解：$(1)U_{1}=IR_{1}=0.3\ \mathrm {A}×20 \ \mathrm {Ω}=6\ \mathrm {V}$
$(2)U_{2}=U-U_{1}=24\ \mathrm {V}-6\ \mathrm {V}=18\ \mathrm {V}$
$R_{2}=\frac {U_{2}}{I}=\frac {18\ \mathrm {V}}{0.3\ \mathrm {A}}=60 \ \mathrm {Ω}$
$(3)W_{2}=U_{2}It=18\ \mathrm {V}×0.3\ \mathrm {A}×2×60\ \mathrm {s}=648\ \mathrm {J}$
