THE FIRST LAW
If you were asked to prove that two and two made four, you might
ﬁnd some diﬃculty, and yet you are quite sure of the fact.
−Sir Arthur Conan Doyle (A Study in Scarlet)
Thermodynamics is a subject that gives us
the foundation needed to deal with energy.
One of the fundamental laws in thermody-
namics is the ﬁrst law of thermodynamics,
which states energy is conserved. Intro-
ducing the ﬁrst law to absolute beginners
is the objective of this chapter.
Conservation of Energy
Conservation of energy means energy cannot be created or destroyed,
but energy can change from one form of energy to another. The ﬁrst law
of thermodynamics is simply the principle of conservation of energy. An
object falling from a tall building gains speed as it comes down, which could
be explained by the principle of conservation of energy as follows. A falling
object losses its potential energy, and the loss in potential energy gets
converted into a gain in kinetic energy. The gain in kinetic energy accounts
for the increase in the speed of the object falling through a height.
Similarly, water falling through a height as in a waterfall attains high
speed. The high speed water, when directed to the blades of a turbine (as
shown in the simpliﬁed diagram of Figure 1.1), sets the turbine blades in
motion, which in turn set the turbine shaft supporting the blades in rotary
motion. The rotating shaft can be used to do useful work. The potential
energy lost by water as it falls through a height is therefore converted into
useful work. We, of course, know that not all the potential energy lost
gets converted into useful work. A part of the potential energy lost, for
example, gets converted into work needed to overcome friction.
Figure 1.1 A simpliﬁed diagram showing how a turbine shaft
is set in motion.
Electricity can be generated in an electric generator in which a coil is
spun between the north and south poles of a magnet or an electromagnet.
To spin the coil, we require the rotation of a shaft, which can be provided by
water falling through a height as discussed above. This is the basic principle
behind hydroelectric power generation. Part of the potential energy lost by
water as it falls through a height is thus converted into electrical energy in
hydroelectric power generation.
The First Law 3
Conversion of heat energy into electrical energy can be achieved using
a steam turbine, the basic working principle of which is shown in Figure
1.2. Water from a water source, such as a river, is pumped into a boiler
operating at a high pressure. The water in the boiler is heated by burning
a fuel, such as furnace oil, in the furnace of the boiler. Steam at high
pressure, generated in the boiler-furnace arrangement, is further heated in
a superheater and directed to strike the turbine blades so as to set them is
motion. The turbine blades are attached to a turbine shaft such that the
moving blades set the shaft in rotary motion. The rotating shaft spins a coil
between the poles of an electromagnet and thereby electricity is produced.
Part of the heat energy released by the fuel burning in the furnace is thus
converted into electrical energy, which is yet another example of conversion
of energy from one form to another.
Figure 1.2 Basic working principle of a steam turbine.
What is Energy?
We spoke of potential energy, kinetic energy and electrical energy, and
we are very familiar with all these forms of energy. However, if we are
asked to deﬁne energy in a clear and simple way, we ﬁnd it diﬃcult to do
so. For example, we can write an equation to calculate potential energy,
but we cannot describe in simple words what potential energy is. We can
write an equation to calculate kinetic energy, but we cannot describe what
kinetic energy is. It is the same with all other forms of energy as well. Even
though, it is diﬃcult to deﬁne energy in a simple way, one could look at
energy as some entity which possesses the capacity to do work.
Nobody really knows the answer to the question why energy is con-
served. Yet, we believe in conservation of energy because our experience
has shown us that energy is always conserved. That is, energy can be
neither created nor destroyed, but energy changes from one form to an-
other. Conservation of energy is therefore taken as a fundamental law, and
is named as the First Law of Thermodynamics.
What is a Fundamental Law?
A law that is in use because nobody has disproved it, is known as a
fundamental law. The ﬁrst law of thermodynamics has never been proved.
Even though the ﬁrst law has never been proved, we use it because no-
body has disproved it either. The ﬁrst law of thermodynamics is therefore
a fundamental law. We will believe in the ﬁrst law of thermodynamics and
continue to use it until someday, someone ﬁnds out that the ﬁrst law of
thermodynamics fails to describe some particular event.
Student: Teacher, you said that we can’t prove the ﬁrst law of thermodynam-
ics, and that we shall have to believe in it. Do you really mean that I
should simply believe that the ﬁrst law of thermodynamics is true?
Teacher: Yes, if you want to continue with learning thermodynamics, you
should take the ﬁrst law of thermodynamics to be a true statement about
how the nature works, until the day someone disproves it.
Student: Please, permit me to ask another question. I am familiar with the
principle of conservation of mass, which states that mass cannot be cre-
ated or destroyed. Can the principle of conservation of mass be proved?
The First Law 5
Teacher: No, the principle of conservation of mass cannot be proved. There is
also the principle of conservation of momentum which cannot be proved
either. They are all fundamental laws.
Student: I have one more question, Teacher. We talk of mass, momentum
and energy as though they are distinctly diﬀerent things like mangoes,
bananas and grapes. I have read something about the Einstein’s theory
of relativity which says, I believe, that mass can be converted into energy.
If Einstein had it right then how could we treat mass and energy as two
Teacher: It is perfectly alright for a beginner in thermodynamics to treat mass
and energy as two distinctly diﬀerent things, and to use the principle of
conservation of mass and the principle of conservation of energy as two
separate principles. When we get interested in learning more about the
sun, the stars or the formidable nuclear reactors, in all of which mass is
being converted into energy, then it would be the right time for us to ﬁnd
out how to apply the conservation principles to such systems. We may
ﬁnd the books listed below to be of great help to us in our quest then.
Born, Max 1951 The Restless Universe. Dover Publications, Inc., New
Yor k .
Feynman, R.P., Leighton, R.B. & Sands, M. 1997 The Feynman Lec-
tures on Physics, Volumes 1, 2 & 3. Narosa Publishing House, New
Venkataraman, G. 1994 At the Speed of Light. Universities Press (India)
Application of the First Law
This book will show you how to apply the ﬁrst law, that is, the princi-
ple of conservation of energy, to thermodynamic systems as they go from
one state to another executing a process, during which the properties of
the system change. I am sure that you are familiar with the words such
as system, state, process and properties, and you know what these words
mean in everyday life. In thermodynamics, however, these words have very
speciﬁc meanings as described in the next chapter. Getting familiar with
thermodynamic terminology in the next chapter is essential to learn ther-
modynamics with ease.
First Law of Thermodynamics
ENERGY IS CONSERVED.
(That is, energy cannot be created or destroyed
but it can change from one form of energy to another.)
“Nooooo..., I am not talking about this kind of
energy conservation .........”