Tuesday, Sept 22, 2009 - APS 3A

Students took out Crystal Structure of Metals worksheet.

I had them do a warm up, asking how far they got, and if they did not finish, where did they get stuck.

Went over the packet in detail as a class.

Did a conclusions section that highlighted the main points of the lab:
Conclusions:
1. Rank the different crystal structures of metals, FCC (face centered cubic), BCC (body-centered cubic), and HCP (Hexagonal Close Packed) in terms of their ductility, most ductile to least ductile:

Most ductile: ________________ ________________ ________________ Least ductile

2. Is it possible to predict crystal structure of a metal based on the physical property of ductility, the ability to be shaped by hammering? ____________

3. Gold has a __________ crystal structure. Iron has a ____________ crystal structure. Which is more easily shaped by hammering? ____________

4. Comparing FCC and BCC, which crystal structure is more tightly packed (has the higher packing factor): ____________

5. Suppose a metal could change its crystal structure from FCC to BCC. For the same number of atoms, would the volume increase, decrease, or stay the same? ____________

6. Suppose you had a thin wire metal, and the metal in the wire changed its crystal structure from FCC to BCC. Would the length of the wire increase, decrease, or stay the same? __________


Showed setup for Iron Wire Demo.
In class 3A we didn't have too much time for the demo so I skimped on having students look for the "physics" in the set-up.

Did the demo. Students made observations.
Students noted the expansion of the wire as it got heated up.
The wire heated to glowing showing that it was hot enough to emit light.

Students were asked to observe what happened to the wire when I shut off the power.
It was a bit subtle, but the wire did shrink a bit, and then expand - showing the phase change from FCC to BCC as it cooled.

Finished with discussion of the Twin Towers on 9/11 and how the phase change in the steel girders could have caused them to become more ductile, along with ~9% uneven shrinkage, adding to the collapse.

I really like this demo because there are just so many ideas shown.

No homework.