解：$(1)P_{ 额 }=U_{ 额 }I_{额 }={ 6 }\ \text{V}×{ 0.8 }\ \text{A}={ 4.8 }\ \text{W}$
$ (2)R_2=2R_1，$
所以$U_1=\frac 13U，$$P_1∶P_1'=U_1^2∶U_1'^2=3\ \text {W}∶0.75\ \text {W}=4，$
所以$U_1=2U_1'，$即$\frac 13U=2×\frac {R_1}{R_1+R_L}U，$得$R_L=5R_1$
所以$P_L=5P_1'=5×0.75\ \text {W}=3.75\ \text {W}$
