Thermodynamic Laws
The Foundation Course on Physics
Professor Vojtěch Mornstein
Glencoe pp. 318 – 355, 368-371
1st Law of Thermodynamics
In general, the laws of thermodynamics are ones of the most
important laws in natural sciences. They are of extreme importance
also for explanation of the processes in living systems.
The 1st Law of Thermodynamics is a special form of the principle of
conservation of energy and is usually written as follows:
DU = W + Q
(or better dU = dW + dQ)
The right way of reading: The internal energy of a thermodynamic
system increases by DU if the surroundings does work W on the
thermodynamic system and heat Q is absorbed by the system
from its surroundings.
Joule experiment
James Prescott Joule (1818–1889)
Transformation of mechanical
energy in heat – one of the crucial
experiments to verify energy
conservation law.
Rotating
paddles in
water.
Temperature
increase is
measured.
Weight moves down
with a constant speed
1st Law of Thermodynamics
The following statement is another valid formulation of the
1st law of thermodynamics:
In a cyclic (reversible) process, the change in the internal
energy of the system equals zero.
The 1st law of thermodynamics therefore provides a complete
definition of internal energy U, one of the basic state
parameters.
The absolute value of the internal energy in real systems is
very difficult to determine (just think of the calculation of
the potential energy of all the particles making up the
system).
• As a result, classical thermodynamics only considers its
changes (DU, dU).
2nd Law of Thermodynamics
• One can recall many processes which are in agreement with the 1st
law of thermodynamics (during which energy is conserved), of which
it is however known by experience that they cannot happen in nature:
– In an isolated system consisting of a colder body and a hotter one, the
hotter body is never heated solely by the cooling of the colder body, i.e.
at the expense of the internal energy of the colder body.
– Gas, which leaves an enclosure and is dispersed in space, will never
spontaneously return into the enclosure.
– A wheel will not be turned by the heat from a hot brake.
– Rubbing hands produces heat but heated palms will not start to rub
mutually ☺.
– Etc.
• These practically impossible processes are characterised by the fact
that they are (better: would be) reversed irreversible processes,
sequences of non-equilibrium states. We know already that their
course cannot be reversed. The law determining the sense or direction
of the processes running in isolated systems (and with sufficient
accuracy in similar ones) is the 2nd law of thermodynamics.
Irreversible processes once again
Dough and
bread
Back to ideal gas: Its free expansion is a
simple example of an irreversible process.
A) A box is divided into two compartments by a wall. There is compressed ideal
gas in equilibrium state in the left compartment. In the right compartment, there
is vacuum.
B) We make someway an opening in the wall, the gas expands in the second part
of the box – an irreversible process is in progress.
C) After certain time, in both parts of the box tmd. equilibrium is reached.
„Ghosts“ in physics: Maxwell´s Demon
• http://www.pitt.edu/~jdnorton/Goodies/exorcism_phase_vol/exorcism_pha
se_vol.html
How to break the 2nd law?!?!
The ghost
separates fast and
slow particles,
temperature
difference
appears. What
has happened to
the 2nd law?
2nd Law of Thermodynamics
2nd Law of Thermodynamics has many mathematical
formulations or wordings. We mention only four:
• 1. It is impossible to make a cyclic (working) engine that
would only extract heat from a reservoir and convert it to
equivalent work, without a certain amount of heat being
transferred from the hotter to the colder body, i.e. it is
impossible to construct the so-called perpetuum mobile of
the second kind.
(Note: A perpetuum mobile of the first kind contradicts the law
of conservation of energy.)
• 2. It is impossible to transfer heat from a colder to a warmer
body without doing work.
• 3. Heat cannot spontaneously flow from a colder to a warmer
body.
Perpetuum mobile (to relax)
Perpetuum mobile of 2nd kind
„seriously“
Can it work forever? Where is the problem? Explanation will be awarded
by some candy tomorrow.
