ATOC 4720: class 11

1
ATOC 4720 class 11

• 1. The first law of thermodynamics
• 2. Joules law
• 3. Specific heats
• 4. Enthalpy

2
1. The first law of thermodynamics
A moving air mass kinetic energy
potential energy
Internal energy
Increase in internal kinetic energy T
Increase in internal potential energy relative

configuration
3
Assume a body of unit mass
Takes heat energy q (joules) thermal
conduction or radiation
External work w
Excess energy q-w
If there is no change in macroscopic kinetic
and potential energy, energy conservation
requires that its internal energy must Increase
by q-w.
4
Internal Energy Before and after
Where

In differential form
This equation states that increment of heat
added to a body is used to do external work by
the body and to increase the internal energy of
the body. This is the first law of
thermodynamics.
5
State 1, V1

1
dWpAdxpdV
State 2, V2
2
6
(No Transcript)
7
2. Joules law
1848, lab experiments
When a gas expands without doing External work,
and without taking in or giving out Heat, the
temperature of the gas does not change.
Ideal gas.
dq0, dw0, gt du0
8
3. Specific heat
dq is given to unit mass gt T to TdT dq/dT
is called specific heat
A specific heat at constant volume
9
dqdu
Joules law du depends only on T, so
10
We may also define
Physics cpgtcv
11
dp0, constant pressure
Cpdq/dT,
12
Cv717 J/deg/kg Cp1004 j/deg/kg, Diff?
First law of thermodynamics
13
4. Enthalpy
If heat is added to a material at constant
pressure, so that the specific volume of the
material increases from a1 to a2, the work
done by a unit mass of the material is p(a2-a1).
Therefore, the heat dq added to a unit mass of
the material at constant pressure is given by