解:$(1)I_{1} =I-I_{2} =1.5\ \mathrm {A}-0.6\ \mathrm {A}=0.9\ \mathrm {A}$
$U=I_{1}R_{1}=0.9\ \mathrm {A}×10 \ \mathrm {Ω}=9\ \mathrm {V}$
$(2)R_{2}=\frac {U}{I_{2}}=\frac {9\ \mathrm {V}}{0.6\ \mathrm {A}}=15 \ \mathrm {Ω}$
$(3)W=UIt=9\ \mathrm {V}×1.5\ \mathrm {A}×10\ \mathrm {s}=135\ \mathrm {J}$