Energy may cross the boundary of a closed system only by heat or work. Heat is energy transferred due solely to a temperature difference, while work is energy expended to lift a weight. Both heat and work are path dependent functions that occur at system boundaries and depend on the process, not the initial and final states. Properties such as temperature and volume are point functions that depend only on the state.

1. Energy Transport by Heat and Work and the Classical Sign Convention
Energy may cross the boundary of a closed system only by heat or work.
Energy transfer across a system boundary due solely to the temperature difference
between a system and its surroundings is called heat.
Energy transferred across a system boundary that can be thought of as the energy
expended to lift a weight is called work.
Heat and work are energy transport mechanisms between a system and its
surroundings. The similarities between heat and work are as follows:
1.Both are recognized at the boundaries of a system as they cross the
boundaries. They are both boundary phenomena.
2.Systems possess energy, but not heat or work.
3.Both are associated with a process, not a state. Unlike properties, heat or
work has no meaning at a state.
4.Both are path functions (i.e., their magnitudes depends on the path
followed during a process as well as the end states. 1

2. Since heat and work are path dependent functions, they have inexact differentials
designated by the symbol δ. The differentials of heat and work are expressed as δQ
and δW. The integral of the differentials of heat and work over the process path gives
the amount of heat or work transfer that occurred at the system boundary during a
process.
2
∫
1, along path
δ Q = Q12 (not ∆Q)
2
∫
1, along path
δ W = W12 (not ∆W )
That is, the total heat transfer or work is obtained by following the process path and
adding the differential amounts of heat (δQ) or work (δW) along the way. The
integrals of δQ and δW are not Q2 – Q1 and W2 – W1, respectively, which are
meaningless since both heat and work are not properties and systems do not
possess heat or work at a state.
The following figure illustrates that properties (P, T, v, u, etc.) are point functions, that
is, they depend only on the states. However, heat and work are path functions, that
is, their magnitudes depend on the path followed.
2

3. • Internal energy: May be viewed as the sum of the kinetic and
potential energies of the molecules.
• Sensible heat: The kinetic energy of the molecules.
• Latent heat: The internal energy associated with the phase of a
system.
• Chemical (bond) energy: The internal energy associated with
the atomic bonds in a molecule.
• Nuclear energy: The internal energy associated with the bonds
within the nucleus of the atom itself.
What is thermal energy?
What is the difference between thermal
energy and heat?
3

4. Property
• Any characteristic that can be ascribed to a
system e.g. volume (V), temperature (T) and
pressure (P).
• Extensive property: depends on the size of
the system e.g. volume, mass
• Intensive property: independent of system
size: pressure, temperature, density
• Non-property: work, heat

5. State
• Condition of the system as described by its
properties.
• Usually only a subset of properties need to
be specified to identify the state of a system.

6. Thermodynamic equilibrium
(TE)
• Undergoes no changes when isolated from its surroundings.
• At an equilibrium state, there is no unbalanced driving
forces between the system and the surroundings and
between parts of the system.
• A system is in TE if following types of equilibrium are
satisfied.
– Mechanical equilibrium: balance of forces
– Thermal equilibrium: system undergoes no changes in properties when
separated from the surroundings through walls that allow passage of heat.
– Phase equilibrium: Mass of each phase within the system does not change
– Chemical equilibrium: chemical composition of the system does not change

7. Process and cycle
• Change that a system undergoes from one equilibrium state
to another equilibrium state.
• Cycle: A process that begins and ends at the same state
e.g. the working fluids of power plants and refrigerators work
in a cycle.

8. Quasiequilibrium process
• Quasiequilibrium or quasistatic process: A process
conducted so slowly that the system is in thermodynamic
equilibrium at every stage of this process.

9. Steady flow process
• Steadynot changing with time
• A process during which the fluid flows through an open
system (control volume) steadily.
• Of engineering importance

10. Temperature and the thermometer
• Temperature is a property which
characterizes “degree of hotness”.
• A thermometer contains a working
substance which undergoes easily
detectable changes in some property
according to the “degree of hotness”.
• Therefore, temperature can be assigned
numerical values based on the reading of
a thermometer.

11. Two bodies in thermal equilibrium
• When a “hot” body is placed in contact with a “cold” body through a
part of their boundary that allows passage of heat, the properties of
the body change initially due to heat transfer between them.
Eventually the heat transfer stops and the properties no longer
change with time. The two bodies have reached thermal equilibrium.

12. Zeroth Law of Thermodynamics
If:
• Body A is in thermal equilibrium with a body C
• Body B is in thermal equilibrium with body C
Then:
• Bodies A and B are in thermal equilibrium with
each other.
• If body C is a thermometer, then body A and
body B are in thermal equilibrium if they have
the same “thermometer reading” i.e.
temperature.
 Two bodies are in thermal equilibrium if they
have the same temperature (there is no need
anymore to place them in contact).

13. Thermodynamic equilibrium
(TE)
• Undergoes no changes when isolated from its surroundings.
• At an equilibrium state, there is no unbalanced driving
forces between the system and the surroundings and
between parts of the system.
• A system is in TE if following types of equilibrium are
satisfied.
– Mechanical equilibrium
– Thermal equilibrium Temperature uniform
– Phase equilibrium
– Chemical equilibrium

14. Pressure and density
• Normal force per unit area exerted by fluid on a
substrate
• Pressure is measured using manometers (rise of
mercury column), Bourdon Tube, pressure
transducers using pressure sensitive resistance,
capacitance, piezoelectric e.m.f. etc.
• Density is the mass per unit volume of a
macroscopic region surrounding a point within
the material.

15. 1-17
Chapter Summary
• The sum of all forms of energy of a system is called total energy,
which is considered to consist of internal, kinetic, and potential
energies. Internal energy represents the molecular energy of a
system and may exist in sensible, latent, chemical, and nuclear
forms.

16. 1-16
Chapter Summary
• The mass-dependent properties of a system are called extensive
properties and the others, intensive properties. Density is mass per
unit volume, and specific volume is volume per unit mass.

17. 1-17
Chapter Summary
• The sum of all forms of energy of a system is called total energy,
which is considered to consist of internal, kinetic, and potential
energies. Internal energy represents the molecular energy of a
system and may exist in sensible, latent, chemical, and nuclear
forms.

18. 1-1
Applications of
Thermodynamics
The human body
Air-conditioning Airplanes
systems
Car radiators Power plants Refrigeration systems

19. 1-2
Crossing Closed-System
Boundries
Energy, not mass, crosses closed-system boundries
• (Fig. 1-13)

20. 1-3
Closed System with Moving
Boundry

21. 1-4
Crossing Control Volume
Boundaries
Mass and Energy Cross Control Volume Boundaries

22. 1-5
System’s Internal Energy
System’s Internal Energy = Sum of Microscopic Energies
• (Fig. 1-19)