Thermodynamics Processes
Stuff gets real
Work Correction
∆U= Q+W
Variable
Q
W
Sign
+
+
What it means
Heat has been transferred into the system
Heat has been removed from the system
Work has been done by the system (work has removed energy from
the system) System is explanded
Work has been done on the system (work has added energy to the
system) System is compressed
Due to the shift in sign convention, we also need to say
W=-p∆V
Practice Problem
Scenario
A system gains heat as work is done on it.
A system is compressed while gaining heat from outside the system.
A system does not lose any heat while its volume is expanded.
A system remains at a constant volume while its temperature increases.
Internal Energy
This is the kinetic energy of a molecule:
If we want to expand this to know the internal energy of a substance, we use
Story Notes. Completed with material from Knapp Notes and Open Stax Physics Textbook
Q
W
We can change the internal energy by:
 Adding (or removing) heat
 Doing work on the system (or having the system do work) – such as changing the volume of the system.
Practice Problem: Increasing the Temperature
The temperature of a gas increases from 293° C to 437°C. What is the ratio of the final internal temperature to its initial internal temperature?
Constant Temperature on a PV Diagram
PV diagrams are diagrams that plot the pressure of a substance on the y axis and the volume of a gas on the x axis.If a substances is acting like an ideal gas, we
can use the ideal gas law to show the relationship between P and V.
P=constant/V
Because P and V are inversely proportional to one another (as one gets larger, the other gets smaller in a closed system).
When you plot inversely proportional variables, you get a hyperbola (see left).
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Isotherms
Each hyperbola looks a little different for different
temperatures.
This diagram shows the PV diagram for a substances at different
temperatures.
Each curve here is called an isotherm- “iso” means “equal” in
Latin.
Practice Problem: Isothermal Process
An ideal gas is initially in a state that corresponds to point 1 on the graph above, where it has pressure p1, volume
V1, and temperature T1. The gas undergoes an isothermal process represented by the curve shown, which takes it to
a final state 3 at temperature T3. If T2 and T4 are the temperatures the gas would have at points 2 and 4,
respectively, which of the following relationships is true?
(A) T1 < T3 (B) T1 < T2 (C) T1 < T4 (D) T1 = T2 (E) T1 = T4
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Four Types of Processes
Isobaric –Constant Pressure
IsothermalTemperature stays the same
Temperature does not change
Internal Energy does not change
Adiabatic- nothing stays the same
Pressure is constant
Volume increases
Isochoric (isovolumetric)
Volume stays the same
Pressure changes
Volume is constant
W=P∆V
W= 0J
∆U=0
Q=0
∆U=Q
Q=W
∆U=-W
What the Graph Looks Like:
What the Graph Looks Like:
What the Graph Looks Like:
Real World Example:
Real World Example:
What the Graph Looks Like:
Real World Example:
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Happens very fast.
No enough time for energy to come in from outside the
system. All energy comes from internal energy.
Real World Example:
Work Done On or By a Gas
Many engines that do work involve pistons and cylinders.
Some assumptions about pistons:
 The piston can move up and down in the cylinder.
 Inside the cylinder, under the piston, is a gas.
 The gas cannot escape around the piston.
 Push down on the piston and the gas is compressed into a smaller volume. +W
 Pull up on the piston and the gas expands into a larger volume. -W
 y
Isobaric Process
How to identify them: Pressure is constant and the volume is increasing
Keep in mind:
Pressure is constant
Volume increases
W=-P∆V
Figure 1- Graph of Isobaric Process
Practice Problem: Temperature in Isobaric Processes
The volume of a gas is doubled while the pressure remains constant.
a) Is the work positive or negative?
b) Is the temperature increasing or decreasing?
b) Is the internal energy increasing or decreasing?
c) Would you expect heat to be added or removed from the system during this process?
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Gas expands at constant pressure
moving piston a distance of  y
Practice Problem: AP Physics 2007B5
2007B5. The figure above shows a 0.20 m diameter cylinder fitted with a frictionless piston, initially fixed in place. The cylinder contains 2.0 moles of nitrogen gas at an absolute
pressure of 4.0 × 105 . Pa. Nitrogen gas has a molar mass of 28 g/mole and it behaves as an ideal gas.
a. Calculate the force that the nitrogen gas exerts on the piston
b. Calculate the volume of the gas if the temperature of the gas is 300 K.
c. In a certain process, the piston is allowed to move, and the gas expands at constant pressure and pushes the piston out 0.15 m. Calculate how much work is done by the gas.
d. Which of the following is true of the heat energy transferred to or from the gas, if any, in the process in part (c)?
_______Heat is transferred to the gas.
_______Heat is transferred from the gas.
_______No heat is transferred in the process.
Justify your answer.
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Isochoric Process
How to Identify them: Volume stays constant
W  PV
 P  0  0
U  Q  W
U  Q  0
U  Q
Figure 2- In an isochoric process, the pressure
changes but the volume does not
Practice Problem: Isochoric expansion
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Isothermal Process
How to Identify it: The isothermal process happens at a constant temperature. The pressure and
volume change, so work is done, but
U  Q  W
0  Q W
U is zero.
Q  W
Figure 3-Isothermal processes follow
the lines of the isotherm
Adiabatic Process
Definition: In an adiabatic process, heat
Q  0
U  Q  W

is neither added or taken away from the system.
U  W
Ways to create an Adiabatic Process
1. The system can be insulated so that heat can neither enter nor leave. Example: Joule’s “heat equivalent”
experiment was adiabatic
2.
The other way to have an adiabatic process (this is actually called a “near adiabatic process”) is to have
it happen very quickly. The process happens so fast that there is no time for heat to be transferred. Example:
The combustion of gasoline in an engine is considered to be adiabatic because each combustion step happens
in a very short time – a few hundredths (or less) of a second
Figure 5- adiabatic process graphs look
like isotherms but they are actually
changing their temperature
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What Happens in an Adiabatic Process
1. A gas that is adiabatically expanded will lose internal energy (U ) and become cooler.
2. A gas that is adiabatically compressed will gain U and become warmer.
Practice Problem: Adiabatic Expansion
Figure 4-Adiabatic Demo
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