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SECTION 2.1 Units of Measurement

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Objective 1. Define SI base units for time, length, mass, and temperature. 2. Explain how adding a prefix changes a unit. 3. Compare the derived units for volume and density.

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S. I. Units We measure things everyday and don’t even notice it. We can use approximations, but scientists need a more exact way. They need to report data that others can reproduce. They need a standard set of units for their measurements.

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S. I. Units Continued… In 1795, the French scientists adopted the metric system. In 1960, an international committee of scientists met up to update the metric system and called it Systeme Internationale d’ Unites, SI.

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Base Units Base Unit: A system of measurement based on an object or event on the physical world. It is independent of other units. There are 7 base units in the SI

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SI Units QuantityBase Unit Timesecond (s) Lengthmeter (m) Masskilogram (kg) Temperaturekelvin (K) Amount of substancemole (mol) Electric currentamphere (A) Luminous intensitycandela (cd)

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Time Units is in Seconds (s) The frequency of microwave radiation given off by a cesium- 133 atom. Many chemical reactions occur in less than a second therefore scientific notation is important.

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Prefixes to SI Units PrefixSymbolFactorScientific Notation Example gigaG gigameter megaM megameter kilok kilometer hectoh hectometer decada decameter none 0-base unit 10 0 meter decid 1/ decimeter centic1/ centimeter millim1/ millimeter microμ1/1,000, micrometer nanon 1/1,000,000, nanometer picop 1/1,000,000,000, picometer

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Length Units are in meters (m) The distance light travels through a vacuum in 1/299,792,458 of a second. Vacuum: a space containing no matter. Close in length to a yard.

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Mass Units are in kilograms (kg) Mass is a measure of the amount of matter. Kg defined by the platinum- iridium metal cylinder kept in France. 1 kg≈2.2 lbs. We use grams (g) or milligrams (mg) most of the time in our experiments.

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Derived Units A unit that is defined by a combination of base units Multiple units to make up the derived units. Examples Speed=meters/second Volume=meters*meters*meters Density=mass/volume

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Volume (m 3 ) The space occupied by an object m 3 represented by a cube whose sides are all one meter in length. We will use cm 3 for solids with regular dimensions. Metric unit we are used to is the liter 1 (L) = 1 cubic decimeter (dm 3 ) 1 liter ≈ 1 quart We will normally use milliliters (mL)

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Density (g/cm 3 or g/mL) A ratio that compares the mass of an object to its volume. Property used to identify an unknown sample D= M/V

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Temperature Hot and cold are relative terms and only refer to qualitative data. Thermometers allow temperature to be measure quantitatively. 3 major temperature scales Fahrenheit (1724) German physicist-Daniel Gabriel Fahrenheit Used mainly only in US Celsius (1742) Swedish Astronomer Anders Celsius Part of the SI system Kelvin (1894) Lord William Kelvin Part of the SI system Introduced absolute zero concept-no negative temp-nothing colder than 0 K.

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Temperature Water’s boiling pt: 212º F, 100º C, K Water’s freezing pt: 32º F, 0º C, K Dry ice’s freezing pt: -108º F, -78º C, 200 K Absolute zero: -459º F, -273º C, 0K

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Converting between temperature scales ºF= 9/5 ºC + 32 OR ºC=5/9 (ºF - 32) K= ºC OR ºC=K What is the relationship between K and ºF? ºF= 9/5 (K-273) + 32 ºF= 9/5 K – ºF= 9/5 K – 459 OR K=5/9 (ºF + 459)

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