chapter 3

1. Dilla University
Collage of Engineering and Technology
School of Mechanical and Automotive Engineering
Engineering thermodynamics
Prepared by: Ashenafi A.
Office: 07
E-mail: abebeashenafi7@gmail.com

2. Outline
o Definition of work
o Work done at the moving boundary of a simple compressible system
o Other systems that involve work
o Definition of heat
o Heat transfer modes
o Comparison of heat and work.
Chapter – Three
Work and Heat
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3. Objective
After the completion of this lecture and tutorial on introduction and fundamental
concepts, students should be able to:
 Define the concept of work, including electrical work and several forms of
mechanical work.
 Examine the moving boundary work or PdV work commonly encountered in
reciprocating devices such as automotive engines and compressors.
 Define the concept of heat and the terminology associated with energy transfer
by heat.
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4. Definition of work:-
o an energy interaction that is not caused by a temperature difference
between a system and its surroundings is called work.
or
o work is the energy transfer associated with a force acting through a
distance.
For instance – a rising piston, a rotating shaft, and an electric wire crossing
the system boundaries are all associated with work interactions.
o Work is also a form of energy and, therefore, has energy units such as kJ.
o The work done during a process between states 1 and 2 is denoted by𝑊12,
or simply W. The work done per unit mass of a system is denoted by w and
is expressed as:
Definition of work
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5. Definition of work
o The work done per unit time is called power and is denoted W. The unit of
power is kJ/s, or kW.
o work is a directional quantity. So work interaction requires the specification of
both the magnitude and direction.
The formal sign convention for work interactions is:
Work done by a system are positive; and work done on a
system are negative.
Or
Another way is to use the subscripts in and out to indicate
direction.
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Fig. 3.1 direction of heat and work

6. Definition of work
o When the direction of a heat or work interaction is not known, simply assume a
direction for the interaction (using the subscript in or out).
A positive result indicates the assumed direction is right. But a negative result
indicates that the direction of the interaction is the opposite of the assumed direction.
So revers the assumed direction.
Note:
Work is not a property. Because a quantity that is transferred to or from a system during
an interaction is not a property since the amount of such a quantity depends on more
than just the state of the system.
o Work is path functions i.e. its magnitude depends on the path followed. Path functions
have inexact differentials designated by the symbol 𝜹.
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7. Cont’d …
o The total work done during process 1–2, is
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Fig. 3.2 different process paths
The integral of 𝛿W is not W2 - W1
(i.e., the work at state 2 minus work
at state 1).

8. Moving boundary work
Moving boundary work: expansion and compression work involved in
automobile engines.
 Quasi-equilibrium (quasi-static) process - is a process during which the
system remains nearly in equilibrium at all times.
Consider the gas enclosed in the piston–cylinder device
Shown in fig. 3.3, and assume it is a quasi-equilibrium
process.
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Fig. 3.3 A gas does a differential amount of work δ𝑊𝑏 as it forces
the piston to move by a differential amount ds.
where; P = initial pressure of the gas, V = initial total
volume , and A = initial cross-sectional area of the
piston.

9. Now, If the piston is allowed to move a distance ds in a quasi-equilibrium manner, the
differential work done during this process is:
The boundary work is positive during an expansion process and negative during a
compression process.
 The total boundary work done during the entire process as the piston moves is
obtained by adding all the differential works from the initial state to the final state:
Cont’d …
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10. Cont’d …
 The area under the process curve on a P-V diagram is equal, in magnitude, to
the work done during a quasi-equilibrium expansion or compression process of a
closed system.
 On the P-v diagram, the area represents the boundary work done per unit mass.
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Fig. 3.4 The area under the process curve on a P-V diagram represents the boundary work.

11. Cont’d …
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Fig. 3.5 The boundary work done during a process depends
on the path followed as well as the end states.
In general, a gas will follow different paths in
a process and each path will have a different
area underneath it, and since this area
represents the magnitude of the work, the
work done will be different value for each
process path.

12. Cont’d …
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13. Cont’d …
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14. Cont’d …
Polytropic Process:
 During actual expansion and compression processes of gases, pressure
and volume are often related by
P𝑣 𝑛
= C,
where n and C are constants. A process of this kind is called a polytropic
process.
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Fig. 3.6 P − V diagram for various
polytropic processes.

15. Cont’d …
 Below is the development of a general expression for the work done during a
polytropic process.
The pressure for a polytropic process can be expressed as
Substituting in to the equation of work:
Since,
 For an ideal gas, the above equation can be written as:
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16. Cont’d …
For the special case of n = 1 the boundary work becomes
For an ideal gas this result is equivalent to the isothermal process.
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17. Cont’d …
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18. Other systems that involve work
There are other forces besides pressure forces, and those forces can also do work.
 A stretching wire stretched by tension force 𝜏 through length change dL. The differential
work is
 A surface with surface tension S. The differential work is
 A system with electrical work where E is the electrical field strength, q is the particle
charge, and x is the distance
 Magnetic work, where the generalized force is the magnetic field strength and the
generalized displacement is the total magnetic dipole moment
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19. Heat
Heat - is defined as the form of energy that is transferred between two systems (or
a system and its surroundings) by virtue of a temperature difference .
It follows that there cannot be any heat transfer between two systems that are at
the same temperature.
Heat is energy in transition. It is recognized only as it crosses the boundary of a
system.
As a form of energy, heat has energy units, kJ.
Adiabatic process - a process during which there is no heat transfer. A process
can be adiabatic when either the system is well insulated so that only a negligible
amount of heat can pass through the boundary, or both the system and the
surroundings are at the same temperature.
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20. Cont’d …
 Heat transfer per unit mass of a system is denoted q and is determined from:
 Formal sign convention for heat interactions
heat transfer to a system is positive; heat transfer from a system is negative.
or
Use the subscripts “in” to represent heat transfer to the system and “out” to
represent the heat transfer out of the system.
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21. Cont’d …
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Mechanisms of heat is transfer
1. Conduction - is the transfer of energy from the more energetic particles (hot) of a
substance to the adjacent less energetic ones (cold) as a result of interaction or
body contact between particles.
2. Convection - is the transfer of heat by the mixing and motion of macroscopic
portions of a fluid. it involves the combined effects of conduction and fluid motion.
3. Radiation - is the transfer of energy due to the emission of electromagnetic waves
(or photons).

22. Comparison of heat and work
Heat and work are energy transfer mechanisms between a system and its surroundings,
and there are many similarities between them:
1. Both are recognized at the boundaries of a system as they cross the boundaries.
That is, both heat and work are 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 depend on the path followed during a
process as well as the end states).
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23. Thank
you!!!