In an isothermal process, temperature remains constant, i.e., ΔT=0.
From, ΔQ=ΔU+ΔW ∵ΔU=0,(∵ΔT=0)
Internal energy is constant for an isothermal process. ∵ΔQ=ΔW i.e., heat supplied in an isothermal change is used to do work against external surrounding.