解：$(1)I_{2}=I-I_{1}=0.5\ \mathrm {A}-0.2\ \mathrm {A}=0.3\ \mathrm {A}$
$U=I_{2}R_{2}=0.3\ \mathrm {A}×20 \ \mathrm {Ω}=6\ \mathrm {V}$
$W_{1}=UI_{1}t=6\ \mathrm {V}×0.2\ \mathrm {A}×10\ \mathrm {s}=12\ \mathrm {J}$
$W_{2}=UI_{2}t=6\ \mathrm {V}×0.3\ \mathrm {A}×10\ \mathrm {s}=18\ \mathrm {J}$
$(2)W=W_{1}+W_{2}=12\ \mathrm {J}+18\ \mathrm {J}=30\ \mathrm {J}$
