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What is Work?

Work

is a force F acting through a displacement x, in the direction of the force.

Compression or expansion of gas (or liquid)

Since F = PA, work can be expressed in terms of pressure and area:

This equation is based on a mechanical definition of work. However, we need a broader definition of work that will allow us to understand other forms of work.

Lesson 4A: page 1 of 30

Ch4, Lesson A, Page 1 - What is Work?

You probably recall the definition of work from your physics class.
Work is defined as a force acting through a distance. You probably even recall that force is a vector and the displacement dx is also a vector.
Work is inner or dot product of these two vectors
The piston-and-cylinder device, shown here, is a closed system.
In this system, the force and the displacement act in the same direction.
The other common situation occurs when the force and the displacement act in opposite directions.
Now, you may be wondering how this ties in with thermodynamics.
Here is the link: pressure acting on a surface applies a FORCE. When the surface, or boundary MOVES, work is being done.
In the diagram shown here, if the fluid in the cylinder expands, the FLUID is doing work on the PISTON.
If the piston pushes down and compresses the gas, then the PISTON is doing work ON the FLUID.
This is a pretty simple system, but they are not always so simple.
Flip the page to see a more generally applicable definition of work.

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