On Friday I was home with a sick kiddo; I left a video and enhanced the homework assignment that was to be due Monday --> changed it to Tuesday and added more problems:
Due Tuesday: Read p. 266-270 and work p. 270#1-6, ALSO p. 882#94-104
I had a couple of questions on the p. 882 problems; I was happy to answer them in class today. Biggest was the usage of Kelvin vs. Celsius. In specific heat problems, they are interchangeable; we are talking about temperature change, and Kelvins and degrees C are the same size; a change of ten units on either scale is identical. Kelvins will be important when referencing absolute temperature, or the quantity of thermal energy in something, OR anytime gases are considered.
Today we went over answers on p. 259's problems that were handed back on Friday, answered questions that were suggested from Tuesday's homework, and had demonstrations on thermal E transfer (HEAT) as well as thermal expansion.
First, I showed that temperature relates to thermal E (microscopic KE) by dropping food coloring into two graduated cylinders; one room temp, and one hot. The food coloring was dispersed and blended rapidly in the hot water due to the increased molecular motion within it. (Temperature is a measure of the average KE of particles within a substance)
Second, we explored thermal insulators; styrofoam, plastic, and paper cups filled with water were subjected to flame. The styrofoam cup stretched outward rapidly and became very thin, seeping water through as it was heated. Plastic did a similar trick, but not as dramatically. Paper became brown but didn't leak through at the site of the flame.
The big question is why did these behave differently, and why did all 3 resist total burn-through?
Styrofoam is the best insulator; it is full of air pockets. Paper is the worst insulator of the three. When heated, the paper cup allows thermal E to transfer directly to the water inside, preventing too much E absorption by the cup and it doesn't burn. The styrofoam cup burned briefly until it became paper thin - losing its air pockets - and then it didn't insulate; it transferred the E straight into the water so it didn't keep burning.
Third, we asked if water was a conductor (we all knew from the reading and the Styrofoam that Air is a great insulator)... yet why do you feel cool if you go outside into cool air? Convection!!!
Is water the same? We heated a test tube of ice water from the bottom. Hot water at the bottom became less dense, moved to the top, melted ice, and boiled, as the whole column of water was stirred and heated. This could be due to conduction, convection, or both! We needed to flip it!
I pinned ice at the bottom of a test tube with steel wool and heated it from the top... the water at the top became less dense and.... stayed right there! A rolling boil was established at the top, maintaining ice at the bottom - that everyone could feel as the hot/cold test tube was passed around. Water isn't a very good conductor.
Finally, most classes got a chance to work with a ball and ring "toy" that you can make change size due to thermal expansion, and a compound bar where we tested conductivity of five metals. Copper was the best, followed closely by Aluminum and brass. Steel wasn't near as good, and last on the list was Nickel, taking a very long time to transfer thermal E just a few centimeters.
Tuesday in class we will discuss how we "feel" temperature, read about the coldest place in the universe and absolute zero, do skits representing different ways HEAT occurs, have notes about specific heat, temperature, conductors/insulators, and thermal expansion/density.
