What is 'WORK' in thermodynamics?????

[6.1] WORK

In thermodynamics, thermodynamic work is the quantity of energy transferred from one system to another. It is a generalization of the concept of mechanical work in mechanics. In the SI system of measurement, work is measured in joules (symbol: J)

According to the First Law of Thermodynamics, it is useful to separate changes to the internal energy of a thermodynamic system into two sorts of energy transfers. Work refers to forms of energy transfer, which can be accounted for in terms of changes in the macroscopic physical variables of the system, for example energy which goes into expanding the volume of a system against an external pressure, by say driving a piston-head out of a cylinder against an external force. This is in distinction to heat energy carried into or out of the system in the form of transfers in the microscopic thermal motions of particles.

The concept of thermodynamic work is a little more general than that of mechanical work, because it also includes other energy transfers, i.e. for example electrical work, the movement of charge against an external electrical field to charge up a battery say, which may or may not necessarily be thought of as strictly mechanical in nature.

Mechanical Work

Thermodynamic Work

[6.11] Differential work done and the work integral

The basic work relationship W=Fx is a special case which applies only to constant force along a straight line. That relationship gives the area of the rectangle shown, where the force F is plotted as a function of distance. In the more general case of a force which changes with distance, the work may still be calculated as the area under the curve. For example, for the work done to stretch a spring, the area under the curve can be readily determined as the area of the triangle. The power of calculus can also be applied since the integral of the force over the distance range is equal to the area under the force curve...................................

[6.1.2] Pressure - Volume work (P-V work)

Gases can do work through expansion or compression against a constant external pressure. Work done by gases is also sometimes called pressure-volume or PV work for reasons that will hopefully become more clear in this section.

Pressure–volume work is the work that is done by the compression or expansion of a fluid. Whenever there is a change in volume and external pressure remains constant, pressure–volume work is taking place. During a compression, a decrease in volume increases the internal pressure of a system as work is done on the system. During an expansion (following figure), an increase in volume decreases the internal pressure of a system as the system does work.

[6.2] Pressure-Volume (P-V diagrams)

A convenient way to visualize these changes in the pressure and volume is by using a Pressure Volume diagram or PV diagram for short. Each point on a PV diagram corresponds to a different state of the gas. The pressure is given on the vertical axis and the volume is given on the horizontal axis, as seen below,

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