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The first law of thermodynamics
Historical notes
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Until 1600 combustion
Industrial Revolution
Newton
Heat: source of mechanical
work
• Carnot and Joule
• Thermodynamics
James Joule (1818-1889)
From energy to heat energy:
Joule's experiment
• The prevailing view of scientists was that the flow of heat
between two bodies at different temperatures consisted in the
transition from the warmer to the colder of a fluid, called caloric.
Until 1700
Benjamin
Thompson
James
Joule
• Called into question the model based on caloric fluid and
understood the nature of heat energy. He suggested that the
friction generated by the work of a body on another body
produces heat.
• Confirmed Thompson ‘s intuition around 1843 showing that
when mechanical work is transformed entirely into heat, the
relationship between work, measured in joules, and heat,
expressed in calories, remains constant in any process.
Joule's experiment
temperature
blades
masses
h
= 4,186
water
1 calorie = 4,186 joule
The First Law of Thermodynamics
A thermodynamic
system is any device
that exchanges heat
and work with the
external environment
The First Law of
Thermodynamics is a
special formulation of
the energy conservation
principle, applied to
thermodynamic systems.
It express the changes
in the internal energy of
a system when it
performs work or it
exchanges heat with the
environment.
Internal Energy
A state
function
The internal
energy is
molecular motion
Measured
in joules
Sum
between
kinetic
energy and
potential
energy
Internal energy U of a system is the sum of the
kinetic energy of molecules and the different
types of potential energy associated with the
bonds between the atoms that compose the
system
kinetic
energy of
molecules
potential
energy of
bond
internal
energy U
• First Law of Thermodynamics
U2>U1
U1=U
U2<U1
Ls
Qa
Lf
Qc
U2=U1+ ∆U
ΔU = Q–L
Quasi-static processes
Piston or elementary
machine
To understand the processes that
involve real thermodynamic
systems, which are rather complex,
we refer to the simple model of
heat engine, consisting of a
cylinder with a sliding piston and
containing a perfect gas.
The perfect gas
• Is the only substance of
which we can describe the
behavior with relatively
simple and accurate laws,
based on measures of
volume, pressure and
temperature, said state
quantities; these, we add
the internal energy U of
an ideal gas, which is all
kinetic and depends only
on the temperature.
U = U(T)
Reversible and irreversible transformations
The real thermodynamic processes give rise in the
PV plane, to a graphic representation which is not
possible to describe mathematically because for a
given volume of fluid, the pressure has different
values. So they are irreversible.
In a very slow process, there are a succession off
equilibrium states. This transformation is called
quasi-static.
The quasi-static transformations can be played in
reverse.
1) isobaric
4)adiabatic
transformations
in the perfect
gas
3)isothermal
2)isochoric
ΔX
pressure
1) Transformation at constant pressure: isobaric
pi=pf
Vi
Vf
volume
pressure
2) Transformation at constant volume: isochoric
pi
L=0
pf
ΔU=Q-L=Q
Vi=Vf
volume
pressure
3) Transformation at constant temperature:
isothermal
pi
ΔU=0
pf
Vi
Vf
volume
ΔU=Q-L
Q=L
4) Transformation without heat exchange:
adiabatic
Q=0,L<0
ΔU=Q-L
ΔU=-L >0
ΔT>0
Q=0,L>0
ΔU=Q-L
ΔU=-L <0
ΔT<0
heat engines
Thomas Savery
(1650-1715)
Thomas Newcomen
(1663-1729)
James Watt
(1736-1819)
Charles Parsons
(1854-1931)
Thermodynamic cycles
Sadi Carnot (17961832)
ΔU= 0  0=Q-L  Q=L
A
C
C
C
The Carnot engine
Heat engine
human body