In this tutorial on graphing, we will examine data taken from an experiment in which the circumferences and radii of several circular objects were measured. The data is displayed in the screen shot to the right. For more information on formatting the data and displaying the text see the previous tutorials. Of course, the equation associated with this data is C = 2pr, or the circumference of a circle is equal to two times pi times the circle's radius. In this experiment, the circumferences and radii are measured. We hope to be able to determine the value for p, which, to six significant figures, is given as 3.14159. It is my firm belief (although not necessarily the belief of all at this University) that beginning laboratory students should plot their data by hand rather than use a computer application to perform this task. However, at this time, some of our lab courses permit computer graphing. Here we show how to use Excel to plot the data. To do so, follow the steps below. It may appear that this is a difficult process, but it is rather straightforward and simple.
Once the graph has been created, take a minute or two to make put the finishing touches on it. For instance, you should always place units under your axis headings. To do this, simply click on the heading, place the cursor at the end of the heading, hit the <ENTER> key and type the units in parentheses. I always delete the legend box. With just one set of data on the graph, the legend is not useful. To delete it, simply click on it and press the <DELETE> key. It is attractive to add appropriate symbols in the title. For instance with our graph, I will change the "pi" in our title to "p". For a reminder of how this is done, see the Displaying Symbols tutorial. Note that it is also possible to change the font style and size of the titles and headings. You should always add a trendline to the graph. That is, make the computer draw the bestfit line to the data. You should also display the equation and the Rsquared value on the graph. Since the relationship between the circumference and the radius is linear, we can expect the plotted data to form a straight line in the form of y = mx + b. To add the trend line, click anywhere on the graph and then click on Chart >> Add Trendline from the menu bar. Since we expect the fit to be linear, select linear fit. It is also possible with Excel to add multiple trendlines to one set of data. For information on that technique see my tutorial on fitting multiple curves on one set of data. To display the equation and Rsquared value on the graph, click on the Options tab. Then place check marks in the appropriate boxes. When the OK button is pressed the best fit line is drawn and the equation of the line and Rsquared value will be displayed on the graph. It will look something like the screen shot to the right. You may move the equation by clicking and dragging it to the desired location. The Rsquared value is actually the square of the correlation coefficient. The correlation coefficient, R, gives us a measure of the reliability of the linear relationship between the x and y values. A value of R = 1 indicates an exact linear relationship between x and y. Values of R close to 1 indicate excellent linear reliability. If the correlation coefficient is relatively far away from 1, the predictions based on the linear relationship, y = mx + b, will be less reliable. For more information about this topic, see the linear regression tutorial. You should find it odd that the equation displayed on the graph is y = 6.179x + 0.2327. After all, we did not measure y's and x's, but rather we measured circumferences (C's) and radii (r's). You should always change the displayed equation to match your measured variables! To change the equation, simply click on the equation and change the variables. The screen shot to the right shows how we made our equation more representative of the experiment. Recall that the governing equation of this experiment is C = 2pr. By doing this step, you are in essence telling your TA that you really do understand what was actually measured and how well the experiment matched the theory. A nice touch to your graph is to decrease the thickness of the bestfit line. The default size is rather thick and often hides the actual data points. To make the line thinner, doubleclick on the trendline and then change its weight to a thinner line. The final result of your efforts is a graph that looks something like the following:
Simply making the graph is not all that is required of the physics student. The real job of the physics student is to determine what physics principles (if any) were verified by the laboratory experiment. You must constantly ask yourself: "What physics principle was this experiment designed to show?" and "Did the experiment actually verify the theory?" In this example experiment, we hoped to show that if we plotted the circumferences of several circles versus their radii, the slope of the resulting graph should equal 2p. We have a very nice graph, but we have not determined if the experiment actually verified the formula C = 2pr! You should expect by now that we can use Excel to compare the experimental slope to the theoretical slope. Another way of stating this is what is our experimental value of p? The screen shot to the right shows how we used Excel to do this. Our slope was determined to be 6.179. (No units, right?!) The theoretical value of the slope is equal to 2p, or p = slope/2. The formula in cell E3 (=E1/2) gives the experimental value of p to be 3.09. The formula in cell E4 gives the percent error between the actual and experimental values. As you can see, an error of only 1.66% indicates that the student performed this lab very carefully! Note that in our percent error formula (cell E4 above) we did not explicitly multiply the result by 100%. Instead, we simply calculated the fraction and then clicked on the Percent Style button, . So here is what the finished worksheet might look like:
Once again, ask your TA if your graphs should be on separate pages or included with the data table as shown above. Once again, make sure that when you print your worksheet you print the gridlines and row and column headings. Review how to do this by visiting the Basic Actions tutorial, section 9. If your worksheets look like this one, you are guaranteed an A for the course!


Copyright © 2000, Clemson University. All Rights Reserved.
