$解：(1)R=\frac {U}{I}=\frac {8\ \mathrm {V}}{0.4\ \mathrm {A}}=20\ \mathrm {Ω}$
$(2)W=Pt=UIt=8\ \mathrm {V}×0.5\ \mathrm {A}× 60\ \mathrm {s}=240\ \mathrm {J}$
$(3)由图15-2-4(\mathrm {a})可知，当I_{R}=0.2\ \mathrm {A}时，I_{L}=I_{R}=0.2\ \mathrm {A}，R_{L}=\frac {U_{L}}{I_{L}}=\frac {1\ \mathrm {V}}{0.2\ \mathrm {A}}=5\ \mathrm {Ω}，R_{总}= R_{L}+R=5\ \mathrm {Ω}+20\ \mathrm {Ω}=25\ \mathrm {Ω}，U=IR_{总}=0.2\ \mathrm {A}×25\ \mathrm {Ω}=5\ \mathrm {V}$
$(4)由图15-2-4(\mathrm {a})可知，当P_{L}=0.6\ \mathrm {W} 时，U_{L}=2\ \mathrm {V}，所以P_{R}=\frac {U_{R}^2}{R}=\frac {(2\ \mathrm {V})^2}{20\ \mathrm {Ω}}=0.2\ \mathrm {W}，P_{L}∶ P_{R}=0.6÷0.2=3∶1$
