume graph. This graph indicates
how pressure (P) of a system varies with its volume (V)
during a thermodynamic process and is known as an
indicator diagram.
ΔW = P ΔV
= Area of a shaded strip ABCD

9. If within the system, there are variations in
pressure or elastic stress, then parts of the
system may undergo some changes.
However, these changes cease ultimately,
and no unbalanced force will act on the
system. Then we say that it is in mechanical
equilibrium.
If a system has components which react
chemically, after some time, all possible
chemical reactions will cease to occur. Then the
system is said to be in chemical equilibrium. A
system, which exhibits thermal, mechanical and
chemical equilibrium, is said to be in
thermodynamic equilibrium. The macroscopic
properties of a system in this state do not
change with time.

10.  Thermodynamic Processes –
 (i) Reversible process: If a process is executed so that all intermediate stages between the initial and
final states are equilibrium states and the process can be executed back along the same equilibrium
states from its final state to its initial state, it is called reversible process. A reversible process is
executed very slowly and in a controlled manner.
 (ii) Irreversible process: A process, which cannot be retraced along the same equilibrium state from
final to the initial state, is called irreversible process.
 All natural processes are irreversible
 (iii) Isothermal process: A thermodynamic process that occurs at constant temperature is an
isothermal process. The expansion and compression of a perfect gas in a cylinder made of perfectly
conducting walls are isothermal processes. The change in pressure or volume is carried out very slowly
so that any heat developed is transferred into the surroundings and the temperature of the system
remains constant. The thermal equilibrium is always maintained. In such a process, ΔQ, ΔU and ΔW
are finite.
 (iv) Adiabatic process: A thermodynamic process in which no exchange of thermal energy occurs is an
adiabatic process. For example, the expansion and compression of a perfect gas in a cylinder made of
perfect insulating walls. The system is isolated from the surroundings. Any amount of heat neither
leaves the system nor enters it from the surroundings. In this process, therefore ΔQ = 0 and ΔU = –ΔW.
 (v) Isobaric process: A thermodynamic process that occurs at constant pressure is an isobaric process.
Heating of water under atmospheric pressure is an isobaric process.
 (vi) Isochoric process: A thermodynamic process that occurs at constant volume is an isochoric
process. For example, heating of a gas in a vessel of constant volume is an isochoric process. In this
process, volume of the gas remains constant so that no work is done, i.e.,
 ΔW = 0
 We therefore get ΔQ = ΔU.

11.  Cyclical processes: There are processes in which, after certain
interchanges of heat and work, the system is restored to its initial
state. In that case, no intrinsic property of the system—including its
internal energy—can possibly change.
 Putting, ΔEint = 0 in the first law yields
 ∴ ΔQ = ΔW (cyclical process)
 Constant-volume processes: If the volume of a system (such as a
gas) is held constant, that system can do no work. Putting W = 0 in
the first law yields
 ΔEint = Q (constant-volume process)
 Free expansions: These are adiabatic processes in which no transfer
of heat occurs between the system and its environment and no
work is done on or by the system. Thus, Q = W = 0, and the first law
requires that
 ΔEint = 0 (free expansion)

12. The Zeroth Law states that if two bodies are in thermal equilibrium with
some third body, then they are also in equilibrium with each other. This
establishes temperature as a fundamental and measurable property of
matter.
The First Law states that the total increase in the energy of a system is
equal to the increase in thermal energy plus the work done on the
system. This states that heat is a form of energy and is therefore subject
to the principle of conservation.
The Second Law states that heat energy cannot be transferred from a
body at a lower temperature to a body at a higher temperature without
the addition of energy. This is why it costs money to run an air
conditioner.
The Third Law states that the entropy of a pure crystal at absolute zero is zero. As explained above,
entropy is sometimes called ‘waste energy,’ i.e., energy that is unable to do work, and since there is no
heat energy whatsoever at absolute zero, there can be no waste energy. Entropy is also a measure of the
disorder in a system, and while a perfect crystal is by definition perfectly ordered, any positive value of
temperature means there is motion within the crystal, which causes disorder. For these reasons, there
can be no physical system with lower entropy, so entropy always has a positive value.

