Question

A kettle transfers 1,500 J of energy, 1,200 J to a thermal energy store and 300 J to a vibrational energy store (sound).  How efficient is the kettle?               ​

Answer 1

Answer:

$\eta = 0.8 = 80\%$

Explanation:

Efficency of a machine is defined as the ratio of "useful" energy over the total energy imput in it. In this case a kettle wants to heat water, so thermal energy is what we need; if we wanted to build some kind of music instrument, we would be looking at the energy generating vibrations.

$\eta = \frac{1200J}{1500J} = 0.8$