A gas does work during isothermal expansion. The source of mechanical energy so produced is the internal energy of the gas itself.

Thermal equilibrium is a state where two or more thermodynamic systems or objects have no net flow of heat energy between them. This happens when the objects are connected by a permeable barrier and have the same temperature.  Thermal equilibrium is an axiomatic concept of thermodynamics. It obeys the zeroth law of thermodynamics.   Here's an example of thermal equilibrium: When you put a mug of hot water into a freezer, the hot water will cool down until it reaches the temperature of the freezer.  The Earth must remain in thermal equilibrium for its temperature to remain constant. This... Show more

Thermal equilibrium is a state where two or more thermodynamic systems or objects have no net flow of heat energy between them. This happens when the objects are connected by a permeable barrier and have the same temperature. 

Thermal equilibrium is an axiomatic concept of thermodynamics. It obeys the zeroth law of thermodynamics.
 

Here's an example of thermal equilibrium:
When you put a mug of hot water into a freezer, the hot water will cool down until it reaches the temperature of the freezer. 
The Earth must remain in thermal equilibrium for its temperature to remain constant. This means that the Earth must radiate just as much heat as it receives from the sun.
 

Related Test: Engineering Physics Practice Test: Thermodynamics, Latent Heat, Specific Heat & Thermodynamics 

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