Thermodynamics I discusses energy analysis of closed systems. It examines moving boundary work from processes like in engines and compressors. The first law of thermodynamics states the principle of conservation of energy for closed systems. The general energy balance applied to closed systems relates the change in internal energy to heat and work. Specific heats at constant volume and pressure are defined and used to calculate changes in internal energy and enthalpy for ideal gases and incompressible substances.

1. Thermodynamics I 1
Chapter 4
Energy Analysis of Closed Systems

2. Homework #4 Due: TBA
2
Problems: 4.5E
4.8
4.24
4.25
4.34
4.42
4.46
4.55
4.59E
4.70E
4.74
4.81
4.89

3. 3
Objectives
• Examine the moving boundary work or P dV work commonly
encountered in reciprocating devices such as automotive engines
and compressors.
• Identify the first law of thermodynamics as simply a statement of
the conservation of energy principle for closed (fixed mass)
systems.
• Develop the general energy balance applied to closed systems.
• Define the specific heat at constant volume and the specific heat at
constant pressure.
• Relate the specific heats to the calculation of the changes in
internal energy and enthalpy of ideal gases.
• Describe incompressible substances and determine the changes in
their internal energy and enthalpy.
• Solve energy balance problems for closed (fixed mass) systems
that involve heat and work interactions for general pure
substances, ideal gases, and incompressible substances.

4. 4
MOVING BOUNDARY WORK
Moving boundary work (P dV work):
The expansion and compression work
in a piston-cylinder device.
The work associated with a
moving boundary is called
boundary work.
A gas does a differential amount of work Wb as it
forces the piston to move by a differential amount ds.
Wb is positive  for expansion
Wb is negative  for compression
For each process we need to determine:
P f V ( )

5. 5
The area under the process curve
on a P-V diagram represents the
boundary work.
The boundary work done during a
process depends on the path
followed as well as the end states.
The net work done during a cycle is the
difference between the work done by the
system and the work done on the system.

6. 6
Constant pressure process
Constant volume process
If the volume is held constant, dV = 0, and the
boundary work equation becomes:
P-V diagram for V = constant
P 1
2
V
P
V
2 1
P-V diagram for P = constant

7. Example- Isothermal Process of an Ideal Gas
7
Discuss Example 4-3 in class:
A piston-cylinder device initially contains 0.4 m3 of air at 100 kPa and
80 C. The air is now compressed to 0.1 m3 in such a way that the
temperature inside the cylinder remains constant. Determine the work done
during this process.
P
mRT
V


8. 8
Polytropic process
C, and n are constants.
n is called polytropic exponent.
Polytropic process of an ideal gas:
When n = 1 :
Schematic and P-V diagram for a polytropic process.

9. Example
9
Discuss Example 4-4 in class:

10. 10
ENERGY BALANCE FOR ALL SYSTEMS
Energy balance for any system
undergoing any process
Energy balance
in the rate form
For constant rates, the total quantities are related to the quantities per unit time
as follows:
Energy balance per unit mass basis
Energy balance in
differential form

11. 11
Energy balance when sign
convention is used (i.e., heat input
and work output are positive; heat
output and work input are negative).
Various forms of the first-law relation
for closed systems when sign
convention is used.
The first law cannot be proven mathematically, but no process in nature is known
to have violated the first law, and this should be taken as sufficient proof.
ENERGY BALANCE FOR CLOSED SYSTEMS
A closed system does not involve any mass flow across its boundaries, the
energy balance for the closed system can be expressed in terms of heat and
work interactions as:

12. 12
For a cycle E = 0, thus Q = W.
ENERGY BALANCE FOR CLOSED SYSTEMS IN
A CYCLE

13. 13
Energy balance for a constant-pressure
expansion or compression process (Example 4-5)
HWU b

For a constant-pressure expansion
or compression process:
An example of constant-pressure process
(example 4-5)
General analysis for a closed system
undergoing a quasi-equilibrium
constant-pressure process. Q is to the
system and W is from the system.

14. Example
14
Discuss Example 4-6 in class: