The surface tension of a liquid is 0.5 N/m. If a liquid film is formed on a ring of area 0.02 m² then its surface energy will be

Given,$\mathrm{surface} \mathrm{energy}=\mathrm{surface} \mathrm{tension}\times \mathrm{area}=0.5\times 0.04=0.02\mathrm{J}$

Surface tension = 0.5N/m

area of the ring = 0.02 m²

The net area is =  2 × surface area since a ring has two open surfaces and the thin film will form on both of them.

$\mathrm{area}=2\times 0.02=0.04{\mathrm{m}}^2$

$\mathrm{surface} \mathrm{energy}=\mathrm{surface} \mathrm{tension}\times \mathrm{area}=0.5\times 0.04=0.02\mathrm{J}$

Hence the correct answer is 0.02J.