Class #1 Ch. 15
Ch. 15 The Laws of Thermodynamics.
Thermodynamics is the study of processes in which energy is transferred as heat and as work.
Internal Energy ---- is the total energy of all the molecules of a substance.
The First Law of Thermodynamics.
We call U the internal energy.
U =Q −W
The is real an extension of the work-energy principle.
It says that the change in the internal energy of a system is equal to the heat added, Q, minus the work
done by the system, W.
The 4 Simple Processes
If we consider a gas contained in a chamber with a movable piston we can define the following 4 types
of processes which can occur.
The first 3 processes concern keeping a single one of the variables in the ideal gas law constant.
P∗V =N ∗R∗T
The 4th process keeps the amount of heat added to the system zero.
1.
Class #1 Ch. 15
Isothermal
Since U , the internal energy if proportional to the temperature, no change in T means no change in U.
It we apply the First Law
U =0=Q−W
so Q =W
Isobaric
Pressure does not change.
From geometry we can see that
W =F∗D= P∗ V
If we use this with the first law
U =Q −W =Q −P  V
so Q =U P V
Isovolumetric
The volume does not change, so
V =0
Since W =P  V , this means that W is zero.
If we now apply the first law, we have
U =Q −W =Q
so Q =U
Adiabatic Q = 0
We can directly apply the first law
U =Q −W =0−W
so
U =−W
3 of these simple processes are represented on a P-V diagram below.
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Class #1 Ch. 15
The adiabatic process is not represented in the above Figure.
If we look at an adiabatic process compared with an isothermal process we will see the below graph.
Point C is always lower than B meaning that an adiabatic process lowers the internal energy as we see
from U =−W and so it ends up at a lower temperature.
Human Metabolism
We consider the application of the first law to an animal like ourselves.
U =Q −W
Since we are almost always in an environment which is colder that we are heat will flow out of our
bodies by conduction and radiation. This means that the Q term is negative. When we move we do
work so both terms on the right hand side are negative. This means that the change in our internal
energy will be negative.
The only way we can avoid running our of energy is to take in energy in the form of food, from which
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Class #1 Ch. 15
we extract chemical energy in the process of digestion.
Example 15-6
To find the latent heat we multiply the mass times the latent heat
To find the word done we use
W =P  V
Finally we apply conservation of energy
U =Q −W
Problem 7
Problem 9
The ideal gas law states that
3
U = n RT
2
4.
Class #1 Ch. 15
Second Law of Thermodynamics
You fill a salt shaker half with salt and then fill the top half carefully with pepper. When you shake it
the grains will mix, and no matter how long you shake it the grains will never separate again.
If you consider the closed system of your muscles shaking the container, energy if conserved, and
would be also conserved if the grains separated. The separation of the grains would not violate
conservation of energy, yet it never happens.
Also if we put 2 objects together which are at the same temperature, we will never see one get hotter
and the other get colder, although this process also conforms to conservation of energy.
The reason neither of these processes occur is that their occurrence would violate the second law of
thermodynamics.
The second law is the strangest law we will encounter in physics. One aspect of its strangeness is that it
has several formulations. One formulation is the following:
Heat can flow spontaneously from a hot object to a cold object, but heat will not flow
spontaneously from a cold object to a hot object.
5.