Question

The average human metabolizes 2000 kcal/day. Some of this energy is converted into work, but the majority is wasted heat. With a skin area of 1.50 m2 , a skin temperature of 27.0o C, and an effective emissivity of e = 0.600, some of this wasted heat is released into the 20.0o C environment via radiation. The rest of the waste heat is removed by evaporating water, either as perspiration or from the lungs during breathing. At core body temperature (37.0o C), the latent heat of vaporization of water is 575 cal/g. The average person requires eight 8 oz. glasses of water per day to stay hydrated; for this problem assume that it is all lost via evaporation. Under these conditions, what is the efficiency of the human body converting energy to work?

Answer 1

heat loss due to radiation is

heat lost due vaporization is

workdone = 2000 - 1.043-769.2 = 1229.757

effiicency = workdone/energy = 1229/2000 = 0.615 i.e 61.5%