We were discussing various basic
concepts of thermodynamics such as work and heat transfer in our previous post.
We have also discussed the concept of microscopic and macroscopic approach in thermodynamics
in our recent post.
Today we will see here the first law of
thermodynamics for closed system undergoing a cycle 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 cycle. After that, in our next
post, we will see first law of thermodynamics for an open system or control volume.
Let us see the following figure, there
is one vessel which is one adiabatic vessel i.e. there will not be any heat
interaction between system and surrounding. Let vessel is contained with some
quantity of water and vessel is equipped with thermometer for measuring the
temperature and paddle wheel too in order to provide the work energy to the
system by the surrounding.
First law of thermodynamics for closed system
Let work energy W is done upon the
system by the surrounding and this work energy, which is done by the
surrounding upon the system, could be easily determined by the fall of weight as
fall of weight will drive the paddle wheel with the help of pulley as displayed
in above figure.
Let the initial temperature of the
system is T1 and when work energy will be provided to the system i.e.
water here by the surrounding by fall of weight mechanism then supplied work
energy will be converted in to thermal energy and hence temperature of the system
will be increased and therefore let the temperature will be changed to T2 and this process is displayed here by path A as shown in following figure.
Let us consider that insulation is removed
now and there could be now heat interaction between system and surrounding. Let
us consider that system is returning back to state 1 to secure the initial
temperature by releasing the heat energy Q to the surrounding via the path B as
shown in figure.
Therefore this is one cyclic process
where system has come to its original position and system has secured the work
energy from surrounding during path A and system has released the heat energy to
the surrounding during the path B.
Mechanical equivalent of heat
According to Joule, work transferred to
the system from surrounding will always be directionally proportional to the
heat energy delivered by the system to surrounding or we can say that Here, J
will be termed as constant of proportionality and it is termed as mechanical equivalence
of heat.
So what we will say for first law of
thermodynamics for a closed system undergoing in a cycle?
When a closed system follows
thermodynamic cycle, net work done upon the system by the surrounding will be
equal to net heat energy released by system to surrounding.
We will see the Carnot cycle and “Firstlaw of thermodynamics for a closed system undergoing a change of state”in our
upcoming posts.
Do you have suggestions? Please write in
comment box
Reference:
Engineering thermodynamics by P.K. Nag
Engineering thermodynamics by Prof S K
Som
Image courtesy: Google
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