Isothermal Process

An Isothermal Process is a constant temperature process.

If the temperature of the system is remains constant then the internal energy of the system must remain constant.  If heat is added to the system then an equal amount of work must leave the system.  If heat is removed from the system then an equal amount of work must enter the system.

Δ U  =  Q + W

0  =  Q + W

Q  =  -W

Isotherm