Units Of Measurement Mrs. Mawhiney

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

SI Units MeasurementUnitSymbol MassKilogramkg LengthMeterM TimeSeconds TemperatureKelvinK QuantityMolemol EnergyJouleJ PressurePascalPa

. 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.

Basic SI units/Derived units Used to generate new Units Volume - space a given quantity of matter occupies Volume - expressed in terms of length - m 3 m 3 - 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

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

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

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

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.

Example: Accuracy and Precision

Equations for Precision and Accuracy 1. Precision 2. Accuracy Absolute Error % AE = (True value-Avg Value) X 100 True Value

Counting Significant Figures Atlantic / Pacific Method a. Absent Decimal- Start on “atlantic” side of number & cross out all zeroes until 1 st 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 1 st nonzero digit Is reached, remaining digits are significant

How many significant figures are in each of the following measurements? 24 mL2 significant figures 3001 g 4 significant figures m 3 3 significant figures x 10 4 molecules 4 significant figures 560 kg2 significant figures

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