13. FIRST LAW OF THERMODYNAMICS
 ΔEint = Eint,f – Eint,i = Q – W
 dEint = dQ – dW
 Rules: The work done on a system is always the negative of the work done by the system, so if we
rewrite
 ΔEint = Eint,f – Eint,i = Q – W
 in terms of the work Won done on the system, we have
 ΔEint = Q + Won
ZEROTH LAW OF THERMODYNAMICS
 If bodies A and B are each in thermal equilibrium with a third body C, then A and B are in thermal
equilibrium with each other.

14.  The Kelvin-Planck’s statement is based on the experience about the
performance of heat engines. (Heat engine is discussed in next
section.) In a heat engine, the working substance extracts heat from
the source (hot body), converts a part of it into work and rejects the
rest of heat to the sink (environment). There is no engine, which
converts the whole heat into work, without rejecting some heat to
the sink. These observations led Kelvin and Planck to state the
second law of thermodynamics as it is impossible for any system to
absorb heat from a reservoir at a fixed temperature and convert
whole of it into work.
 Clausius statement of second law of thermodynamics is based on
the performance of a refrigerator. A refrigerator is a heat engine
working in the opposite direction. It transfers heat from a colder
body to a hotter body when external work is done on it. Here
concept of external work done on the system is important. To do this
external work, supply of energy from some external source is
necessary. These observations led Clausius to state the second law of
thermodynamics in the following form.
 It is impossible for any process to have as its sole result to transfer
heat from a colder body to a hotter body without any external
work.

15.  The third law of thermodynamics just says that you
cannot reach absolute zero through any process that uses
a finite number of steps. Which is to say, you cannot get
down to absolute zero at all. Each step in the process of
lowering an object’s temperature to absolute zero can get
the temperature a little closer, but you can’t get all the
way there, unless you use an infinite number of steps,
which isn’t possible.
 The Third Law of Thermodynamics is concerned with the
limiting behaviour of systems as the temperature
approaches absolute zero.
 Most thermodynamics calculations use only entropy
differences, so the zero point of the entropy scale is often
not important.

16.  The Third Law states, the entropy of a perfect crystal is zero
when the temperature of the crystal is equal to absolute zero
(0 K). The crystal must be perfect, or else there will be some
inherent disorder. It also must be at 0 K; otherwise, there will
be thermal motion within the crystal, which leads to disorder.
 The Third Law of Thermodynamics was first formulated by
German chemist and physicist Walther Nernst. In his book, ‘A
Survey of Thermodynamics’ (American Institute of Physics,
1994), Martin Bailyn quotes Nernst’s statement of the Third
Law as, ‘It is impossible for any procedure to lead to the
isotherm T = 0 in a finite number of steps’. This essentially
establishes a temperature absolute zero as being
unattainable in somewhat the same way as the speed of light
c. Theory states and experiments have shown that no matter
how fast something is moving, it can always be made to go
faster, but it can never reach the speed of light. Similarly, no
matter how cold a system is, it can always be made colder,
but it can never reach absolute zero.

17. There is a field of ultra-low-temperature
research, and every time you turn around
there’s a new record low. These days,
nanokelvin (nK = 10−9 K) temperatures
are reasonably easy to achieve, and
everyone is now working on picokelvins
(pK =, 10−12 K). The record-low
temperature was achieved 1999 by the
YKI-group of the Low Temperature
Laboratory at Aalto University in Finland.
They cooled a piece of rhodium metal to
100 pK, or 100 trillionths of a degree
Celsius above absolute zero besting the
previous record of 280 pK set by them in
1993.
While a temperature of absolute zero does not
exist in nature, and we cannot achieve it in the
laboratory, the concept of absolute zero is
critical for calculations involving temperature
and entropy. Many measurements imply a
relationship to some starting point. When we
state a distance, we have to ask, distance from
what? When we state a time, we have to ask,
time since when? Defining the zero value on the
temperature scale gives meaning to positive
values on that scale. When a temperature is
stated as 100 K, it means that the temperature
is 100 K above absolute zero, which is twice as
far above absolute zero as 50 K and half as far
as 200 K.

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