The Principles
and Concepts of
Thermodynamics
The Foundation Course on Physics
Professor Vojtěch Mornstein
Glencoe pp. 318 - 355
Thermodynamics
Thermodynamics (TMD) is a part of physics which is concerned with
transformations of heat and other forms of energy in TMD systems.
Why to get acquainted with TMD in medicine?
• Heat is produced inside our bodies. To keep ideal body temperature we
need some thermoregulation.
• Heat production accompanies and characterises chemical a biochemical
reactions. Enthalpy H and Free enthalpy G are of special importance in
chemistry.
• Heat can be used for different procedures in physiotherapy, we can even
speak about thermotherapy (like baths, sauna, diathermy, ultrasound
etc.)
• Intense heating of tissues is used in surgery to kill e.g. tumours, or to
stop some bleedings – we speak about thermal ablation and
electrocauterisation. Hyperthermia can kill tissues only by temperature
increase by 20 °C!
• Measurement of temperature is an important diagnostic method for
centuries.
• Thermography belongs among imaging methods.
Remember: heat = kind of energy
3
Example: Activation of body by various
thermotherapeutic methods - hydrotherapy
Whirling bath increases blood
supply and metabolism.
Alternating
application of
sharp hot and cold
water jets – a
method with
activation effect.
Sauna: hot (80 - 100°C) air of
low humidity (10-30%) followed
by cooling in cold water. Steam
bath: about 45°C, up to 100%
humidity.
4
Baths, paraffin etc.
• Heat can be delivered not only
in water bath (which is most
common). Quite popular are
also peloids (mud baths).
• Turf can be used as well.
• Advertisement: Tell your
parents about spas in
Czechia!☺
• For treatment of e.g. an
arthritic knee, it can be helpful
to use even molten paraffin
with initial temperature up to
60 - 77 °C.
Extreme cold – Cryotherapy Chamber
5
Action of dry and very
cold air (up to -160°C!)
during some minutes
followed by aerobic
exercise.
6
Short-wave diathermy – an example of electric
tissue heating. The disc electrodes are not in direct
contact with the skin, there is insulating gap instead.
Heat is produced by an
alternating electric
field! In this field, the
charged molecules are
put in motion, heat
arises due to friction.
Hyperthermia
• Thermal ablation of a tumor – high frequency currents are
used. It is kind of surgery.
Thermography – Infrared cameras
Sophisticated method of temperature measurement –
visualisation of temperature maps.
A thermoimage of a patient´s feet one day before and one day
after „revascularisation“ (renewal of full blood supply). The colours
are not real, it is a way how to display different temperatures.
Let us go on!
• Thermodynamics is not easy!
Thermodynamics as science
• Development: 19th century – steam engines, combustion
engines, turbines.
• At the beginning of 20th century it became solid foundation of
Physical chemistry.
• It is the key to understanding uniqueness of life, and answer
to questions e.g.:
• What is the difference between an animated and unanimated
matter?
• This question can be answered in the so-called nonequilibrium
thermodynamics.
JAMES WATT
19.1.1736 - 19.8.1819
Milestones
Otto von Guericke (1650) - first vacuum pump and demonstrated a vacuum („Magdeburg
hemispheres“).
Robert Boyle with Robert Hooke noticed a correlation between pressure, temperature,
and volume a little later.
In 1697 Thomas Savery built the first engine driven by steam.
At the University of Glasgow James Watt (1781) increased steam engine efficiency.
Sadi Carnot, the "father of thermodynamics", published Reflections on the Motive Power
of Fire (1824), a discourse on heat, power, energy and engine efficiency (Carnot cycle!). It
marked the start of thermodynamics as a modern science.
The first thermodynamic textbook was written in 1859 by William Rankine.
The first and second laws of thermodynamics emerged simultaneously in the 1850s,
primarily out of the works of W. Rankine, R. Clausius, and William Thomson (Lord Kelvin).
The foundations of statistical thermodynamics were set by physicists such as James Clerk
Maxwell, Ludwig Boltzmann, Max Planck, Rudolf Clausius and Josiah Willard Gibbs.
