Wednesday, January 17, 2007

• Chapter 1 review
  
• Ponderable:  What constitutes a good theory?
• A good theory is one that is useful.
  
• Ponderable:  Why is the period of revolution for Earth around the Sun not one day?
• While this relative motion would explain day and night (assuming that the Earth does not rotate), it would not explain how we get seasons on an annual (yearly) frequency.
  
• Chapter 2 - Atoms
• Discuss:  In Fig. 2.12, the length scales range from 10^25 m down to 10^-25 m, with human size (~1 m) right in the middle.  Is this just chance, or is there a reasonable explanation for this occurrence?
• This could be a coincidence, but most likely it is a consequence of our ability to measure the very large and small approximately equally.
• Question:  What is the age of a typical student in seconds?
  
• (20y)(365 d/y)(24 h/d)(60 m/h)(60 s/m) = 600 million seconds = 600 Ms = 6e8 s
• Discuss:  Suppose someone offers you a billion one-dollar bills if you are willing to first count them.  Should you accept this offer?
• Assume that you can count the bills at an average rate of 1 per second, then it would take about 31 years without breaks to count this money!  I would not accept this offer; would you?
  
• Estimate the number of molecules in a drop of water.
• One drop of water is about 0.1 mL, which has a mass of 0.1 g.  The molecular weight of water is 18 g/mol (since oxygen has 8 protons and 8 neutrons, and there are two protons from hydrogen).  Therefore, one drop of water is about (0.1 g)/(18 g/mol) = 0.006 mol.  One mole = 6.02e23, so there are approximately 3e21 molecules in one drop of water.
  
• If the nucleus of a hydrogen atom were the size of a baseball, how far away would the electron typically be?
• The nucleus is ~10e-15 m, while the atom (electron cloud) is ~10e-10 m.  So if the nucleus were ~10 cm in diameter, then the electron cloud would be about 100 000 times bigger, or 10 km in diameter!  Clearly, atoms are mostly empty space!
• Why is ice less dense than water?  (Hint: see figures 2.2 and 2.7 in the textbook.)
• The open ring structure of ice makes it less dense, as can be seen in the figures.