Question

An ideal gas expands adiabatically to three times it's original volume.in doing so,the gas does 720joules of work..How much heat flows from the gas ?

Answer 1

In an adiabatic process, the system is insulated from its environment so that although the state of the system changes, no heat is allowed to enter or leave the system. So from the 1-st law of thermodynamics it follows that

$$Q=\Delta {{U}_{\operatorname{int}}}+W=0$$

where Q and W are the heat exchanged by the system and the work done by or on the system, $\Delta {{U}_{\operatorname{int}}}$ is the change in internal energy. Then

$$W=-\Delta {{U}_{\operatorname{int}}}$$

Thus if the gas does 720joules of work, then this is due to the internal energy of the gas, so heat flows from the gas is -720joules.