解：$(2)R_{ 1 }=\frac {U_{ }^2}{P_{加热 }}=\frac {({ 220 }\ \text {V})^2}{{ 2000 }\ \text {W}}={ 24.2 }Ω$
$ R_{ 总 }=\frac {U_{ }^2}{P_{保温 }}=\frac {({ 220 }\ \text {V})^2}{{ 800 }\ \text {W}}={ 60.5 }Ω$
$ R_2=R_总-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 }\%$
