解：?$(2)R_{ 1 }=\frac {U_{ }^2}{P_{加熱 }}=\frac {({ 220 }\ \text {V})^2}{{ 2000 }\ \text {W}}={ 24.2 }Ω$?
?$ R_{ 總 }=\frac {U_{ }^2}{P_{保溫 }}=\frac {({ 220 }\ \text {V})^2}{{ 800 }\ \text {W}}={ 60.5 }Ω$?
?$ R_2=R_總-R_1=60.5\ \mathrm {Ω}-24.2\ \mathrm {Ω}=36.3\ \mathrm {Ω}$?
?$ Q=cm?t=4.2×10^3\ \text{J/(kg}·℃) ×{ 30 }\ \text{kg}×(55℃-25℃)={ 3.78×10^6 }\ \text{J}$?
?$ W=P_加t={ 2000 }\ \text {W}×{ 35×60 }\ \text {s}={ 4.2×10^6 }\ \text {J}$?
?$ η=\frac {Q}{W}×100\%=\frac {{ 3.78×10^6 }\ \text{J}}{{ 4.2×10^6 }\ \text{J}}×100\%={ 90 }\%$
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