$解：(1)P_{ 额 }=U_{ 额 }I_{额 }={ 6 }\ \text{V}×{ 0.8 }\ \text{A}={ 4.8 }\ \text{W}$
$ (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}$
