We were discussing Boyle’s law and Charles law in
our previous post. Today we will try to understand here the first law of
thermodynamics.
If a system follows a complete thermodynamics cycle,
there will be a net work which will be done on the system or done by the
system.Â
Let us consider one thermodynamic system which is following a complete
thermodynamic cycle and net work is done by the system.
As we are aware with
law of conservation of energy that energy can’t be created or destroyed but it
could be converted from one form of energy to another form of energy. Therefore
system will require a mechanical energy for doing this work.
Now let us concentrate over the system that we have
assumed above. System is following a complete thermodynamic cycle i.e. system
has reached to its original state or we may say that internal energy of the
system will be constant. Therefore system will not provide the mechanical
energy by itself for doing the work. For doing the work, system will require
here heat energy.
According to law of conservation of energy, net work
done by the system will be equivalent to the net heat supplied to the system.
Now, we can define here the first law of thermodynamics
When a closed system follows a thermodynamic cycle,
net work done by the system on its surrounding will be equal to the net heat
supplied to the system from its surrounding.
Therefore, for a closed system that follows a
thermodynamic cycle, we will have following relationship as per first law of
thermodynamics.
Figure shows a thermodynamic cycle
∑d
Q = ∑ d W
∑d Q: Net heat supplied to the system from its
surrounding
∑d W: Net work done by the system on its surrounding
First law of thermodynamics is applicable to
reversible as well as irreversible processes also.
First law of thermodynamics for a non cyclic process
When we consider first law of thermodynamic process
for a non cyclic process, there will be one more term involved i.e. internal
energy of the system.
Let us consider a system is undergoing with change of
state in a non cyclic process and heat and work transfer, both are involved in
this process. Net transferred energy will be accumulated within the system.
Figure showing a system, where Q is the heat energy
supplied to the system from surrounding and work energy W is the work which is
done by the system on surrounding. Net transferred energy i.e. (Q-W) will be
accumulated within the system. This accumulated energy will be termed as change in internal
energy of the system.
According to first law of thermodynamics, we will
have
Let us consider a system where there is more energy
transformation takes place as shown in figure
According to first law of thermodynamics, we will
have following equation here
Q2 + Q3 – Q1 = ∆E +
(W2+ W3-W1-W4)
∆E or ∆U indicates change in internal energy of the
system
Sign convention for heat and work
Understanding of sign convention for heat and work is
quite important to secure the correct equation.Â
Heat supplied to the system
will be taken as positive and heat rejected from the system will be considered
as negative.
Work done by the system will be considered as
positive and work done on the system will be considered as negative.
We will discuss the second law of thermodynamics in
our next post
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