Thermodynamics is a branch of physics
that deals with the energy and work of a system.
Thermodynamics deals
only with the large scale response of a system that we can observe
and measure in experiments. As aerodynamicists, we are most
interested in the thermodynamics of propulsion
systems.
The Zeroth Law of Thermodynamics
introduces the concept of thermodynamic equilibrium, in which
two objects have the same temperature. If
we bring two objects that are initially at different temperatures
into physical contact, they will eventually achieve thermal
equilibrium. During the process of reaching thermal equilibrium, heat
is transferred between the objects.
The amount of heat transferred is
proportional to the temperature difference between the objects and
the
heat capacity
of the object. The heat capacity is a
constant that tells how much heat is added per unit temperature rise.
The value of the constant is different for different materials. Heat
is always transferred from the object at the higher
temperature to the object
with the lower temperature.
For a gas, the heat transfer is related to a change in
temperature. The temperature, pressure,
and volume of the gas determine the state
of the gas. Heating a gas changes the state of the gas. But the state
of a gas can be changed in a wide variety of ways. On another slide,
we show how work done on a gas also changes
the state of the gas. The amount of work that a gas can do depends on
both the initial and final states and on the process used to make the
change. In the same way, the amount of heat transferred in changing
the state of a gas also depends on the initial and final states and
the exact process used to change the state. So, while the temperature change
may be the same for two processes, the heat transfer is not normally the same.
The value for the heat capacity is different for different processes.
Different processes
result in different amounts of heat transfer and work. The effects of
both heat flow and work are combined in the First
Law of Thermodynamics.
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