ch20 - The Big Ideas-Chapter 20(Serway and Beichner Physics...

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The Big Ideas—Chapter 20 (Serway and Beichner, Physics for Scientists and Engineers, 5 th Edition) AJM:2/17/01 1 Section 1 Heat, like work, refers to a transfer of energy from one system to another. Objects do not “have” heat; they “have” internal energy. The internal energy of an object may be changed via mechanical processes (i.e., those that involve work ) or via thermal processes (i.e., those that involve heat .) Note that, although heat is a flow of energy from an object at a higher temperature to an object at a lower temperature that occurs as a result of the temperature difference, temperature itself is not a measure of heat . For instance, we can “warm” something (i.e., increase its temperature) by doing a suitable amount of work on it. An archaic unit of heat—the calorie—created at a time when people thought heat itself was a sort of fluid that objects “contained” is still used today even though we now know that it is simply an alternate unit of energy. 1 cal 4.186 J Section 2 The “heat capacity” of an object is the change in internal energy ( not necessarily heat!) per unit change in temperature. By definition it is an extensive quantity—i.e., it depends on the amount of “stuff.” The “specific heat” of a substance is the change in internal energy per unit mass per change in temperature. By definition it is an intensive quantity—i.e., it depends only on the identity of the substance. These two quantities have very unfortunate names!! For instance, the heat capacity of an object has nothing specifically to do with “heat” (it has to do with changes in internal energy which may be the result of heat or work) nor should it be taken to imply that objects have a finite capacity for “heat” (or even for changes in internal energy!) The “heat capacity” of an object is just the sum of the heat capacities of its constituent parts with heat capacity of each part depending on its own mass and the “specific heat” of the substance of which it is composed.
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