A motor operating on `120 V` draws a current of `2 A` . If the heat is developed in the motor at the rate of `9 cal s^(-1)`, what is the efficiency?

Explanation

`P_("in") = 12 xx 2 = 240 W, heat loss = 9 cals^(-1) = 9 xx 4.20 = 378 Js^(-1)`
` P_("out") = 240 - 37.8 = 202.2 W`
Effiency `eta = (P_(out)/(P_("in")) xx 100 = 84.25%`