Plot and look Measuring Uncertainty. Complete the table QuantityUsual symbolUsual unit Power Pressure Frequency Electrical charge Stress Strain Young.

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Presentation transcript:

Plot and look Measuring Uncertainty

Complete the table QuantityUsual symbolUsual unit Power Pressure Frequency Electrical charge Stress Strain Young modulus Spring constant Celsius temperature

Complete the table QuantityUsual symbolUsual unit PowerPW PressurepPa FrequencyfHz Electrical chargeQC StressσNm -2 StrainεFraction or % Young modulusEPa Spring constantkNm -1 Celsius temperature θ oCoC

Learning objectives Be able to present data warts and all Be able to round off significant figures sensibly Be able to use a histogram of frequency of occurrence of a single reading repeated many times Be able to make judgements about data Be able to consider the uncertainty of experimental work

What is uncertainty? The uncertainty of an experimental result is the range of values within which the true value may reasonably be believed to lie

Steps in uncertainty 1.Human error 2.Systematic error 3.Resolution 4.Random error 5.Combining errors of resolution and random error

Measuring breaking stress of cotton We are going to look at an example of dealing with measuring a quantity that varies form sample to sample. The stress needed to break a material does not have an exact single value. This is because one sample will not be identically the same as another.

Plot and look Set up as shown. Length of cotton 10cm Pull the force meter until the cotton snaps Increase the tension 0.5N at a time Record the breaking force Repeat as often as possible in 15 mins. cotton Force meter

Gather every-ones data onto one table Plot a frequency histogram Calculate the mean and estimate the range (spread) of your data for the breaking force. Convert the range to a ±% of the mean value NOW THINK: how is the variation you see accounted for? Is the uncertainty in the force measurement enough to explain it? Or is it the samples or the material.

Measure the cross sectional area of your sample and then find the breaking stress How uncertain are you about the cross- sectional area. Decide where the greatest % variation comes from. Now make a claim about the breaking stress and its range for your material