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# Physlet® Physics 2E: Thermodynamics

## Chapter 19: Heat and Temperature

The study of the relationship between heat, work and internal energy, described macroscopically by temperature, is the focus of thermodynamics. Heat is the energy transferred due to the temperature difference between objects and/or the energy transferred when work is done to change the internal energy of an object. The 2nd law of thermodynamics declares that heat flows naturally from objects of higher temperature to objects at lower temperature. We categorize the mechanism of heat transfer in terms of conduction, convection and radiation. As we study what happens when heat is added to materials, we explore state changes (melting-freezing, vaporizing-condensing) and temperature increases in solids and liquids. As the temperature increases, we also quantitatively describe the expansion of materials.

## Chapter 20: Kinetic Theory and Ideal Gas Law

The connection between the macroscopic quantities of temperature and pressure and the microscopic quantities of internal energy and momentum is the subject of the kinetic theory of gases. We use the ideal gas as a model system to explore the connections between macroscopic and microscopic quantities. Often, we will use pressure-volume diagrams (PV diagrams) to provide a description of a thermodynamic process to show the work done and the change in internal energy associated with an input of energy.

## Chapter 21: Engines and Entropy

One application of thermodynamics is the transfer of thermal energy into work in an engine. In general, this cyclic process involves an exchange of heat with two reservoirs, heat in at a high temperature and heat out at a low temperature, resulting in net positive work from the process. We use PV diagrams to describe the work done and heat exchange in each step of the cycle. There is a limit to the work you can get out of an engine compared to the net thermal energy you put in. Conservation of energy (first law of thermodynamics) says that you cannot get more energy out than you put in, i.e., you can only break even. But the second law of thermodynamics says that you cannot even break even, you can only lose. The second law of thermodynamics says that, as time goes forward, entropy (disorder) increases. It takes energy to decrease entropy. In this chapter we will pull together the ideas of work, heat, and entropy to show how everything fits together in simplified engines.

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