Determining Uncertainties from Graphs (AQA A Level Physics)

Using Error Bars

• The uncertainty in a measurement can be shown on a graph as an error bar

• This bar is drawn above and below the point (or from side to side) and shows the uncertainty in that measurement

• Error bars are plotted on graphs to show the absolute uncertainty of values plotted

• Usually, error bars will be in the vertical direction, for y-values, but can also be plotted horizontally, for x-values

Representing error bars on a graph

Determining Uncertainties from Graphs

• To calculate the uncertainty in a gradient, two lines of best fit should be drawn on the graph:

• The ‘best’ line of best fit, which passes as close to the points as possible

• The ‘worst’ line of best fit, either the steepest possible or the shallowest possible line which fits within all the error bars

The line of best fit passes as close as possible to all the points. The steepest and shallowest lines are known as the worst fit

• The percentage uncertainty in the gradient can be found using:

• The percentage uncertainty in the y-intercept can be found using: