Chapter 3 – Heat and Work
An engine is a device that extracts energy from some source and converts that energy into
useful work.
In aircraft engines, the energy source is the fuel, and the useful work that is
produced is used to push the aircraft forward.
In order to understand how energy is
converted into useful work and to understand why aircraft engines are built the way they
are, we need to study a subject called
thermodynamics
.
Thermodynamics is a branch of science concerned with the conversion of heat into work
and vice versa.
You are probably very familiar with the ease in which we can turn work
into heat.
For example, you can feel your hands warm up when you rub them together
rapidly.
On the other hand, converting heat into work is not usually a simple task.
Heat and work are both manifestations of one thing  energy – and we use the same units
to measure both.
The SI unit is called the
Joule
(abbreviated
J
), and the English unit is
called the
British Thermal Unit
(abbreviated
BTU
).
To convert from one system to the
other, use the conversion:
1 BTU = 1055.1 J
Energy, work, and heat are terms you probably use quite often, and you probably have a
qualitative idea of what they are.
In our study of the thermodynamics of aircraft engines,
we will assign a precise definition to these terms.
Work
Work can be defined in a number of ways depending on the context.
In the
thermodynamic context, we usually mean
mechanical
work which is defined as the
product of force and distance moved by the point of application of the force, i.e., force
times distance.
W = F · d
(3.1)
Actually, this definition is somewhat simplified, but you will learn more about that later
in your physics course.
Since we are multiplying a force times a distance, we need to
introduce a few more units here.
In the SI system, a force has the units of
Newtons
(abbreviated
N
), and distance has the units of
meters
(abbreviated
m
).
One newtonmeter
equals one Joule.
1 Nm = 1 J
Example:
A man pushes a block across the floor from Point A to Point B.
The force he applies is a
steady 100 N and Point A is 30 meters away from Point B.
How much work has the man
done?
22
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View Full DocumentSolution:
Work = force x distance
J
Nm
J
Nm
m
N
3000
3000
30
100
=
=
×
Heat
Heat is not so easily defined.
If we add heat to a substance, then usually the temperature
increases.
If we denote the amount of heat by Q (Joules) and the temperature rise
∆
T
(measured in appropriate units), then we can often say that Q is proportional to
∆
T.
Mathematically, we can state that as follows:
Q
α
∆
T
(3.2)
What I have shown here is a proportion.
In order to turn this into an equation, we need a
constant of proportionality
.
We can get this constant by performing experiments in
which we carefully add a known quantity of heat to a substance and measure the
temperature.
We can do this over a wide range of heat inputs and plot the temperature
response on a graph.
If the temperature really is proportional to the input, our graph will
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 Spring '08
 Abbitt
 Absolute Zero, Joule

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