Class 7.2: Graphical Analysis and Excel
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Transcript of Class 7.2: Graphical Analysis and Excel
Class 7.2: Graphical Analysis and Excel
Solving Problems UsingGraphical Analysis
Learning Objectives Learn to use tables and graphs as
problem solving tools Learn and apply different types of graphs
and scales Prepare graphs in Excel Be able to edit graphs Be able to plot data on log scale Be able to determine the best-fit
equations for linear, exponential and power functions
Exercise Enter the following table in Excel
You can make your tables look nice by formatting text and borders
Independent Variable, x
Dependent Variable, y1
Dependent Variable, y2
1 1 1
2500 10 50 5000 100 100
7500 1000 150
10000 10000 200
Axis Formats (Scales) There are three common axis formats:
Rectilinear: Two linear axes Semi-log: one log axis Log-log: two log axes
Length (km)
1 km 10 km
Log scale:
1 km
Length (km)
Linear scale:
Use of Logarithmic Scales A logarithmic scale is normally
used to plot numbers that span many orders of magnitude
1 10 100 1000 10000
Creating Log Scales in Excel
Exercise (2 min): Create a graph using x and y1 only.
New Graph
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Creating Log Scales in Excel Now modify the graph so the data
is plotted as semi-log y This means that the y-axis is log scale
and the x-axis is linear. Right click on the axis to be
modified and select “format axis”
Creating Log Scales in Excel
On the Scale tab, select logarithmic
“OK” Next, go to Chart
Options and select the Gridlines tab. Turn on (check) the Minor gridlines for the y-axis.
“OK”
Result: Graph is straight line.
New Graph
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Exercise (8 min) Copy and Paste the graph twice. Modify one of the new graphs to be
semi-log x Modify the other new graph to be
log-log Note how the scale affects the
shape of the curve.
Result:semi-log xNew Graph
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Result: log-log
New Graph
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Equations The equation that represents a straight line
on each type of scale is: Linear (rectilinear): y = mx + b Exponential (semi-log): y = bemx or y = b10mx
Power (log-log): y = bxm
The values of m and b can be determined if the coordinates of 2 points on THE BEST-FIT LINE are known: Insert the values of x and y for each point in the
equation (2 equations) Solve for m and b (2 unknowns)
Equations (CAUTION) The values of m and b can be
determined if the coordinates of 2 points on THE BEST-FIT LINE are known.
You must select the points FROM THE LINE to compute m and b. In general, this will not be a data point from the data set. The exception - if the data point lies on the best-fit line.
Consider the data set:X Y1 42 83 104 125 116 167 188 199 20
10 24
Team Exercise (3 minutes) Using only the data from the table,
determine the equation of the line that best fits the data.
When your team has completed this exercise, have one member write it on the board.
How well do the equations agree from each team?
Could you obtain a better “fit” if the data were graphed?
Which data points should be used to determine the equation of this best-fit line?
Example of Best-Fit Line
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X, Independent Variable (No Units)
Y, D
ep
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Which data points should be used to determine the equation of this best-fit line?
Example of Best-Fit Line
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X, Independent Variable (No Units)
Y, D
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Comparing Results How does this equation compare
with those written on the board (i.e- computed without graphing) ?
CONCLUSION: NEVER try to fit a curve (line) to data without graphing or using a mathematical solution ( i.e – regression)
What about semi-log graphs? Remember, straight lines on semi-
log graphs are EXPONENTIAL functions.
mx)bln()yln(
)eln(*mx)bln()yln(
)eln()bln()yln(
)e*bln()yln(
e*by
mx
mx
mx
What about log-log graphs? Remember, straight lines on log-
log graphs are POWER functions.
)xln(*m)bln()yln(
)xln()bln()yln(
)x*bln()yln(
x*by
m
m
m
Example Points (0.1, 2) and (6, 20) are taken from
a straight line on a rectilinear graph. Find the equation of the line, that is use
these two points to solve for m and b. Solution:
2 = m(0.1) + b a)20 = m(6) + b b)
Solving a) & b) simultaneously:m = 3.05, b = 1.69
Thus: y = 3.05x + 1.69
Pairs Exercise (10 min)
FRONT PAIR: Points (0.1, 2) and (6, 20) are taken from a straight
line on a log-log graph. Find the equation of the line, ie - solve for m and b.
BACK PAIR: Points (0.1, 2) and (6, 20) are taken from a straight
line on a semi-log graph. Find the equation(s) of the line, ie - solve for m and
b.
Interpolation Interpolation is the process of estimating a
value for a point that lies on a curve between known data points Linear interpolation assumes a straight line
between the known data points One Method:
Select the two points with known coordinates Determine the equation of the line that passes
through the two points Insert the X value of the desired point in the
equation and calculate the Y value
Individual Exercise (5 min) Given the following set of points,
find y2 using linear interpolation.(x1,y1) = (1,18)(x2,y2) = (2.4,y2)(x3,y3) = (4,35)
Assignment #13 DUE: TEAM ASSIGNMENT See Handout