BRAVO! You got it right!

The emissivity of the human body is about 0.95 and the surface area of an average male is about 1.75 m². Normal body temperature is 37^oC but skin temperature is slightly lower, 35^oC = 308 K.

In the previous Q&A we found that this typical man emits radiation at the rate of about 850 W.

If this same typical man is in surroundings that are at 25^oC, he also absorbs heat. We need temperatures in units of Kelvins, 25^oC = 298 K.

What is the rate at which this man absorbs heat?

The rate at which heat is emitted is given by

R_asorb = e A T_surr⁴

R_asorb = (0.95)(5.67 x 10 ^{- 8} W/m² K⁴)(1.75 m²)(298 K)⁴

35^oC = 298 K

That means the net rate is R_net = R_emit - R_asorb

R_net = 848 W - 740 W

R_net = 108 W

Hence, you may already have heard that people produce about as much heat as a one-hundred watt light bulb. That is what we have just calculated.

(c) 2000, Doug Davis; all rights reserved.