Calculate $\Delta U$ for a system undergoing an endotermic process in which 15.6 kJ of heat flows and where 1.4 kJ of work is done on the system.

Solution:

\begin{equation} \Delta U=q+w \end{equation}

where $q=15.6\;kJ$ and $w=1.4\;kJ$

\begin{equation} \Delta U=15.6+1.4=17.0\;kJ \end{equation}

The system increases its energy in 17 kJ.