A method is used sometimes in open air camps, of keeping food warm overnight. In the evening, food is heated in the metal pot which is then placed in the wooden box and surrounded by loosely packed hay. The next morning when the metal pot is removed from the hay, the food is still warm.
a) Explain how the loosely packed hay reduces heat loss from the food
b) State with a reason what is likely to happen if the hay is tightly packed.
The difference in temperature between one thing and another creates the conditions where heat transfer may occur, the heat moving from a higher temperature area to a lower temperature area. Heat transfers by three methods: Radiation, Convection, and Conduction. In the case of Radiation, a hot object emits infrared radiation, which carries the heat away. In Convection, the air around the hot object is heated and moves away, while cooler air moves in to replace it, and the cycle continues. In Conduction, heat is distributed from the hot to cool by the vibration of molecules in contact with the hot object.
By loosely packing the hot pot with hay, you're creating many "air pockets" around the pot. Hay, being a poor conductor of heat, meaning its molecules won't vibrate very much relative to the metal pot it surrounds, will instead help insulate the pot and keep it warm. Packing it loosely also slows down the rate of convection, since the air around the metal pot can't circulate, and remove the warmer air near the pot and cool it down.
If you tightly pack the hot pot, more hay will be in contact with the metal pot, and it will lose heat more rapidly through conduction. Hay is a better conductor of heat than air, which is the same as saying that air insulates better than hay.