Uncertainty in measurement

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Presentation transcript:

Uncertainty in measurement

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

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?

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?

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

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 12.0-12.9 13.0-13.9 3 14.0-14.9 8 15.0-15.9 4 16.0-16.9 Volume of HCl (cm3 ) Number of groups in this range 12.0-12.9 3 13.0-13.9 4 14.0-14.9 15.0-15.9 1 16.0-16.9 Draw histograms of these results. Which are more precise? Which show the greatest uncertainty?

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