Is the problem influenced by the number of particles?
Original state
Equal distribution
final state
Non-equal distribution
Interesting vids
• https://www.youtube.com/watch?v=Y9nOV7u
8Y4A
• https://www.youtube.com/watch?v=Tay3-
2WKQ5Y
Entropy
4. There is an additive* state parameter/function S, entropy,
defined by the relation:
where DS is change in entropy and Q is the amount of heat added
to the system at temperature T. The inequality applies to
irreversible processes. In a reversible process:
Entropy is the result of the efforts to find a thermodynamic
quantity (a state parmeter) that would include heat transferred
into or outside the system. Heat is not a state function.
T
Q
S D
T
Q
S =D
*Additive state pameters: we can summarise their values in individual compartments of the
system. Additive/non-additive = extensive/intensive. Example: energy/temperature
Entropy
In a thermally isolated system (e.g. during an adiabatic
process, Q – amount of heat added - equals zero) the
following relation applies:
DS  0
As a result, the second law of thermodynamics is
sometimes referred to as the increase-in-entropy law. The
change in entropy in an isolated system can be only
positive, hence entropy can only increase. Processes in an
isolated system tend towards establishing thermodynamic
equilibrium, so that under thermodynamic equilibrium
entropy reaches its maximum.
Combined Formulation of the 1st and
2nd law
• For a reversible process we can formulate Q = T.DS and W =
-p.DV (taking into consideration only the volume work of
the system). If we pass the following for Q in the first law of
thermodynamics, we obtain:
DU = T.DS - p.DV.
• This equation is sometimes referred to as the combined
formulation of the first and the second law of
thermodynamics.
• The third law of thermodynamics is mentioned merely for
the sake of completeness: No finite process can bring the
system to the temperature of absolute zero.
The Carnot cycle (optional)
Carnot cycle – a theoretical heat engine
The most efficient heat engine cycle is the Carnot cycle, consisting of two isothermal
processes and two adiabatic processes. The Carnot cycle can be thought of as the most efficient
heat engine cycle allowed by physical laws. When the 2nd law of thermodynamics states that not
all the supplied heat in a heat engine can be used to do work, the Carnot efficiency sets the
limiting value on the fraction of the heat which can be so used.
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/carnot.html
Red area
quantifies
the useful
work done.
Heat was
transformed
into work
Carnot cycle in T(S) diagram
Q = TDS
QH = THDS
QC = TCDS
QH – QC = heat absorbed which can
be changed in useful work
Efficiency =
=
H
CH
H
CH
H
CH
T
TT
ST
STST
Q
QQ −
=
D
D−D
=
−
Carnot cycle
• (Carnot’s theorem) The efficiency of any heat engine operating
between two specified temperatures can never exceed the efficiency
of a Carnot engine operating between the same two temperatures.
• Maximum achievable efficiency of this engine η (eta) could be equal
to 1 (i.e. 100%) but only if the lower temperature T2 = 0 K. Efficiency
of real heat engines (e.g. diesel engines) does not exceed about 50
%.
• The heat engine is a system that converts heat or thermal energy—
and chemical energy—to mechanical energy, which can be used to
do mechanical work.
• 25 percent for most automotive gasoline engines.
• 49 percent for a modern coal-fired power stations
• 60 percent for a steam-cooled gas turbine.
Entropy and disorder (optional)
• We can easily imagine that absorption of heat in a system
can influence its structure – the increased thermal motion
can disintegrate molecules, for example.
• Overmore, it can be relatively easily shown that any
increase of entropy is connected with increase of disorder.
• Disorder has higher probability compared with an ordered
state.
• Thus there is an equation showing relation between
entropy and probability of a thermodynamic state. It is very
famous formula called Boltzmann principle:
DS = k ln(P1/P2)
The 2nd Law of Thermodynamics
• How to explain these jokes?
(Jokes of physicists are
sometimes difficult to
understand.)
Version 2020