In an isothermal process for an ideal gas:

1. 'Isothermal' literally means at constant temperature: ΔT = 0. 2. Since U depends only on T for an ideal gas, ΔU = 0 in an isothermal process. 3. First law then gives Q = W (heat absorbed equals work done). _Source: syllabus Physics Ch 11, §11.7 Thermodynamic Processes_