解：?$(1)U_{1}=\frac 13U，$?所以?$R_{1}=\frac 13(R_{1}+R_{2})，$?得?$R_{2}=2R_{1}=10\ \mathrm {Ω}$?
?$(2)I_{ }=\frac {U_{ }}{R_{1 }+R_{ 2 }}=\frac {{ 3 }\ \text {V}}{{ 5 }Ω+{ 10 }Ω}={0.2 }\ \text {A}$?
?$Q_{ 1 }=I_{ }^2R_{ 1 }t_{ }=({ 0.2 }\ \text {A})^2×{ 5 }Ω×{ 60 }\ \text {s}={ 12 }\ \text {J}$?
