Uncertainties in Measurement

1. Uncertainties in Measurement

2. 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.

3. What causes uncertainty ? • There are three causes of uncertainty we need to know about in Higher Physics. • Random uncertainty • Systematic uncertainty • Reading uncertainty

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.