Aaron Keller

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Why is the Earth Still Hot Inside? part of Earth and Space Science:Summer 2010:Activities
In this activity the teacher brings to students' attention that the Earth is still very hot inside. Deep mines are noticeably hotter the deeper you go (up to 30º per km of depth). Scientific investigations have determined that the upper mantle has a temperature of 1200ºC - 1400ºC and that the solid inner core has a temperature of up to 7000ºC. The Earth is also very old: currently estimates put its formation at 4.53 to 4.47 billion years ago. So if it was hot at its formation and all this time has passed, why is it still hot inside now? Part of the answer is that radioactive isotopes in the core and mantle continually produce heat in the interior of the Earth. But the primary reason is that heat loss from a body is less efficient when the surface area to volume ratio is low. To investigate this problem students will be given a set of materials to look at cooling rates. They will heat glass spheres (marbles) in a hot water bath at 100ºC and transfer them to cold water in a styrofoam cup. They will then record the temperature of the water in the cup as a function of time. In this way they will generate both a total change in temperature and a total time during which the glass transfers heat to the water. Through discussion and discovery students will be led to repeat the experiment with glass spheres of a different size but the same total mass (the number of spheres will change, but the mass will be held constant). In this way students can be led to the idea that the ratio of surface to volume has important consequences in the transfer of heat. [image right border]