Physics 7b
Midterm Exam #1
Fall 2004
Name:__________________________Disc. Section_______________
SID__________
#1______
#2______
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M
w
of O
2
= 32 g/mol,
k
b
=1.38x10
-23
J/K,
N
A
=6x10
23
The weight of a 1kg mass on the Earth’s surface is 2.2pounds.
The relative weight of each problem is indicated next to the problem number.
Most credit will be given for algebraic work.
Please do not insert numerical values
until you have a final algebraic answer inside a box
. If you don’t know a particular
constant, use a symbol to get partial credit. It is almost impossible to award partial credit
when a student has inserted numbers too early.
If you get stuck on one problem, go on to
the next and come back to the difficulties later in the exam period.
Do not leave early
until you have completed everything. Do not quit! Never, never, never quit!!

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1.
(20 pts) PG&E charges for natural gas in a unit called a therm.
1 therm = 10
5
BTU.
1BTU= heat to raise the temp of one pound
of water by 1
o
F.
The gas price is $0.67/therm.
The price of electricity is $0.14/kW hr
When buying energy from PG&E, what is the ratio of cost per Joule for gas compared to
electricity?