1. **Problem:** A gas is compressed, and 80 J of work is done on it. During compression, 20 J of heat is released to the surroundings. Find its internal energy. 2. **Formula:** The First Law of Thermodynamics states that the change in internal energy $\Delta u$ is given by: $$\Delta u = Q + W$$ where $Q$ is the heat added to the system and $W$ is the work done on the system. 3. **Important rules:** - Heat released to surroundings means heat leaves the system, so $Q$ is negative. - Work done on the system is positive. 4. **Given:** - Work done on gas, $W = +80$ J - Heat released, $Q = -20$ J 5. **Calculation:** $$\Delta u = Q + W = -20 + 80 = 60 \text{ J}$$ 6. **Answer:** The internal energy increases by 60 J.