You decide to use your body as a Carnot heat engine. The operating gas is in a t

Question

You decide to use your body as a Carnot heat engine. The operating gas is in a tube with one end in your mouth (where the temperature is 37.0 C) and the other end at the surface of your skin, at 30.0 C.

Part A)What is the maximum efficiency of such a heat engine? Would it be a very useful engine? (2.26%)

Part B)Suppose you want to use this human engine to lift a 2.60 kg box from the floor to a tabletop 1.40 mabove the floor. How much must you increase the gravitational potential energy? (35.71J)

Part C)Suppose you want to use this human engine to lift a 2.60 kg box from the floor to a tabletop 1.40 mabove the floor. How much heat input is needed to accomplish this? (1580J)

Part D)How many 350 calorie (those are food calories, remember) candy bars must you eat to lift the box in this way? Recall that 80.0 % of the food energy goes into heat.

I've done Parts A, B, and C as seen from the answers above but I do not know how to do Part D. So far I've guessed .0054, .001077, and 1.35 but those are wrong.

Explanation / Answer

given

Th = 37 C = 273.16 + 37 = 310.16 K

Tc = 30 C = 303.16 K

a. maximum efficiency = 1 - Tc/Th = 0.0225689

b. m = 2.6 kg

h = 1.4 m

dE = mgh = 35.7084 J

c. heat input = U

dE = n*U ( where n is efficienty)

U = 1582.19496 J

d. 350 cal = 1.464e+6 J

hence

n*1.464*10^6 * 0.8 = U

n = 0.00135 number of candy bars

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