Maths for Chemists
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Equation of a straight line graph
Getting on the straight, if not the narrow!
In applied science when it often helps to try to resolve a numerical solution to a straight line graph.
There are many reasons for this. One is very practical: if you have a set of experimental results and you know they are related by a straight line graph, it is relatively easy to choose the best straight line to draw. Curves are much more difficult!
There are several ways of describing a straight line graph by means of an equation. In A-level Chemistry we only need one of them. This is usually written:
y = mx + c
Explain, please
First of all, let's be clear about what the various terms mean in the equation: y = mx + c.
• 'y' means 'whatever you decide to plot on the y axis' (the vertical one).
• 'x' means 'whatever you decide to plot on the x axis'.
• 'c' means the reading on the y axis at the point where the graph crosses it. (i.e. the value of y when x = 0). For a given set of values of x and y, c has a constant value.
• 'm' is the crucial one as far as we are concerned. 'm' is the gradient of the graph.
If m is so important, how can you find it?
You can find the value of m by measuring a change in y and dividing it by the corresponding change in the value of x. The steeper the graph is, the bigger is the value of m.
The value of m is positive when the straight line rises from left to right. It is negative if the line falls from left to right. Assuming that the axis and scales are drawn up in the usual way.
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updated: 31 January 2007
