How do you find q for isothermal processes?

1 Answer
Jul 24, 2016

In isothermal processes, we know that by definition, the temperature is constant.

The internal energy #DeltaU# only depends on temperature for ideal gases, so #DeltaU = 0# in an isothermal process. Therefore, the first law of thermodynamics becomes:

#cancel(DeltaU)^(0) = q + w#

And we just get:

#color(blue)(-q = w = -PDeltaV)#

where:

  • #q# is the heat flow in #"J"#.
  • #w# is the expansion/compression work in #"J"#.
  • #DeltaV# is the change in volume in #"L"#. Note that #"8.314472 J"/("0.083145 L"cdot"bar")# is a convenient conversion unit.
  • #P# is a single pressure. It tends to be in #"bar"# or #"atm"#.

If #"atm"#, you use #"8.314472 J"/("0.082057 L"cdot"atm")# for the conversion unit.