heat and work, thermodynamics note,thermodynamics note chapter3

1. [R Gnyawali/ P Timilsina] Page 1
Chapter 3: Heat and Work
Energy and Energy transfer:
Energy can be defined as the capability to produce an effect. It is a system quantity that describes
the thermodynamics state of a system. Energy can exist in numerous forms such as thermal,
mechanical, kinetic, potential, electric, magnetic, chemical, and nuclear, and their sum
constitutes the total energy (E) of a system. The magnetic, electric, chemical, nuclear and
surface tension effects are significant in some specialized cases only and but in thermodynamic
they are usually ignored. In the absence of such effects, the total energy of a system consists of
the kinetic, potential, and internal energies.
Energy can be stored within a system and can be transferred (as heat or work) from one to
another.
The energy that a system possesses as a result of its motion relative to some reference frame is
called kinetic energy (KE). The energy that a system possesses as a result of its elevation in a
gravitational field is called potential energy (PE). Internal energy includes the microscopic
forms of energy which result from the molecular motions and it is a thermodynamic property of
the system. It is denoted by U.
Energy of a system can be categorized into two titles.
1) Stored Energy (K.E.,P.E., Internal Energy, Chemical, Nuclear):
The forms of energy already discussed, which constitute the total energy of a system, can
be contained or stored in a system.
2) Transient Energy (Heat, Work):
The forms of energy not stored in a system. These energies are recognized at the system
boundary as they cross it, and they represent the energy gained or lost by a system during
a process. The only two forms of transient energy are heat transfer and work transfer.
Hence, Total energy of a system (E) = KE + PE + U
Specific energy (e) =
Units of Energy: Joules (J), Calorie (cal), British Thermal Unit (Btu)
Energy Transfer:
Work Transfer:
Work is defined as force F acting through a distance in the direction of the force.
W =
But in Thermodynamics, Work is said to be done by a system if the sole effect on the
surroundings (external to the system) could be the raising of a weight.

2. [R Gnyawali/ P Timilsina] Page 2
Units: J (N/m)
Sign convention:
Work transfer is the mechanism by which energy is transferred across a boundary between
system, by reason of the difference in pressure (force) of the two systems, and in the direction of
the lower pressure.
Work done of a simple compressible system
Shaded Area under P-V curve as shown in diagram below.
It is possible to take a system from state 1 to state 2 along
many quasi-static paths such as A, B or C. Since the area
under each curve represents the work for each process, the
amount of work done during each process not only is
function of the end states of the process but also depends
on the path that is followed in going from on state to
another. For this reason, work is a path function; or in
mathematical form denoted by , an inexact differential.
The properties (P, V, T, h, s) are point function.
Heat transfer:
Heat is defined as the form of energy that is transferred across boundary of a system (without
transfer of mass) by virtue of the temperature and in the direction of lower temperature.
System
W
(+ve) System
W
(-ve)
P
V
1
2
A
B
C

3. [R Gnyawali/ P Timilsina] Page 3
Sign Convention:
Similarities between Heat and Work
1) Heat and work are both transient phenomenon. Systems never possess heat or work, but
either crosses the system boundary when a system undergoes a change of state.
2) Both are boundary phenomenon. Both are observed only at the boundary of the system.
3) Both are path function.
Dissimilarities
1) Heat transfer is the energy interactions due to temperature difference but work can be
considered as energy transfer due to the reasons other than temperature difference.
2) In a stable system (no moving parts) there can’t be work transfer but heat transfer is
possible.
3) The sole effect external to the system can be reduced to rise of a weight for work transfer
but other effects are observed for heat transfer.
4) Heat is regarded as disorganized transfer mechanism and low grade of energy whereas
work is considered as organized mechanism and high grade of energy.