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There is another similar question about change in internal energy for isothermal processes. Hopefully this gives some insight: Internal energy, or U, is a state function dependent on temperature. This means that for a process that is isothermal (delta T is 0), even if it is an expansion of a gas, the delta U will equal 0 because the internal energy does not change. Because delta U = q + w, even if work is done, q will be the exact opposite to ensure that delta U is 0. To clarify: delta U does not refer to the same quantity as work.
If you are referring to an isothermal process, a gas can expand (-w) if it is heated (+q) for the same amount of energy. The energy lost by expanding is supplied by heat, and the change in internal energy is 0.
Delta U is the change in the internal energy if the system and that is equal to Q (heat added to the system) - W (work done by the system). If delta U is zero, the heat added to the system is the same as the work done by the system. The amount of heat that is added to the system has the same amount of energy as the amount of expansionary work done by the system to the surrounding. The work energy lost to the surrounding by the expansion of a gas is basically replaced by heat energy into the system, resulting in a delta U value of 0.
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