What is the change in internal energy in joules for a system that releases 154 j of heat and does 125 j of work on its surroundings?

The change in internal energy of the system is given by the first law of thermodynamics:

where 
Q is the heat added to the system
W is the work done by the system

In this problem,  (with a negative sign because the heat is released by the system (not absorbed)), and the work is , with a positive sign since it is performed by the system. Therefore, the variation of internal energy of the system is


So, the internal energy of the system has decreased.

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