Sunday, September 13, 2009

Chem and Phy Sci- Graphing

Lee Marek
1. Use graph paper (home-made graph paper is unacceptable) ink or pencil. Better yet do it on Excel, other spread sheet programs or some graphing program.

2. Plan ahead! Note the largest value number to be plotted on each axis and make sure your scale is large enough so that you use up at least half of the paper in both directions.

3. Label the axis and give units to those labels in parenthesis: LABEL (UNITS) as in distance (cm).

4. All graphs should have a title. A good title that always works is "y" as a function of "x" as in (A). The independent variable is usually plotted on the horizontal (x) axis. "Distance as a Function of Time" is a good title for graph (B).

5. Most graphs should start at the origin (x = 0, y = 0). There are exceptions like graphing temp. If your lowest temp. is 51o C start at 50o C. This is because 0o C has no special meaning (0 cm means no distance, 0o C doesn't mean no temperature).

6. Pick a logical scale, counting by .1, 1, 2, 5, or 10 etc., not 3, 5, 7, 9, etc.

7. Put a small circle around each data point lightly in pencil. This is necessary due to the uncertainty in the data. Excel will do this for you.

8. Look at your points. Draw the best straight line or smooth curve that goes through as many points as possible. Point to point connections are usually not used in science but are quite valid for plotting irregular data which does NOT display any regularity - like gold prices. Try to miss as many points above the line as you draw as below. Excel will do this for you, as will many graphing programs.

9. If 2 or more lines are plotted on a graph a key is needed. It should be placed on the right side and toward the top if possible. A different color ink must be used for each line. Again Excel will do this for you.

10. If you have a straight line though the origin you have a direct proportion (A). When one variable goes gets larger so does the other. In graph (B) you have an inverse relationship. When one value becomes larger (say pressure) the other (volume) gets smaller.

Here is a quick check list:


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