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3、 cd 6.0.2A 3V 0.5A 7.5 3 8 11.DADC 12 15.DDBB 16.BC 17 BD 18. 19. 1 4 2 2 . 3 4 4 . 2 20. Q =c m t =4.2 10 3 J/(Kg ) 250 10 -3 Kg (60-20) =4.2 10 4 J(3 ) Q =Q =4.2 10 4 J(1 ) t =Q / c m =4.2 10 4 J/ 0.46 10 3 J/(Kg ) 100 10 -3 Kg =913 (4 ) t= t +60 =973 (1 ) 21. (1) IA1 I1 U R1 4 V 0.2 A (3 ) (2)(b) 0.40 A 2.0 A A 0.40 A I2 I I1 0.4 A 0.2 A 0.2 A R2 U IR2 0.4 V 0.2 A (3 ) A 2.0 A I2 I I1 2 A 0.2 A 1.8 A R2 U IR2 4 V 1.8 A (3 )