We were discussing various basic concepts of thermodynamics
such as work energy transfer in thermodynamics in our recent post. We have also discussed the concept of enthalpy in the field of
thermal engineering.
Today we will see here the first law of thermodynamics
for closed system undergoing a change of state with the help of this post.
Let us see here first basics of first law of thermodynamics
As we have already discussed that first law of thermodynamics deals with
the law of conservation of energy and according to law of conservation of
energy, energy can’t be created or destroyed but also it could be converted
from one form of energy to another form of energy.
We can also say that energy will always be
conserved. Heat and work, these are two different forms of energy. If heat
energy is provided to the system by the surrounding then system may provide the
work energy to the surrounding and similarly if work is being done upon the
system then system may deliver the energy in terms of heat to the surrounding.
Hence, this is the basic concept of first law of
thermodynamics. Now we will see here the first law of thermodynamics for a
closed system undergoing a change of state. After that, in our next post, we
will see first law of thermodynamics for an open system or control volume.
As we have discussed in our previous post “first law of thermodynamics for a closed system undergoing a cycle” that the algebraic
summation of all energy transfer i.e. heat energy transfer and work energy transfer
across the system boundaries will be zero. Or we can say that for a closed
system undergoing a cycle, we will have following equation according to the
first law of thermodynamics.
First
law of thermodynamics for a closed system undergoing a cycle
But we must note it here that if system is not
undergoing in a cyclic process but also we have one system which is undergoing
a change of state then in that case above equation will not be applicable as
above equation is only valid for a system which is going under a cycle.
So, what will be the equation of energy transfer across the system boundary according to the first law of
thermodynamics for a closed system under a change of state?
Let we have one closed system which is undergoing a
change of state and energies i.e. work energy and heat energy both are crossing
the system boundaries. Net energy will be stored within the system and we must
note it here that energies stored within the system will be termed as internal
energy of the system or also termed as energy of the system.
Let us see the following figure, Q heat energy
enters the system from surrounding and work energy W leave the system or we can
also say that system is doing work W on the surrounding by taking Q amount of
heat energy from the surrounding.
So, what will be the net amount of energy stored
within the system during this process?
(Q-W) will be the net energy and it will be
accumulated within the system and hence there will be increment in internal
energy of the system as (Q-W) amount of energy will be stored within the system
during this process.
Therefore according to the first law of
thermodynamics, we have following equation for above thermodynamic process
where system is under a change of state.
Q-W = ∆E
Where, ∆E is increase in internal energy of
the system during the process
Let we have one system where multiple energies are
crossing the system boundary in a process as shown in following figure,
Now we will remind here the concept of “Sign convention used for heat and work energy transfer across the system boundary “and we will write here the
equation of energy transfer across the system boundary according to the first
law of thermodynamics for this process.
Q1 + Q2-Q3-W1+W2+W3 = ∆E
We must note it here that we will not secure the
absolute internal energy for this process, but also we will always secure here
the change in internal energy or change of system energy.
We will see another topic “Energy a property of the system” in our next post.
You must find out and read the following quite useful engineering articles and these are as mentioned below.
Do you have suggestions? Please write in comment box
Reference:
Engineering thermodynamics by P.K. Nag
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