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Graphs
The results of an experiment are often used to plot a graph.
A graph can be used to verify the relation between two variables and, at the same time, give an immediate impression of the precision of the results.
When we plot a graph, the independent variable is plotted on the horizontal axis.
Think of the independent variable as the cause and the dependent variable the effect.

If one variable is directly proportional to another variable, then a graph of these two variables will be a straight line passing through the origin of the axes.
So, for example, Ohm's Law has been verified if a graph of voltage against current (for a metal conductor at constant temperature) is a straight line passing through (0,0).
Similarly, when current flows through a given resistor, the power dissipated is directly proportional to the current squared.
If we wanted to verify this fact we could plot a graph of power (vertical) against current squared (horizontal).
This graph should also be a straight line passing through (0,0).

The "Best Fit" Line

The best-fit line is the (in this case, straight) line which passes as near to as many of the points as possible.

By drawing such a line, we are attempting to minimize the effects of random errors in the measurements.

So, if the points look like these...

then the best fit line should be something like this...

Notice that the best-fit line does not necessarily pass through any of the data points.

In this case I have drawn the line through the origin but even this is not necessarily the case: a best fit line that does not pass through the point (0,0) might be suggesting that there was a systematic error in the experiment.

If systematic error has been ruled out one can perhaps consider the origin to be an especially accurate point.

To Measure the Slope of a Graph
The slope of a graph tells us how a change in one variable affects the value of another variable.
To measure the slope, proceed as follows.

 1 Find out what one small square represents vertically. For example, one small square might represent 0.01volts or 10metres etc. This is the vertical scale. 2 Find out what one small square represents horizontally. For example, one small square might represent 0.002Amps or 0.5seconds etc. This is the horizontal scale. 3 Count the number of small squares, Ny between y1 and y2 (see diagram), and work out the change that this figure represents. vertical change = Ny × (vertical scale) 4 Count the number of small squares, Nx between x1 and x2 (see diagram), and work out the change that this figure represents. horizontal change = Nx × (horizontal scale) 5 Now, the slope of the graph is defined as and must, of course, be stated in the appropriate units

(x1, y1) and (x2 , y2) can be the co-ordinates of any two points on the line but for best precision, they should be as far apart as possible.

In this graph, it is clear that y decreases as x increases so in this case, the slope is negative.

Error Bars

Instead of plotting points on a graph we sometimes draw lines representing the uncertainty in the measurements.

These lines are called error bars and if we plot both vertical and horizontal bars we have what might be called "error rectangles", as shown here.

In the graph shown here:
x was measured to ±0.5s and
y was measured to ±0.3m

Once we have represented the uncertainties on the graph (by drawing the error bars/rectangles) we can see that there must exist a range of possible values for the slope of the graph.

The best-fit line could be any line which passes through all of the rectangles.

If we assume that the line passes through zero, the maximum and minimum slopes of lines which are consistent with these data are about 1.06ms-1 maximum and about 0.92ms-1 minimum.

However, notice that if we have no reason to believe that the line must pass through the origin, the range of possible slopes of best fit line increases greatly.

Measuring the Slope at a Point on a Curved Graph
Often we will plot results which we expect to give us a straight line.
If we plot a graph which we expect to give us a smooth curve, we might want to find the slope of the curve at a given point; for example, the slope of a displacement against time graph tells us the (instantaneous) velocity of the object.

To find the slope at a given point, draw a tangent to the curve at that point and then find the slope of the tangent in the usual way.

This method is illustrated on the graph on the next page.
A tangent to the curve has been drawn at x = 3s.

The slope of the graph at this point is given by
Δy/Δx = (approximately)6ms-1

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