# Measurements, SI Units and Significant Figures

The International System of Units is the most widely used system of measurements and considered to be the standard system for science and industry. There are seven base units which can be found in the chart below.

 Dimension Unit Unit Symbol Formula Symbol Length Metre m d, r ,s, l Time Second s t Mass Kilogram kg m Temperature Kelvin K T Amount of Substance Mole mol n Electric Current Ampere A I Luminous Intensity Candela cd IL

Just as there are many different units of measurement, there are also many different tools of measurement. Some examples of these different tools include metre sticks, balances, scales, tape measures, thermometers and voltmeters. Some tools are very accurate and some are not. Scientists are always concerned with being both accurate and precise in their measurements. Accuracy and precision are different from each other and both very important.

Accuracy is a measure of how close a measurement is to the true value of the quantity being measured.

Precision is a measure of how close a series of measurements are to one another.

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# Significant Figures

Significant figures are the digits in a number that carry actual meaning in the precision of that number.  Significant figures are important when writing and interpreting numbers because the amount of significant figures will indicate the precision of a number.

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# When writing or interpreting numbers you must keep in mind the following rules:

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## 18

both the 1 and the 8 are significant in this number so this number has 2 significant figures

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# The rest of the rules deal with zeros…

## 1008

both of the zeros in the middle of this number are significant so this number has 4 significant figures

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## 0.00108

the three zeros at the beginning of this number are not significant. Those three zeros are simply holding the place of the decimal so this number has 3 significant figures

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## 1.0800

The zeros at the end of this number (the trailing zeros) are significant because there is a decimal in the number so this number has 5 significant figures

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## 10800

The zeros at the end of this number (the trailing zeros) are not significant because there is not a decimal place in the number so this number has 3 significant figures

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