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"Error" in measurement doesn't usually mean "mistake'.  It means the difference between the value found by measurement and the "true value' of the quantity. eg an object that has a mass of 120 g may be shown to weigh 130 g on an imperfect weighing machine.                 True weight:           120 g                 Measured weight:   130 g                 Error:                     +10 g Measurement errors arise because of inevitable imperfections in the measuring instrument and limitations of the human eye. Errors come in all sizes, and sometimes you need to decide if the error in your measurement is so big that it makes the measurement useless. (see examples below) Errors can be positive or negative. An electric current might be measured as

 

Examples

The effective size of the error depends on the actual size of the error the size of the measurement itself Example 1 Measuring a Line Actual length of line:                     11 cm Length of line when measured:     12 cm Error is (Measured Length - Actual Length) Error is (12 cm - 11 cm)  = 1 cm. The error expressed as a fraction of the actual size is Example 2 Measuring the height of a person Actual height is 1.72 cm = 1270 mm If the error in measurement is only 1 mm, then expressing this as a fraction of the actual size

 

Have a Go Problem 1 Voltage is measured with a multimeter. A particular multimeter is being tested. True voltage of the multimeter:  224 V. Measured voltage:  220 V. Calculate the actual error and the percentage error. You will notice that in this example the error is a negative value Problem 2 Another multimeter is being tested. True voltage of the multimeter:  150 V Measured voltage:  153 V Calculate the actual error and the percentage error. In this case the error has a positive value.

 

Practice Questions Question 1 Answer 1.3 hectares Question 2 answer + or - 0.2%   Question 3 answer + or - 0.2 cm   Question 4 answer + or - 32.2 sec   Question 5 answer + or - 0.2%   Question 6 answer + or - 1.2%

 

 

Solution 1 Actual Error = Measured  Voltage -.True Voltage                     = 220 - 224 V                     = (-) 4 V back to Have a Go

Solution 2 Actual Error = Measured Voltage- True Voltage                    =   153-150 V                    =   (+) 3 V The multimeter is slightly less accurate than the one in the previous problem (This had an accuracy of 1.8%) back to Have a Go

 

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