Thermodynamics is a branch of physics
which deals with the energy and work of a system.
only with the large scale response of a system which we can observe
and measure in experiments. An understanding of thermodynamics was
very important for the Wright brothers in the design of their
In our observations of the work done on
(or by) a gas, we have found that the amount of work depends not only
on the initial and final states of the gas
but also on the process (or path) which produces the final state.
Similarly the amount of heat transferred into (or
from) a gas also depends on the initial and final states and the
process which produces the final state. Many observations of real
gases have shown that the difference of the heat flow into the gas
and the work done by the gas depends only on the initial and final
states of the gas and does not depend on the process
which produces the final state. This suggests the existence of an
additional variable, called the internal energy of the gas,
which depends only on the state of the gas and not on the process which produces
The internal energy is a state variable, just like the temperature or
the pressure. The first law of thermodynamics defines the change in internal
energy (E) from state 1 to state 2 as equal to the difference of
the heat transfer (Q) into a system and
the work (W) done by the system.
E2 - E1 = Q - W
We have emphasized the
words "into" and "by" in the definition. Heat removed from a system
would be assigned a negative sign in the equation. Similarly work
done on the system is assigned a negative sign.
The internal energy is just a form of energy like the potential
energy of an object at some height above the earth, or the kinetic
energy of an object in motion. The first law is really just a statement of the
conservation of energy for a thermodynamic system.
In the same way that potential energy
can be converted to kinetic energy while conserving the total energy
of the system, the internal energy of a thermodynamic system can be
converted to either kinetic or potential energy. Like potential
energy, the internal energy can be stored in the system.
Notice, however, that heat and
work can not be stored or conserved independently since they depend
on the process.
The first law of thermodynamics allows for many
possible states of a system to exist, but only certain states are
found to exist in nature. The
of thermodynamics helps to
explain this observation.
If a system is fully insulated from the outside environment, it is
possible to have a change of state in which no heat is transferred into the
system. Scientists refer to a process which does not involve heat
transfer as an adiabatic process.
It is also possible to have a change of state in which no work is done by
the system and only heat is transferred.
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