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Uncertainties in Measurement

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What is uncertainty When you are making a measurement, there is always some degree of uncertainty in your result. A person who says they are 1.94 m tall is unlikely to be exactly that height and could be up to half a centimetre taller or smaller. We say their height is 1.940 ± 0.005 m. The uncertainty is ± 0.005 m.

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What causes uncertainty ? There are three causes of uncertainty we need to know about in Higher Physics. Random uncertainty Systematic uncertainty Reading uncertainty

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More about Random Uncertainty This is caused by random and unpredictable changes in the experimental conditions. These are equally likely to make the results of the measurements too big or too small. Repeated measurements can help reduce the effects of random uncertainty.

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More about Systematic Uncertainty Systematic effects are caused by faulty equipment or faulty experimental technique. Systematic effects have occurred when the results of the measurements are all too big or too small. A graph which is a straight line but does not go through the origin as expected is likely to be due to systematic effects.

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More about Reading Uncertainty This is a measure of how well a scale can be read. For an analogue scale, the uncertainty is ± half of the smallest scale division. For a digital scale, the uncertainty is ± 1 in the least significant digit.

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Example of an analogue scale The voltmeter on the left has an analogue scale. The smallest scale division is 1 volt. The uncertainty from reading this scale is ± 0.5 volts.

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Example of a digital scale The voltmeter on the left has a digital scale. The least significant digit is the 8. The uncertainty from reading this scale is ± 0.1 volts.

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Uncertainty in Measurement Professor Bob Kaplan University Department of Science 1.

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