解：?$(1) S_{1}$?閉合?$，S_{2}$?斷開
?$I_{ }=\frac {U_{ }}{R_{ 1}+R_{2 }}=\frac {{ 6 }\ \text {V}}{{ 10 }Ω+{ 20}Ω}={ 0.2}\ \text {A}$?
?$P_{ 低 }=I_{ }^2R_{ 2 }=({ 0.2 }\ \text {A})^2×{ 20 }Ω={ 0.8}\ \text {W}$?
?$(2) S_{1}、$??$S_{2}$?均閉合
?$R_{ }=\frac {R_{2}R_{3}}{R_{2}+R_{3}}=\frac {{ 20 }\ \mathrm {Ω}×{10 }\ \mathrm {Ω}}{{ 20 }\ \mathrm {Ω}+{10 }\ \mathrm {Ω}}={ \frac {20}3 }\ \mathrm {Ω}$?
?$P_{ 高 }=\frac {U_{ }^2}{R_{ }}=\frac {({6 }\ \text {V})^2}{{ \frac {20}3 }Ω}={ 5.4 }\ \text {W}$?
?$(3)$?較好的電路是?$(\mathrm )，$?因為在電路?$(\mathrm {a})$?中?$，$?低溫擋時?$R_{1}$?要發(fā)熱?$，$?浪費電能
