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Why Is The Metric System The Preferred System Of Measurement For Science

l>Lesson I: The Metric System and SI Units

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A meter is 3.37 inches more than a yard.

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A liter is 1.82 fluid ounces more than a quart.
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A dime weighs about one gram.
IntroductionThe metric system is the preferred system of scientific units for several reasons:The majority of countries in the world employ the metric system of measurement.The prefixes attached to metric units carry the same meaning for all base units.The metric system is based upon powers of ten, which is convenient because:A measurement in the metric system that is represented by a rational number remains a rational numberafter metric unit conversion. (For example, 250 mm = 25 cm = .25 m). In contrast irrational unit systems, such as the English system, do not have the same property (For example, 250 inches = 20.8333… ft = 0.0039457… mile)Because metric units are decimal-based, they are easily converted by moving the decimal point.

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The terms mass and weight are often used interchangebly in chemistry. Strictly speaking, weight is the force (F) associated with a given mass (m) as it is accelerated (a) by gravity: F = m &#215 aThe English System unit of mass is the slug, which when multiplied by the acceleration of gravity (32 ft/sec2) gives the weight in pounds.Length
Metric Base UnitsThe metric system uses the following base units:Unit of MeasurementNameAbbreviation
Meter m
Mass Gram g
Volume Liter L

Frequently the above units are too small or more often too large to appropriately scale the measured quantity. It is then necessary to subdivide or expand our measurement unit. This will be discussed in the section on prefixes.

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The liter is not the SI unit of volume Volume is a unit derived from length. The volume (V) of a cube that has length (l) = 1.0 cm, width (w) = 1.0 cm, and height (h) is given by:V = l &#215 w &#215 h = 1.0 cm &#215 1.0 cm &#215 1.0 cm= 1.0 cm3By definition, 1.0 cm3 = 1.0 mL
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It follows from dimensional analysis that 1.0 dm3 = 1.0 L.

SI Units
The metric-based Syst&#232me International or SI units are used to standardize the report or calculation of scientific quantities:

Physical QuantityName of UnitAbbreviationLength
Meter m
Mass Kilogram kg
Temperature Kelvin K
Time Second s
Amount of Substance Mole mol Electric Current Ampere I Luminous Intensity Lumen Iv

The SI units are used to construct all other units (these are called derived units). Some examples:

PropertySymbolDimensionsName
Velocity v m &#183 s-1
Area A m2
Frequency v s-1 Hertz (Hz)
Force F kg &#183 m &#183 s-2 Newton (N)
Energy E kg &#183 m2 &#183 s-2 Joule (J)
Exa-
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Outside the United States, the word “meter” is spelled “metre” and the word “liter” is spelled “litre.”

Metric Prefixes

To change the scale of the base units, prefixes are attached. A prefix represents a factor by which the base unit must be multiplied. Metric prefixes are listed below (The prefixes most-commonly used in chemistry are listed in red):PrefixSymbolDecimalValuePower of Ten

E 1,000,000,000,000,000,000 1018
Peta- P 1,000,000,000,000,000 1015
Tera- T 1,000,000,000,000 1012
Giga- G 1,000,000,000 109 Mega- M 1,000,000 106 Kilo- k 1,000 103 Hecto- h 100 102 Deka- da 10 101 (no prefix) 1 100 Deci- d 0 .1 10-1 Centi- c 0 .01 10-2 Milli- m 0 .001 10-3 Micro- &#181 0 .000001 10-6 Nano- n 0 .000000001 10-9 Pico- p 0 .000000000001 10-12 Femto- f 0 .000000000000001 10-15 Atto- a 0 .000000000000000001 10-18
In addition to the base metric units, many other scientific quantities also employ this system of prefixes:

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1 Mb = 1 Megabyte = 1 &#215 106 byte
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1 Food Calorie = 1 kcal= 1 kilocalorie = 1 &#215 103 calorie
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1 ns = 1 nanosecond = 1 &#215 10-9 second
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1 pf = 1 picofarad = 1 &#215 10-12 farad
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1 kWH = 1 kiloWatt&#183hour = 1 &#215 103 Watt&#183hour
The metric prefixes can be employed to scale the base units so that they can represent anything from a very large numeric value (for example by using prefixes such as Exa, Tera, or Mega) to a very small numeric value (for exampleby using prefixes such as Atto, Femto, or Pico). Scaling the unit up or down when reporting measurements is good practice because:It can be used to keep the numeric value of measurements to reasonably-sized numbers, say between values of 0.01 to 100.It can be used to remove ambiguous place-holding zeroes in measurments, so that the number of significant figures are properly represented.The prefixes instantly convey how many decimal place moves are required of the numeric value. For example, using the scientific notation value of the metric prefixes):1 cm = 1 centimeter = 1 &#215 10-2 meter = 0.01 meter1 kg = 1 kilogram = 1 &#215 103 gram = 1000 gram1 &#181L= 1 microliter = 1 &#215 10-6 liter = 0.000001 literFor further explanation of unit interconversion using the metric prefixes as conversion factors, visit the dimensional analysis page. In addition to the standard metric prefixes, the &#197ngstrom is also a commonly used unit that arises in measurements on the atomic scale:1 &#197ngstrom = 1 &#197 = 1 &#215 10-10 meter
| Metric Units Exercises | Main Lesson Page | Email Dr. Parkinson |

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