Problem: You are in a bathosphere going deep below the ocean surface.
The bathosphere has a volume of 524
m3. At your current location, the sphere is losing heat at a constant
rate of 817 W. The current pressure
in the sphere is 1 atm, and the current temperature is 25 oC. What
is the pressure going to be one hour
later? (You can assume that the air acts as an ideal gas with a molar
specific heat of 20.8 J/mol K.)
SITUATION AND VARIABLES: (All variable definitions
require words or clearly labeled diagrams. Where useful, diagrams are included
in this section.)
C = molar specific heat = 20.8 J/mol K
FIND: final pressure (the
students clearly identify the quantity they are finding/question they are
answering) ANSWER: 9.7 x 104 Pa
STRATEGY: (all concepts used in the problem
are discussed here. I usually provide a full list of "possible" concepts
to the class. Also, I suggest
that students use the form: "Used -----
to find ---- " and/or "Used ----- because -----". The latter would
be used in cases such a elastic collisions:
"Used conservation of mechanical energy because
the collision is elastic." I do not require this, though I point out not
using this format will potentially make
it harder for the TAs to decide what you did.)
Used the ideal gas law to find the number of moles.
Used definition of heat flux to compute the heat.
Used the relation between heat and temperature change (Q = nCDT)
to find the temperature change.
Used the ideal gas law to find new pressure.
Though it would be nice to have the strategy
follow the logical order that the equations are used in, I have not required
TYPICAL GRADING BREAKDOWN FOR A 20 Point PROBLEM:
2 - 3 points for SITUATION (depending on the
1 point for FIND
2 - 3 points for ANSWER (I always assign 1 for
numerical value, 1 for direction, 1 for units)
roughly 1/2 remaining points for STRATEGY and
1/2 for CALCULATIONS, though some
problems are heavier in one area than the other.
I treat all 5 sections as completely independent,
except in the following sense.
Points are taken off for "incorrect" strategies
that are NOT attempted in the calculation box.
"incorrect" strategies that are attempted in
the calculation box are treated as neutral and receive no credit.
"correct: strategies that are not attempted or
carried out incorrectly are given credit in the strategy box,
but appropriate points dedutcted in the calculation.