Question
An electric drill transfers 200 J of energy into a useful kinetic energy store. It wastefully transfers 92 J of energy to the thermal store of the drill and the surroundings . What is the efficiency of this drill? Give your answer as a percentage to 2 significant figures.
Answer
4.6
(281 Votes)
Xerxes
Master · Tutor for 5 years
Answer
200 / 292 * 100 ≈ 68.49 ≈ 68
Explanation
The efficiency of a device or system can be defined as the ratio of useful output energy (or power) to input energy (or power). It essentially gives us an idea of how well a system or device converts the energy (or power) it gets into the energy (or power) we want, without wasting any. Efficiency is usually expressed as a percentage. Mathematically, it can be calculated as Efficiency (%) = (Useful output energy/Input energy) x 100.In this particular question, we have been given 1. Useful output energy = 200J and 2. Input energy = Useful output energy + Wasteful energy = 200J + 92J = 292J. Now let's plug these values into the equation of efficiency and do the calculation.