During the years 1873-76 the American physicist Gibbs published a paper On the
Equilibrium of Heterogeneous Substances, in which he showed how thermodynamic
processes, including chemical reactions, could be graphically analyzed to determine if a
process would occur spontaneously.
The Thermodynamic System
• Definition:
– Thermodynamic system: A region of space bounded by arbitrary
surfaces which delineate the portion of the universe we are interested
in. In other words: an arbitrary object, a body greater than a molecule.
We can distinguish:
– Isolated system: one which cannot exchange particles or energy with
its environment.
– Open system: one which can exchange both particles and energy with
its environment.
– Closed system: can exchange energy but no particles.
The Equilibrium State
• An isolated system always reaches an equilibrium state
in which it does not macroscopically change. Open
systems do not reach the equilibrium state in general.
• LIVING SYSTEMS ARE OPEN SYSTEMS
• Thermal equilibrium is a special case of an equilibrium
state.
• Two systems in thermal equilibrium have the same rate
of thermal energy exchange between them. They are at
the same temperature in any given point.
Thermodynamics – the State Parameters
• Quantities describing a TMD system in equilibrium are called state
parameters.
• In a case of thermal equilibrium, the temperature is a determinant
quantity/state parameter. However, it is not sufficient for full
description of a thermodynamic system.
• A defined set of state parameters is necessary for full description of
a TMD system. It can be volume V, pressure p, temperature T,
concentration of substance c, internal energy U, enthalpy H …..
• These parameters are mutually related to each other in the
equations of state.
• The simplest thermodynamic system is the ideal (or perfect) gas.
• Equation of state for ideal gas (you may already know) – it is the
universal gas law:
pV = nRT
[Pa, m3, mol, J·K-1·mol-1, K]
What is the ideal (perfect) gas?
• A perfect gas is a theoretical gas that differs from real
gases in a way that makes certain calculations easier. In
most cases the perfect gas is the same as an ideal gas.
In particular, intermolecular forces are neglected,
which means that one can neglect many complications
that may arise from the Van der Waals
(intermolecular) forces. The particles of the perfect gas
have no volume of its own but they can elastically
collide and share their kinetic energy!
• (Van der Waals forces are, in principle, attraction forces
of electric character acting between all molecules.)
Equation of State for a Real Gas (optional)
P – pressure, V – volume, n – amount of
substance in moles, R – molar gas constant, a, b –
semi-empirical constants. In the real gas the
molecules are attracted (thus lowering the gas
pressure) and have their own volumes (real gas
cannot be compressed to infinitely small volume).
Van der Waals equation:
Reversible and Irreversible Thermodynamic
Process
• Reversible TMD processes are represented by a sequence
(series, train) of very close equilibrium states which differ only
slightly in values of some thermodynamic state quantities. This
sequence could be “reversed”, and then the same process
would proceed backwards. The reversible processes are only
approximations of real ones. They do not take place in nature.
• In other words, the reversible process is the one after which a
second process could be performed so that the system and
surroundings can be restored to their initial states with no
change in the system or surroundings.
• Irreversible processes are sequences of non-equilibrium
states, and they cannot be “reversed”.
Reversible and Irreversible
Thermodynamic Process
• Cyclic process: the initial and final states of the system are
identical (but not necessarily the state of surroundings)
• Sign convention: energy given to a system and work done
by an external force on the system are considered to be
positive. Energy transferred from the system to its
surroundings and work done by the system on its
surroundings are considered to be negative.
Most of the formulas/equations in this presentation are fully
valid only for reversible processes. Mathematical approach to
irreversible processes is much more complex.
Irreversible Processes
Back to Kinetic Theory
Before we will deal with thermodynamic processes
and laws we should finish the explanation of the
two fundamental thermodynamic quantities –
the pressure and temperature.
The following explanation is based on the
description of the movement of individual
molecules in a perfect gas enclosed in a vessel.
We also need to average physical properties of
these molecules.
Pressure of a gas
• First, we assume constant amount of
a gas in an thermally insulated vessel.
• For maximum simplification, the
particles will move only along the x-
axis.
• The particles collide sometimes with
the perfectly elastic walls, and have
to change their momentum (Dp)
because they reverse their
movement.
