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 150Calories. (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 37.2 °C and that of herenvironment is 15.2 °C. The emissivity of her skin is 0.74 andits surface area is 1.2 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?

Number

Units

The number of significant digits is set to 3; the toleranceis +/-2%

Chapter 13, Problem 29

Explanation / Answer

      P = Ae (T4 -T04 )       = (1.2m2 ) (0.740(5.67*10-8 ) (310.24 -288.24 )       = 118.8 J/s we know that power P = Q/t so time t = Q/P = (150) (4186J ) / 118.8 J/s =5285.35 s or 1.5 hr

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