Exp 4 (VRT) help

[robert zellmer]

I'm getting a number of questions concerning exp 4 (VRT) and sig. fig.,
particularly about those for your times.

I know a lot of you use your phones (stopwatch feature) to time things
(which actually caused some problems for exp 3).  I know these
stopwatches give times to fractions of a second.  However, do you really
believe you were that accurate in deciding exactly when the color 
disappeared?
I think you are lucky if your times are accurate to ± 1 sec. Most 
likely, your
overall accuracy is to the units place (e.g. if you recorded 15 secs for the
75 deg C run you are likely only accurate to the units place).

Remember from a previous e-mail you can't use times like h:mm:ss.
You can use minutes or seconds for your times, it doesn't matter. Using one
or the other simply shifts your line up or down but doesn't change the 
slope.
Even so, the number of s.f. will be different for the various times.  If 
you were
timing in seconds or convert to seconds most of you will have about 2 
s.f. for
time for the first 2 or 3 time and temp readings.  Then the rest will 
generally
have 3 s.f., maybe 4 for the last reading. In the last reading you 
probably got
something between 15-20 minutes.  This would be 900-1200 seconds.  If you
had recorded this in seconds you would still be accurate only to about  
+ or - 1
second (units place in seconds).   That means your times will have a 
different
number of s.f. and ln(t) will have different numbers of decimal places.  
Remember,
the number of decimal places in the ln(t) will be the same as the number 
of s.f. in
the time.

When you graph, since your ln(t) will generally have 2-4 decimal places 
(mostly 3)
just us 3 decimal places on the ln(t) axis.  You will have to format the 
axis by
right-clicking the axis and then choose "format axis" and choose 
"number" with
3 decimal places.

The s.f. for you temperature values (in kelvin of course) will depend on
how accurately you recorded the temperatures.  You should have recorded
the temps to 1 decimal place in lab but some of you may have recorded your
temps to only the units place.  Use whatever s.f. you got in the lab 
while doing
the exp.  You can't simply add another s.f. to the temperatures to get 
the correct
number of s.f.  Your temps in kelvin will wind up with either 3 or 4 
s.f. (depending
on whether you recorded them to 1 decimal place).  This is the 
addition/subtraction
rule.  You can possibly gain a s.f. in the kelvin temp compared to Celsius.

Since your ln(t) and temp will generally have 3 or 4 s.f. you can safely
report your slope and Ea to 3 s.f.  Even though some of the time values have
only 2 s.f. you can gain one from the graph for the slope. Don't worry about
the s.f. on the graph for the slope if it has more than needed as long 
as you
report it on the report sheet to the correct s.f.  If the slope on the 
graph doesn't
have at least the number of s.f. you need you can format it so it will.  
Right click
on the equation for the line and format the equation (as you would for 
the numbers
in the table to give the correct s.f.). Make sure you have enough s.f. 
for the slope
from the graph in order to get the correct number of s.f. for Ea.

Make sure your graph conforms to the instructions which have been given all
semester.  Some of the graphs for exps 1 and 3 are not even close to 
what they
should have been (more than one to a page, not taking up pretty much the 
whole
page, incorrect s.f., missing units, non-descriptive titles and labels, 
etc.)

As stated in the rubric your sample calculations should include a sample 
calculation
for the slope of the line.  If you use Excel you still need to show a 
sample calc. for
the slope from your line using points from the line (not your data 
points). However,
use the Excel slope to calc. Ea.

Dr. Zellmer
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