An electric iron of resistance $20 \Omega$ takes a current of $5.0$ A. Calculate the thermal energy, in joules, developed in $30 \mathrm{~s}$.
$$
\begin{array}{l}
\text { Energy }=\mathrm{P} t \\
\text { Energy }=I^{2} R t=(5 \mathrm{~A})^{2}(20 \Omega)(30 \mathrm{~s})=15 \mathrm{~kJ}
\end{array}
$$