Uncertainties in Measurement

What is uncertainty

• When you are making a measurement, there is always some degree of uncertainty in your result.

• A person who says they are 1.94 m tall is unlikely to be exactly that height and could be up to half a centimetre taller or smaller.

• We say their height is 1.940 ± 0.005 m.

• The uncertainty is ± 0.005 m.

What causes uncertainty ?

• There are three causes of uncertainty we need to know about in Higher Physics.

• Random uncertainty

• Systematic uncertainty

• Reading uncertainty

More about Random Uncertainty

• This is caused by random and unpredictable changes in the experimental conditions.

• These are equally likely to make the results of the measurements too big or too small.

• Repeated measurements can help reduce the effects of random uncertainty.

More about Systematic Uncertainty

• Systematic effects are caused by faulty equipment or faulty experimental technique.

• Systematic effects have occurred when the results of the measurements are all too big or too small.

• A graph which is a straight line but does not go through the origin as expected is likely to be due to systematic effects.

More about Reading Uncertainty

• This is a measure of how well a scale can be read.

• For an analogue scale, the uncertainty is ± half of the smallest scale division.

• For a digital scale, the uncertainty is ± 1 in the least significant digit.

Example of an analogue scale

The voltmeter on the left has an analogue scale. The smallest scale division is 1 volt. The uncertainty from reading this scale is ± 0.5 volts.

Example of a digital scale

The voltmeter on the left has a digital scale. The least significant digit is the 8. The uncertainty from reading this scale is ± 0.1 volts.