Graphical Skills

On their own numbers do not mean a lot.  A table of numbers can be confusing.  A graph allows us to see a picture of how the numbers relate to each other.

1. Always use a sharp pencil and a ruler.
2. Draw the axes
3. Label the axes with the quantity and the units
4. When you plot Quantity 1 against Quantity 2, you put Quantity 2 on the horizontal axis.
5. Look for the highest value in each range.  You calibrate (put numbers on) your axes to the nearest convenient step above your highest value.
6. Use a sensible scale.
7. Plot your points with crosses (+ or ×).  Points get lost.
8. Join your points with a line, but not dot-to-dot!

It can be difficult to decide whether a set of results is a straight line or a curve.  If it’s clearly a straight line, draw your line of best fit with a ruler.  If the graph is a curve, then try to make a smooth curve.  A flexi-curve can help you with this.

If a point is way out from the rest, then it’s probably an anomalous result.  If you can, recheck the data or do that part of the experiment again.  If not, ignore it.

The table below shows some data to plot:

This graph is nonsense.  Can you see why?  Although graphs drawn like this are quite useless, they are depressingly common.

  The correct graph is shown below:

Notice:

• Axes labelled with quantities and units;

• Scales are sensible;

• Line of best fit drawn through the points.  No dot-to-dot.

When we read a point off the graph within a range, we are interpolating.  When we extrapolate a graph, we are extending it beyond the plotted range, making a reasoned guess as to where the line is going to go.

In the Exam:

It is important that you understand that the exam not only tests knowledge with understanding, but also tests a range of other skills.  One of these is to be able to interpret and transfer data from a variety of sources such as tables and graphs.  Therefore graphical skills are essential for success and are widely used to decide whether a candidate gets a grade A or B, or at the other end, an E or U.

  Therefore you will need to get a lot of practice with graphs!

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