• The whole „round trip“ between the
opposite walls takes Dt for each
particle.
• The Fx bar and vx bar symbols denote
average values of respective
quantities (force and velocity).
Impulse – Force theorem:
𝐹𝑥Δ𝑡 = Δ𝑝 = 2𝑚𝑣 𝑥
(Total change of velocity of the
particle colliding with the wall is 2vx
because the particle has to stop and
then gain an opposite velocity.)
Pressure of a gas
We can derive the time for the whole „round trip“ of a particle:
t
L
vbecause
v
L
t x
x D
==D
22
Thus, after substitution, the average force exerted by one particle
on the wall is:
𝐹 𝑥 =
∆𝑝
∆𝑡
=
2𝑚𝑣 𝑥
2𝐿
𝑣 𝑥
=
𝑚𝑣 𝑥
2
𝐿
For N molecules with the same mass m we can write:
( )
L
vvvvm
F Nxxxx
x
22
3
2
2
2
1 ...+++
=
And also:
𝑣 𝑥
2
=
𝑣1𝑥
2
+ 𝑣2𝑥
2
+ 𝑣3𝑥
2
+. . . 𝑣 𝑁𝑥
2
𝑁
Pressure of a gas
Hence the average force of all molecules exerted on the wall is:
L
vmN
F x
x
2
=
We have considered movement of the molecules only along the x-axis which would be
possible only in a „one-dimensional“ gas. In 3D space the kinetic energy of particles is
equally distributed in individual dimensions. We can say the same about the force
acting on the walls. So the total force F has to be one third of the Fx.
L
vmN
FF x
2
3
1
3
1
==
Pressure p is defined as
F/A, where A is the force
action area. In our case it
is the area of the wall.
A × L is the volume V of
the gas. Thus (bars are
omitted): V
mNv
LA
mNv
A
F
p
33
22
===
Pressure and the Universal Gas Law
Transferred from previous slide:
V
mNv
LA
mNv
A
F
p
33
22
===
The velocity v in above formula is called mean quadratic velocity. It can be shown
that the same velocity can be expressed also:
m
kT
v
3
=
where k is the Boltzmann constant (1.38064852·10-23 m2·kg·s-2·K-1 i.e. ….. J·K-1 )
and T is the Kelvin temperature.
Now we can substitute in the formula for pressure where N = nNA (n is the number
of moles, NA is the Avogadro constant).
kTnNpV
V
kTnN
m
kT
V
mnN
p A
AA
===
3
3
NA k = R
R is molar gas constant!
So we have: pV = nRT - the Universal Gas Law!
Tired?
Work done by / on thermodynamic
systems
Gas and piston system (heat engines)- This kind of work can be called mechanic or
volumetric
• In thermodynamic systems doing mechanical work the acting force can be
expressed as the product of pressure p and surface area A, on which the
pressure is exerted, that is F = p.A. Let us imagine that the piston face area A
moves along a very short trajectory Ds, thus increasing gas volume (see Fig).
The volume change DV is calculated as DV = A.Ds. If the gas volume
increases against external forces mechanical work is done, which is equal to
the product of force and the displacement along which the force is exerted,
hence:
W = F.Ds = p.A.Ds = p.DV.
Work is not
a state
parameter!!
Work done by / on thermodynamic
systems
a) electric work (done by galvanic cell or nerve cell
membrane)
W = Q.U ,
where W is the work necessary to transfer an electric charge Q
between places with potential difference U*
b) chemical work (crucial for chemistry or
biochemistry)
W = m.Dn,
where W is the work necessary to increase or decrease amount of a
chemical compound Dn in a chemical reaction. m is chemical potential.
*Potential difference = voltage (denoted as U, V or E)
Other important quantities:
Kelvin temperature is a quantity which indicates the average
kinetic energy WKS of the particles in a system e.g., for an ideal
monatomic gas (k is the Boltzmann constant):
Internal energy of the system is the sum of all kinetic and potential
energies of all particles forming the system.
Heat (thermal energy) is the part of internal energy of the system
which can be exchanged between systems as a result of their
different temperatures.
Heat is not a state parameter!!
Version 2020