Question

lily's hairdryer converts 30j of electrical energy into heat, 10j into sound and 60j into kinetic energy. how efficient is it?

Answer 1

The efficiency of a machine is a measure of the extent to which the input energy is well used for an specific output usage and function. So in case or hair drying machine only the useful energy is the kinetic energy which makes air to blow and make our hair dry and heat energy which makes air hot so that,

In Hairdryer

Heat energy heating the air.

Kinetic energy of the fan that blows the air.

Waste energy = Sound energy.

(E) ?efficiency = useful power/ input power (× 100%)

Here total input power is electrical energy which is 100j

So that as in question it is also given that,

Only 10j is sound energy so ,

E =(30+60)j /100j × 100 = 90 %

Hence the efficiency of the hairdryer is 90%