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P
2
P
1
V
1
= V
2
V No work is done, because the area under curve is zero. Because the gas
can not do work, if heat is added, it is completely used to increase internal energy of the system.
W = 0 In term of first law of thermodynamics,
Q = ∆U + W
Q = ∆U + 0
Q = ∆U
4. Adiabatic Process
In adiabatic process, there is no heat transferred to or from system. That is,
Q = 0 This condition ocuurs on a thermally isolated system. For real life
condition, we only approximate adiabatic process if the changes occur rapidly enough. Because if you wait long enough, there will be heat
transferred to or out of system. The curve of this process is called adiabat. During an adiabatic
process, al thermodynamics coordinates P, V, T change. For example, if a pressure reduced, the gas expands. However there is no heat flows to the
gas. So, the change of internal energy is equal to work done by system, but change of internal energy is negative internal energy decrease.
Q = ∆U + W
0 = ∆U + W
- ∆U = W
To be wrong is
nothing, unless you
continue to remember
it
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Because the internal energy and temperature decrease, the process is cooling process. Similarly, an adiabatic compression process is warming
process. We need to state other relationship in adiabatic process. An important
fa to is the atio of the gas’s ola spe ifi s heat, defined by quantity ϒ = C
p
C
v
Where C
p
is specific heats at constant pressure, and C
v
is specific heats at constant volume. For monoatomic and diatomic gas, the value of
ϒ is about 1.67 and 1.40. The volume and pressure at any two points on an adiabatic are related by
P
1
V
1 ϒ
= P
2
V
2 ϒ
The work done by an ideal gas during an adiabatic process can be shown to be
W = P
1
V
1
– P
2
V
2
The curve of adiabatic is similar with the curve of isothermal process, but for adiabatic is steeper.
Balcony
Conceptual Example : Exhaling : Blowing Hot or Cold?
The air in your lungs is warm. This can be demonstrated by putting your forearm near your mouth and blow air with your mouth opened wide. If
you repeat this with your lips puckered, the air will feel : a warmer b cooler c the same
Reasoning and Anwer
To answer the question you must try it and you will surprised to find that the answer is b.
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When you blow with mouth opened, you gush a warm air. However, when you blow with lips puckered, the stream of air is compressed. Then the air
expand, doing work againts the atmosphere. The process is approximately adiabatic, because it takes place quickly. Because Q = 0,
∆U = -W. Therefore
∆U is negative, and the temperature decrease.
Follow-Up Exercise
Even during winter days with snow on slopes, It is common in the Rocky Mountain to experiance blast of warm air coming down the slopes. Explain
how this winds could experience a significant rise in temperature while there is still snow and ice on the ground.
Hint : the process occurs is adiabatic.
The Attic No ou k o a out the od a i s p o esses. Let’s sa e it i the atti .
Thermodinamics Processes
The Kitchen In order to understand about the second law of thermodynamics, you
can do a simple experiment below. place cold water in a glass
pour hot water to the same glass observe what happen, if necessary check the mixed water with
your finger. Process
Characteristics Result
First Law of Thermodynamics
Isothermal
T = Constant ∆U = 0
Q = W
Isobaric
V = Constant W = P
∆V Q =
∆U + P ∆V
Isochoric P = Constant
W = 0 Q =
∆U
Adiabatic Q = 0
∆U = -W
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What happen to the mixed water? Is it still cold or hot? Why it can be happen?
Reading Room
D. The Second Law of Thermodynamics
