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This text is meant to accompany class discussions. It is not everything there is to know about the basics of torque and the two conditions of equilibrium. It is meant as a prep for class. More detailed notes and examples are given in the class notes, presentations, and demonstrations (click here.)
Questions Click for the questions that go with this reading
Defining our System


To examine and understand how energy flows, a gas system needs to be defined. Our system is shown below.

The container houses a monotomic gas. The container does not need to be a rectangular shaped cube. It could be a car, room in a house, hot air ballon, cooler, soap bubble, your lungs, etc. This container is just a simplification of the system. Thermal energy can be added or removed from the container. The lid can slide up and down without friction. This means the gas inside can expand or contract. The space outside of the container is called the, "surroundings," or, "environment."


Mechanical Work and Gases

The 5 minute video below will give you a small review of mechanical work, the signs and prepositions we use as well as how it applies to gases. It is important to understand how the signs of work depend on the view point and the prepositions used to describe it.

This video can be found at:


PV Diagrams

A PV diagram is a visual way of representing information. A PV diagram contains or can be used to deduce information about a gass's pressure, volume, temperature, number of moles, type of process, work by the gas, work by the surroundings, gass's internal energy and in many cases the flow of thermal energy into and out of the gas. This is a PV diagram...

Example PV diagram


Here is video describing what you can do with a PV daigram and partially how it is used.

This video can be found on YouTube at


  • Question
  • Solution on paper
  • Video Solution

2.00 moles of a gas is in a container when it expands. As it expands, the state of the gas changes and follows the path shown below.

Calculate the following values:

  1. Internal energy for the initial state.
  2. Work done by the gas as the gas moves from the initial state to the final state.
  3. Work done by the surroundings as the gas moves from the initial state to the final state.
Solutions on notebook paper
[Video Solution Coming soon]
Readng Quiz
Readng Quiz
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by Tony Wayne ...(If you are a teacher, please feel free to use these resources in your teaching.)