解:(1) m =ρV = 1.0 × 103 kg/m3 × 10 -3 m3 = 1kg,
Q吸=cm( t - t0 ) =4.2 × 103 J/( kg . ℃) ×1 kg × (100℃ -20 ℃) = 3. 36 × 105 J.
(2) W =Pt =2 000 W ×200 s =4 ×105 J.
=3.36 ×105 J/(4 ×105 J) =84%.
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