This document discusses the first law of thermodynamics and Joule's experiment that verified it. It contains the following key points: 1. Joule conducted an experiment in 1840-1849 where he used a paddle wheel apparatus to convert the potential energy of falling weights into heat energy in a liquid. He found that the work input was always proportional to the heat generated. 2. This established the first law of thermodynamics - that energy cannot be created or destroyed, only converted from one form to another. 3. The first law has consequences called corollaries, including that a perpetual motion machine of the first kind is impossible as it would produce work without an equivalent energy expenditure.

1. Unit No:01
Thermodynamics
Prof. Yash B. Parikh
M.Tech (Computer Integrated Manufacturing)
B.E.(Mechanical Engineering)
Assistant Professor
Department of Mechanical Engineering

2. First Law of Thermodynamics
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 This law of thermodynamics is a particular statement
of the general principles of law of conservation of
energy applied to heat & work transfer.
 The law states that, “Energy can neither be created
nor destroyed but it can only be converted from one
form to another”.
 Though the law can not be prove analytically, it has
never been disproved & on the contrary many
experiments have verified its statement.

3. Joule’s Experiment
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 During the year (1840 – 1849), James Joule analyzed
the statement of conservation of energy by
arrangement shown in the fig.

4. Joule’s Experiment
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 It consists of a paddle wheel arrangement with the
liquid filled in an insulated container.
 When the weight falls, it supplies the work energy
equals to its potential energy to paddle wheel &
causes to rotate it.
 This causes the fluid to heat up due to friction
between the paddle wheel & the fluid.
 As a result of work transfer the temperature of the
liquid rises, which is measured with the help of the
thermometer.

5. Joule’s Experiment
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 The system has undergone a process (1-A-2) as
shown.

6. Joule’s Experiment
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 Now the heat is transferred from liquid to
surrounding till the system returns to its original state
of pressure & temperature.
 The heat transferred is shown by process (2-B-1).

7. Joule’s Experiment
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 With such experiment Joule concluded that in every
case the work input W was always proportional to
heat transfer Q at the end of the cycle.
 Mathematically we can write,
ʘʃ d’W = J ʘʃ d’Q
Where, J is a constant known as mechanical
equivalent of the system.

8. Joule’s Experiment
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 In M.K.S unit J = 4.187 kJ/k cal
 In S.I unit J = 1 J/Nm
 Since W and Q are both measured in J or Nm we can
write,
ʘʃ d’W = ʘʃ d’Q

9. Corollaries of First Law
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 The first law of thermodynamics has the number of
important consequences, which forms corollaries.
(A corollary is a statement that follows readily from
a previous statement.)
 There are three main corollaries of first law,
Corollary 1 (Perpetual Motion Machine 1)
 A machine operating on a cycle and producing work
continuously or periodically without absorption of
energy at the same time is known as perpetual
motion machine.
 Therefore, the first law can also be stated in
following terms,

10. Corollaries of First Law
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 “It is impossible to construct a perpetual motion
machine of first kind”.
i.e. no machine working over a cycle can produce
energy without corresponding expenditure of energy.
 Converse of PMM 1 is also true.
i.e. there can not be any machine which would
consume work energy without some other form of
energy appearing simultaneously.

11. Corollary 2
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 While studying thermodynamics we come across with
processes rather than cycles.
 When a system executes a process, the net heat
transfer equals to sum of net work transfer and
change in stored energy, dE.
d’ Q = d’ W + dE
 This is known as First law for a process.

12. Corollary 3
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 For an isolated system, there are no mass & energy
transfer, i.e. dQ = 0 & dW = 0.
Therefore, (dE)isolated = 0
 Total energy of an isolated system remains constant.