Final answer:
When work is done on a system and heat is added, the internal energy can still decrease because of the signs of work and heat exchanges.
Step-by-step explanation:
In thermodynamics, when work is done on a system, it can increase the system's internal energy. However, if heat is also added to the system, the internal energy can decrease. This may seem counterintuitive, but it can be explained by understanding the signs of work and heat exchanges.
Work done on a system is considered positive, while work done by a system is considered negative. On the other hand, heat added to a system is also considered positive, while heat removed from a system is considered negative. So, if work done on the system is greater than the heat added, the overall effect is a decrease in internal energy.