The use of graphs

In experimental physics, graphs have three main uses. The first is to determine the value of some quantity, usually the slope or the intercept of a straight line representing the relation between two variables. Although this use of graphs is often stressed in elementary teaching of practical physics, it is in fact a comparatively minor one. Whether we obtain the value of the slope of a straight line by the method of least squares or by taking the points in pairs (see chapter 4), we are of course not using the graph as such, but the original numbers. The only time we actually use the graph to determine the slope is when we judge or guess the best line through the points by eye. This is a crude method – though not to be despised on that account – and should only be used as a check on the result of a more sophisticated method, or when the value of the slope is not an important quantity in the final result.

The second use of graphs is by far the most important. They serve as visual aids. Suppose, for example, the rate of flow of water through a tube is measured as a function of the pressure gradient, with the object of determining when the flow ceases to be streamlined and becomes turbulent.