Question

Many of the houses have fireplaces that are
15 percent efficient and use 15 kg of wood to
heat a room to 20°C (68°F). Wanting to reduce
their ecological footprint, several of the home-
owners are considering switching to woodstoves
that are 75 percent efficient. If they do change to
using a woodstove, how many kilograms of wood

Answer 1

By using a woodstove, the kilograms of wood required to

heat a room to 20°C is 3 kg

Efficiency of a machine

• Efficiency is the percent of work put into a machine (input work) to that of the work done by the machine (output work).

• Efficiency of a machine is always less than 100% because the output work is always less than the input work because some of the input work is used to overcome friction.

Efficiency = work output/work input * 100%

Efficiency of fireplaces = 15%

Amount of food required to heat a room to 20°C = 15 kg

Assuming efficiency of 100%, amount of wood required will be X kg:

X = 15% * 15 kg

X = 2.25 Kg

By switching to woodstoves which are 75% efficient, amount of wood required will be Y kg:

Y = 2.25 kg / 75%

Y = 3 kg

Therefore, by using a woodstove, the kilograms of wood required to

heat a room to 20°C is 3 kg

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