class01lecturenotes

class01lecturenotes - Class 1 Wednesday January 7,2010...

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Class 1, Wednesday, January 7,2010 Reading: lS-:I How many molecules of air are there in a volume of space equal to that of a sugar-cube? One sugar cube is tcm 3 We will, however, use the MKS system of units in this class. With approximately 10 19 molecules per cm 3 , how many are there in 1 m 3 ? A volume of 1 m 3 is called a "unit volume". There are hence one million times more molecules in 1 m 3 than in 1 cm 3 of space. This amounts to 106'1019=1025 molecules of air in one unit volume of space. Of course, this is an approximate count. In particular, this number also depends on a variety of other factors, such as your location. On the other hand, there are approximately 100 Billion stars in one galaxy, and about 100 Billion galaxies similar to our own in the entire visible universe. Hence, there are about 10 22 stars in the visible universe. These two simple estimates show that there are 1000 times more molecules of air in one unit volume of space than there are stars in the visible universe! The space that we perceive as empty contains the air that we breathe, and it is actually quite crowded. This is counter-intuitive. Indeed, for centuries, philosophers were troubled by the idea of empty space. As we will see later, the 17 tti century German physicist and inventor Otto von Ouericke, put an end to this so-called abhor of the emptiness (horror vacui). What is actually going on in this unit volume filled with classroom air? Imagine a gas enclosed in a container with a piston. The gas molecules move about in random directions and at random speeds. They are free to move as they please. A gas behaves much like a swarm of insects. Occasionally, the molecules collide with each other, or, if they are enclosed, collide with the walls of the container. Let's assume that the gas is locked up in a cylindrical container sealed with a piston. We can move the piston up and
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