# 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.
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### Ch 4, 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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