HW 5

Here are some of the common mistakes on homework 5:

On number 20, many people did not account for the units - you need to change Cp into m^2/(s^2*C). (Cp for water = 4.184X10^3 m^2/(s^2*C)). Or you could have changed the kinetic energy into kJ/kg. Make sure you pay attention to the signs (temperature increased in this problem).

On 21, the problem was a liquid. Therefore you cannot use the ideal gas equations for work. Also, although there was a small change in volume, there was a very large pressure change so you cannot say that there was no work done. Also, be careful you copy the equations done correctly. The second term in kappa was multiplied by P, so kappa depended on P. Therefore when you take the integral, you cannot pull kappa outside of the integral.

On 22, many people struggled with taking the derivatives. The first step was to solve the equation for V in part a and P in part b. Then differentiate. The chain rule and multiplication (or quotient) rules were necessary on part b. Many people first solved for beta and used other properties to get dP/dT. While this was correct, I believe it was a little more difficult than solving for (dP/dT)v directly. Also, the problem specified that V should not be in the answer, so you should have substituted in what V equaled in the end.

On 23, make sure you say specifically how you solved for the answer. It was OK to use your calculator or computer, but make sure you say how you got the answer. Also, make sure your answers make sense - all three methods should have given similar results. If you got something way different, make sure to check it.

On 24 and 25, you can not assume ideal behavior or constant density for the liquid. The point of these problems was to use the more detailed equations in the text. There were mutliple ways of solving these problems (RK, Lee-Kesler, generalized correlations if appropriate), but ideal gas was not a valid assumption.