1. Uncertainty in measurement

2. Objectives
To understand why repeat experiments do not give the same answer each time
To distinguish between precision and accuracy
To be able to represent a set of results as a histogram and determine how precise they are
Calculate the mean from a set of readings
To calculate the range of a set of reading
To calculate the measure of uncertainty from a set of readings
More problems using moles

3. Whenever experiments are done, the results obtained will be
different each time.
This is partly due to the resolution of the equipment used
What is the resolution of the measuring
cylinder in the picture?

4. However, even if our equipment seems perfect, random error
cannot be avoided. This means that we will get a slightly
different answer every time we do an experiment.
How can we reduce the effect of random error and get
a more accurate answer?

5. If our answer is close to the true value our results are
accurate
If the results of repeats of an experiment are very close together
our results are
precise

6. Two groups measured the volume of hydrochloric acid needed to
react with 25.0cm3 sodium hydroxide.
Volume of HCl (cm3)
Number of groups in this range 3 8 4
Volume of HCl (cm3 )
Number of groups in this range 3 4 1
Draw histograms of these results. Which are more precise?
Which show the greatest uncertainty?

7. Imagine you repeated an experiment and got results of
52, 54, 62, 51, 55
One of these results in anomalous. 62
Calculate the mean of these results 53
Calculate the range of these results
55-51 = 4