Engineering Question #2329

Travis Cheeseman, a 31 year old male from Bloomington, IND. asks on October 23, 2004,

Ok, this is my project: I have put roughly two cups of rice in a sock and threw it in the microwave for four and a half minutes, it kept its warmth for roughly two hours. Do you know of anything that is as cheap and safe, but will hold heat longer then that!!

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The answer

John Jones answered on October 26, 2004

One way to hold heat for a long time is to use a material with a high specific heat. The specific heat of rice is about 1.8 kJ/kg.K. Pearl barley is better, with a specific heat of about 2.8 kJ/kg.K, and buckwheat is better still, at 3.2-3.8 kJ/kg.K. (Water has a specific heat of 4.2 kJ/kg.K.).

To do much better than this, you would need to use a material that changed state from solid to liquid at about the temperature that you want to maintain. You proabably won't find such a material in your kitchen, but physiotherapy stores may carry packets of a specially formulated gel that changes state at a little above body temperature, for use in heat treatment of sprains. This gel can stay hot longer than rice, since it stores the latent heat associated with its change from solid to liquid in addition to the heat associated with its elevated temperature.

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