Multiple-ConceptExample 8 reviews the approach that is used in problems such ast

Question

Multiple-ConceptExample 8 reviews the approach that is used in problems such asthis. A person eats a dessert that contains 170 Calories. (This"Calorie" unit, with a capital C, is the one used by nutritionists;1 Calorie = 4186 J. See Section 12.7.) The skin temperature of theperson is 37.4 °C and that of her environment is 20.8 °C.The emissivity of her skin is 0.78 and its surface area is 1.3m2. How many hours would it take her to emit anet radiant energy from her body that is equal to theenergy contained in this dessert?

Explanation / Answer

   from the theory we have the equation
   Pnet = e A (T4-To4) and
   Pnet = Q / t
   on equating we get
   t = (Q / Pnet)
     =  [Q / e A(T4 -To4)]
   from the given
   Q = (170 C) (4186 J / 1 C)
   T = 37.4oC + 273 K
      = ......... K
   To = 20.8oC + 273 K
        = ....... K
   solve for t
   t = ...... s

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