A coil develops $800 \mathrm{cal} / \mathrm{s}$ when $20 \mathrm{~V}$ is supplied across its ends. Compute its resistance.
$$
\mathrm{P}=(800 \mathrm{cal} / \mathrm{s})(4.184 \mathrm{~J} / \mathrm{cal})=3347 \mathrm{~J} / \mathrm{s}
$$
Then, because $\mathrm{P}=V^{2} / R$
$$
R=\frac{(20 \mathrm{~V})^{2}}{3347 \mathrm{~J} / \mathrm{s}}=0.12 \Omega
$$