A 100 watt immersion heater is placed in a pot containing 1 litre of water at 20ºC. How long will it take to heat water to boiling temperature if 20% of the available energy is lost to surroundings?

Given P of the heater is 100W

Heat required to boil water

$\mathrm{ms}\left({\mathrm{t}}_2-{\mathrm{t}}_1\right)=1\times 4200\times 80$$\mathrm{mass} \mathrm{of} 1 \mathrm{liter} \mathrm{of} \mathrm{water} \mathrm{is} 1 \mathrm{kg} \mathrm{and} \mathrm{specific} \mathrm{of} \mathrm{water} \mathrm{is} 4200 \mathrm{J}/\mathrm{kg}$

$20 \% \mathrm{is} \mathrm{loss}$

$\mathrm{so} \mathrm{heat} \mathrm{required} 4200\times 80=\mathrm{P}\times \mathrm{t}\times \frac{80}{100}$

$\mathrm{so} \mathrm{t}=\frac{4200\times 100}{100}=4200 \sec =70 \min$