Editing Isothermal process (section)

== Details for an ideal gas ==
[[File:Ideal gas isotherms.png|thumb|'''Figure 1.''' Several isotherms of an ideal gas on a p-V diagram, where p for pressure and V the volume.]]

For the special case of a gas to which [[Boyle's law]]<ref name="Klotz"/> applies, the product ''pV'' (''p'' for gas pressure and ''V'' for gas volume) is a constant if the gas is kept at isothermal conditions. The value of the constant is ''nRT'', where ''n'' is the number of [[Mole (unit)|moles]] of the present gas and ''R'' is the [[ideal gas constant]]. In other words, the [[ideal gas law]] ''pV''&nbsp;=&nbsp;''nRT'' applies.<ref name="Klotz"/> Therefore:

: <math> p = {n R T \over V} = {\text{constant} \over V} </math>

holds. The family of curves generated by this equation is shown in the graph in Figure 1. Each curve is called an isotherm, meaning a curve at a same temperature ''T''. Such graphs are termed [[indicator diagram]]s and were first used by [[James Watt]] and others to monitor the efficiency of engines. The temperature corresponding to each curve in the figure increases from the lower left to the upper right.