Units Of Measurement

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Units Of Measurement

• Mrs. Mawhiney

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Measurement of mass, length and volume

• In the United States, we use a fairly awkward
  system of measurement for most things - the
  English system Scientists use the metric and SI
  systems of units for the measurement of physical
  quantities
• This system using standard units based on very
  precisely known properties of matter and light
• Prefixes are used in from of the units to
  indicate powers of ten

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SI Units
Measurement Unit Symbol
Mass Kilogram kg
Length Meter M
Time Second s
Temperature Kelvin K
Quantity Mole mol
Energy Joule J
Pressure Pascal Pa
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. Base Units

• Mass - the quantity of matter that a sample
  contains
• Note that weight is a measure of the attraction
  of gravity for a sample and it varies depending
  on the distance of the mass to a planet or moon
• Scientists often speak imprecisely of the
  weight of an amount of substance. They really
  mean mass.

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Basic SI units/Derived units

• Used to generate new Units
• Volume - space a given quantity of matter
  occupies
• Volume - expressed in terms of length - m3
• m3 - an inconveniently large volume, so we use
  liter (L one cubic decimeter)
• We often use a mL (1 cubic centimeter) for more
  manageable amounts of matter

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Converting between units

• The standard method to convert between two
  different units is the factor-label or
  dimensional analysis method
• Dimensional analysis converts a measurement in
  one unit to another by the use of a conversion
  factor
• Conversion factors are developed from
  relationships between units

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Measurements and Units

• Measurement - determines the quantity,
  dimensions or extent of something
• 1.Consist of two parts
• a. a numerical quantity (1.23)
• b. a specific unit (meters)
• Unit - a definite quantity adapted to as a
  standard of measurement

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Features of Measured Quantities

• When we measure a number, there are physical
  constraints to the measurement
• Instruments and scientists are not perfect, so
  the measurement is not perfect (i. e., it has
  error)
• The error in the measurement is related to the
  accuracy and the precision of the measurement

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Accuracy and Precision

• Accuracy how close the measurement is to the
  true value (of course we have to know what the
  true value is)
• Precision is a measure of how closely
  individual measurements agree with one another.

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Example Accuracy and Precision
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Equations for Precision and Accuracy

• 1. Precision
• 2. Accuracy
• Absolute Error
• AE (True value-Avg Value) X 100
• True Value

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Counting Significant Figures

• Atlantic / Pacific Method
• a. Absent Decimal- Start on atlantic side
  of number cross out all zeroes until 1st
  nonzero digit is reached, remaining digits are
  significant
• b. Present decimal- start on the pacific
  side of the number cross out all zeros until
  the 1st nonzero digit Is reached, remaining
  digits are significant

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How many significant figures are in each of the
following measurements?
24 mL
2 significant figures
3001 g
4 significant figures
0.0320 m3
3 significant figures
6.400 x 104 molecules
4 significant figures
560 kg
2 significant figures
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Scientific notation and significant figures

• 1. When using scientific notation the base
  must be written with the correct number of
  significant digits
• 2. All zeroes are significant when using
  scientific notation