Work
In thermodynamics, work performed by a closed system is the energy transferred to another system that is measured by mechanical constraints on the system . Thermodynamic work encompasses mechanical work (gas expansion, ) plus many other types of work, such as electrical. As such, thermodynamic work is a generalization of the concept of mechanical work in mechanics. It necessarily excludes energy transferred between systems as heat, which is modeled distinctly in thermodynamics. For closed systems, energy changes in a system other than as work transfer are as heat.
Fig 1
An isobaric expansion of a gas requires heat transfer during the expansion to keep the pressure constant. Since pressure is constant, the work done is PΔV.
Heat and Work
Heat transfer (often represented by Q) and doing work (W) are the two everyday means of bringing energy into or taking energy out of a system. The processes are quite different. Heat transfer, a less organized process, is driven by temperature differences. Work, a quite organized process(as in gas expansion), involves a macroscopic force exerted through a distance. Nevertheless, heat and work can produce identical results. Both heat and work can cause a temperature increase.
Heat transfer into a system, such as when the Sun warms the air in a bicycle tire, can increase its temperature, and so can work done on the system, as when the bicyclist pumps air into the tire. Once the temperature increase has occurred, it is impossible to tell whether it was caused by heat transfer or by doing work.
This uncertainty is an important point. Heat transfer and work are both energy in transit—neither is stored as such in a system. However, both can change the internal energy of a system. Internal energy is a form of energy completely different from either heat or work.