## Definition and Fundamental Expression of Work

### Definition :

Thermodynamic work is conceptually related to mechanical work, but it specifically refers to the energy transfer that occurs when a system performs work on its surroundings (or vice versa). Unlike general mechanical work, the focus here is on the system and how it interacts with external forces.

In classical mechanics, the infinitesimal work dw done when a force F moves an object by a small displacement dh is expressed as:

dW = F . dh dW=F.dh

In thermodynamics, pressure is defined as the external force applied per unit area on a movable boundary, such as a piston. This gives:

where Pext is the external pressure and A is the area of the movable surface. When this force causes a displacement, it leads to a change in volume dV, resulting in the thermodynamic expression for work:

and

F = P ext . A F=P_{ext}.A
A . dh = − dV A.dh=-dV

So,

dW = − P ext . dV dW=- P_{ext}.dV
Illustration Video