Q吸=cm(t-t0)=4.2×10³J/(kg.℃)*2kg*(100℃-20℃)=6.72×10ˆ5J
Q放=W=Pt
η=Q吸/Q放=Q吸/Pt得
t=Q吸/Pη=6.72×10ˆ5J/(2000w*60%)=560s