Refrigerator - Thermodynamics Video Lecture - Class 11

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thermodynamics part 15 is brought to you
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that you have watched all the videos
from part 1 to part 14 before going
ahead with part 15 we have discussed the
basics of heat engine let us see how
does a refrigerator function now the fun
the way or the functioning of a
refrigerator is just the reverse of a
heat engine how we will see that it
works reverse of a heat engine so now if
you see the schematic diagram what do
you see for heat engine this was the hot
reservoir so heat engine used to take
heat from the hot reservoir do some work
on the environment and then you returned
the remaining amount of heat to the cold
reservoir right but in case of
refrigerator what happens it extracts
heat or it takes heat from the cold
reservoir and then some work is done on
the refrigerator refrigerated it doesn't
in this case the system doesn't do work
work is done on the system and then the
amount of heat is transferred to the hot
reservoir so if we see that Q - let me
keep the convention saying otherwise you
might get confused later suppose Q 2 is
the amount of heat which it takes from
the cold reservoir W work is done by the
surroundings on the system and then the
system returns Q 1 amount of it to the
hot reservoir so in this case if you try
to write down it in mathematical form
you can write it as Q 2 plus W becomes
equal to Q 1 right not just similar to
efficiency of a heat engine there is
something called Co P of a refrigerator
which tells how good the refrigerator
functions Co P stands for coefficient of
performance so Co P basically is denoted
by alpha and mathematically it is the
ratio of the
eat which the system takes from the cold
reservoir to the amount of work which is
done on the system so it basically
checks how nicely the system is able to
extract heat from the cold reservoir so
a point to be noted here is that this
value alpha can be greater than one
because the amount of heat which the
system will take from the Holden server
can be greater than the amount of work
which is drawn on done on the system so
as alpha can be very well greater than
one so fat by energy conservation we can
write this alpha or COP in terms of Q 1
and Q 2 Q 2 so how do we do that we can
see from the figure if you see the
diagram which I drew in the last page
what was it it was something like this
it is suppose this is the refrigerator
this is my cold reservoir it takes
amount of heat Q to some amount of work
W is done on the system and the system
returns q one heat to the hot reservoir
right so looking at this we can say that
Q 2 plus W gives Q 1 Q 2 is the amount
of it which therefore ejected X or
absorbs similarly W is the work which is
done on there if you'd written so this
both will add up to form Q 1 which is
the heat that is transferred to the hot
reservoir right so from this we can say
W is equal to Q 1 minus Q 2 so we
replace this value of W in the
expression for alpha so alpha becomes Q
2 divided by Q 1 minus Q 2 thus we
expressed alpha in terms of Q 1 and Q 2
now this w can another important point
to note here is the work done on the
refrigerator can never be equal to 0
because the refrigerator refrigerator
will not function if there is no work
done on it so this work done will never
be equal to 0 that is Q 1 and Q
we'll never be equal to 0 so if bourbon
is never equal to zero that means Q 1
minus Q 2 will also never be equal to 0
that which again implies that if this is
never equal to 0 that means alpha will
never be infinite so what does it mean
it means that the value of Co P that is
alpha can be very very greater than 1
but it cannot reach infinity if there
has to be a limit in its maximum value
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