We often cut a watermelon in half and put it into the freezer to cool it quickly. But usually we forget to check on it and end up having a watermelon with a frozen layer on the top. To avoid this potential problem a person wants to set the timer such that it will go off when the temperature of the exposed surface of the watermelon drops to 3°C. Consider a 25-cm-diameter spherical watermelon that is cut into two equal parts and put into a freezer at 212°C. Initially, the entire watermelon is at a uniform temperature of 25°C, and the heat transfer coefficient on the surfaces is 22 W/m2·K. Assuming the watermelon to have the properties of water, determine how long it will take for the center of the exposed cut surfaces of the watermelon to drop to 3°C. Answer: 46.7 min

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A: a) The given temperature in kelvin is TK=T°C+273=128+273=401K

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A: See below

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Q: heat transfer rate

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A: Solution: Given that T0 =  -40 °F = 233.15 K T = 86 °F = 303.15 K

Calorimetry

Measuring the heat transferred during chemical reactions or other physical processes is known as calorimetry. It comes under the branch of thermochemistry.