A kettle transfers 1500 J of energy, 1200 J to a thermal energy store and 300 J to a vibrational energy store (sound). How efficient is the kettle?

Answer: 0.8

Explanation:

The efficiency is defined as the ratio of total imput energy that is actually utilized to the end of the device.

So if we have a total transfer of 1500j of energy, and 1200j are used to heat the thing inside the kettle, 300j are not used to the actual function of the kettle.

So the efficiency is n = 1200j/1500j = 0.8

This means that a 80% of the energy imput is actually used to heat the kettle.