A –Level Physics: Accuracy + Uncertainty

Objectives: Independent thinking To identify the causes and sources of random and systematic errors To distinguish the words accuracy and precision from one another To plan effective and detailed experiments To explain how errors can be reduced To take measurements and consider the uncertainty of the apparatus Independent thinking Application of mathematical concepts in a practical context Use of instruments and equipment

Accuracy and precision An accurate value is judged to be close to the true value. Accuracy cannot be quantified and is influenced by both systemic and random errors Precision is the consistency between values obtained by repeated measurements. They are ‘precise’ if clustered together. It’s influenced by only random errors Random errors These are errors that are due to the experimenter. The size of these errors depends on how well the experimenter can use the apparatus. Systematic errors These are errors which are due to the apparatus, and may result from either faulty apparatus, badly calibrated apparatus or a zero error.

This one has unclustered results and they’re not accurate! Random and Systemic Errors: ….What they look like in graphs These one are nicely clustered as the same instrument was made throughout. Not accurate These results are not precise as there was error in reading the instrument These results are precise and accurate as the instrument was read well and correctly calibrated This one has unclustered results and they’re not accurate!

Significant Figures and Decimal Places: because they’re very different… https://www.youtube.com/watch?v=te89vNO5fhk As you watch the lovely lady go through sig figs and decimal places, make notes. Then pick some numbers and get your neighbour to change them into 3sig figs , 2 decimal places etc… 10mins

Completing Practical Experiments: A how-to guide Identify the variables that you wish to measure. Identify and control other variable to give a fair test. Decide on suitable apparatus to measure the chosen variables. Consider any safety aspects of the experiment. Decide how you will collect the data – manually or electronically. Carry out the experiment. Present your date in a table if appropriate. Plot a suitable graph if required. Repeat readings if necessary. State a clear conclusion. Identify sources of error and suggest how they may be minimised. ..

Presentation of graphs Correct title given Points plotted correctly and clearly Best fit curve drawn Gradient calculated accurately Anomalous point recognized and checked Axes labelled with quantity and units © Hodder & Stoughton 2015

The two extremes of gradient are shown in the example. Uncertainties in graphs For example: In this we have an uncertainty for quantity of value A as +/-1, as such the error bars indicate this Quantity A Quantity B 1 3 4 5 2 7 6 10 9 8 35 30 40 45 x m2 m m1 When drawing graphs error bars are used to show the range of uncertainty of a given reading. The final line should pass though all these. The two extremes of gradient are shown in the example.

Tasks Go through the pages again, this time making extensive notes Read carefully through pages 10-23 of the text book. It takes you through a single experiment and how it would be undertaken accurately and precisely Sit at one of the tables set up with 2p pieces and micrometers (6 people max per table), and work through the sheets provided slowly and methodically. This will help you practice what you have read Go through the pages again, this time making extensive notes Include: Working out of the ‘example’ questions Taking measurements and uncertainties Resolutions Percentage uncertainty Percentage difference 7.5ml is 7.5cm3, therefore is 7.5x10-6m3, as density is mass/volume, density works out at 8000kgm-3.