Multiple-Concept Example 8 reviews the approach that is used inproblems such as

Question

Multiple-Concept Example 8 reviews the approach that is used inproblems such as this. A person eats a dessert that contains 160Calories. (This "Calorie" unit, with a capital C, is the one usedby nutritionists; 1 Calorie = 4186 J. See Section 12.7.) The skintemperature of the person is 36.5 °C and that of herenvironment is 18.7 °C. The emissivity of her skin is 0.76 andits surface area is 1.6 m2. How many hours would it takeher to emit a net radiant energy from her body that isequal to the energy contained in this dessert?

Explanation / Answer

energy E = 160 cal = 160 * 4186 J The skin temperature of the person is t = 36.5 °C = 36.5+273 = 309.5 K Temperature of her environment is t ' = 18.7 °C =18.7 + 273 =291.7 K The emissivity of her skin e = 0.76 surface area is A = 1.6 m2 net radiant energy from her body = the energy contained in this dessert                     eA (T^4 -T'^4 ) t = 160 * 4186 where = Stefan constant = 5.67*10^-8 Wm^-2k^-4 plug the values we get time t   value inseconds So, time in hours = (t / 3600 ) hrs net radiant energy from her body = the energy contained in this dessert                     eA (T^4 -T'^4 ) t = 160 * 4186 where = Stefan constant = 5.67*10^-8 Wm^-2k^-4 plug the values we get time t   value inseconds So, time in hours = (t / 3600 ) hrs

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