Q=Cm(t2-t1)
=4200*1*20 (m=ρ水V水=1kg）
=84000J
(2)
1/R並=1/R1+1/R2
R並=0.9R2
P加熱=U^2/R並
500=48400/0.9R2
R2=107.55Ω
R1=9R2=968Ω
P保溫=U^2/R1
=48400/968
=50W
(3)
W=P加熱*t加熱+P保溫*t保溫
=500*3*60+50*1.5*60
=90000+4500
=94500J