CONCEPTUAL PRUSIS PRACTICE Chapter 17 Change of Phase Our Earth's Hot Interior A major puzzle faced scientists in the 19th century. Volcanoes showed that Earth is molten beneath its crust. Penetration into the crust by bore holes and mines showed that Earth's temperature increases with depth. Scientists found that heat flows from the interior to the surface. They assumed that the source of Earth's internal heat was primordial, the afterglow of is fiery birth. Measurements of cooling rates indicated a relatively young Earth-some 25 to 30 millions years in age. But geological evidence indicated an older Earth. This puzzle wasn't solved until the discovery of radioactivity. Then it was learned that the interior is kept hot by the energy of radioactive decay. We now know the age of Earth is some 4.5 billion years-a much older Earth. Al rock contains trace amounts of radioactive minerals. Those in common granite release energy at the rate of 0.03 joule/kilogram-year. Granite at Earth's surface transfers this energy to the surroundings as fast as it is generated, so we don't find granite warm to the touch. But what if a sample of granite were thermally insulated? That is, suppose the increase of internal energy due to adioactivity were contained. Then it would get hotter. How much? Let's figure it out, using 790 joule/kilogram-kelvin as the specific heat of granite. Calculations: 1. How many joules are required to increase the temperature of 1 kg of granite by 1000 K? PHYSICS ... PHYSICS ... Q = cmat 2. How many years would it take radioactive decay in a kilogram PHYSICS .... of granite to produce this many joules? Questions: 20 1. How many years would it take a thermally insulated 1-kg chunk of granite to undergo a 1000 K increase in temperature? 2. How many years would it take a thermally insulated one million-kilogram chunk of granite to undergo An electric toaster stays hot a 1000 K increase in temperature? while electric energy is supplied, and doesn't cool until switched off. Similarly, do you think the 3. Why are your answers to the above the same (or different)? energy source now keeping the Earth hot will one day suddenly switch off like a disconnected toaster - or gradually decrease over a long time? 4. (True] [False] Ener