Measurements: Precision, Accuracy and Uncertainty.

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Presentation transcript:

Measurements: Precision, Accuracy and Uncertainty

Recording Measurements Always record 1 more digit than the smallest division of the instrument This last digit is an estimate so it is said to be uncertain. The ones before that are certain. All measurements have a degree of uncertainty

Measurement of Distance Top ruler Smallest unit is 0.1 so it is 2.5something Guess one more number (the “something”) Could be 2.54 or 2.55 etc…

Measurement of Distance Bottom ruler Smallest unit is 1 so we know it is 2.something Guess one more number (the “something) Could be 2.4 or 2.5, etc…

Practice

Measurement of Liquids Measure from the bottom of the meniscus

Measurement of Liquids Mercury (Hg) has a convex meniscus (B), for which you would measure from the top Concave/convex determined by surface tension between the liquid and the container

Convex Meniscus of Mercury

Practice

More Practice Hebden page 32 #48 Hebden page 34 #50

Uncertainty All measurements have a degree of uncertainty E.g. 3.14±0.03 – I’m sure the measurement lies between 3.11 ( ) and 3.17 ( ) Uncertainty must have the same decimal place as the measurement – Why?

Expressing Uncertainty Use your own judgment I will be flexible in marking Just make sure the decimal places are correct and your value is reasonable

Practice Add uncertainty to the previous questions Hebden page 35 #51 & 52

Precision & Accuracy Precision measures the agreement between results of repeated measurements – i.e. how close measurements are to each other – More decimals = more precision! Accuracy measures the agreement between a measurement and the accepted value – i.e. how close a measurement is to the accepted value (e.g. the speed of light is 2.998x10 8 m/s)

Precision & Accuracy