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