class06lecturenotes

class06lecturenotes - Class 6 Monday Reading In our first...

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Class 6, Monday, January 18, 2010 Reading: In our first class we talked about liquids and gases on a microscopic level as well, but t only very briefly. On a microscopic level, what is the distinction between these three states of matter? In a gas, each molecule or atom has enough energy to overcome the weak attractive forces between the basic particles. As a result gas-particles can move around freely and get far away from each other. In a liquid, the constituent particles are about as closely packed as they can be, but their energy is still large enough to allow them to eventually move away from their initial neighbors. They can vibrate and spin and jostle themselves out of a given position and slip away. This energy is, however, too small to allow them to move more than a distance equal to about their diameter at a given time. In a solid, particles are also packed together about as far as they can be, but the energy per particle is too low to allow them to change neighbors. Particles in solids behave much like chickpeas in an egg-carton. They live in their individual potential well of their molecular bonds. They can vibrate and spin, i.e. roll up and down the potential well, but they can't ever get away from their "original" neighbors (unless the solid is given a sufficient amount of energy). v A number of physical quantities can be introduced to describe macroscopic systems. Apart from the ones we already know, p, V, P, N, n, we can introduce the temperature T, and the thermal energy, Em. Not all of these parameters are independent of one another. For example, the density is related to the volume tt.rd the mass. A similar relationship exists between Nand n, and we will study more such relationships over the coming week of classes. Only two of these parameters can be specified independently. All the others follow from these two. Indeed, the whole science of thermodynamics is concerned about finding out everything else from just two state-variables! The Temperature In the macroscopic view, the temperature is a measure of how "hot" or "cold" something is. This simple and rather superficial definition is, however, good enough to allow us to build an instrument for measuring the temperature, a thermometer. We will see later how temperature is linked to the thermal energy of a system. (, I
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How do you build a simple thermometer? Following Celsius' strategy, we can find some property of a material that varies linearly with the temperature. For example, the volume of certain liquids, such as Mercury, increases linearly when heated up, and decreases when cooled. When placed in a cylindrical receptacle, we also assume that this receptacle does not change in size considerably when heated or cooled. This way, the cross-sectional area stays constant, and so we established a linear relationship between the column of Mercury in the tube and the temperature. With this issue settled, we can try to find two fiducial points at which we establish the height, and assign each an arbitrary
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class06lecturenotes - Class 6 Monday Reading In our first...

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