SECTION 2.1 Units of Measurement
Mar 31, 2015
SECTION 2 .1
Units of Measurement
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.
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.
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.
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
SI Units
Quantity Base UnitTime second (s)Length meter (m)Mass kilogram (kg)Temperature kelvin (K)Amount of substance mole (mol)Electric current amphere (A)Luminous intensity candela (cd)
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.
Prefixes to SI Units
Prefix Symbol Factor Scientific Notation
Example
giga G 1000000000109 gigameter
mega M 1000000 10 6 megameter
kilo k 1000 103 kilometer
hecto h 100 102 hectometer
deca da 10 10 1 decameter
none none 0-base unit 100 meter
deci d 1/10 10-1 decimeter
centi c 1/100 10-2 centimeter
milli m 1/1000 10-3 millimeter
micro μ 1/1,000,000 10-6 micrometer
nano n 1/1,000,000,000 10-9 nanometer
pico p 1/1,000,000,000,000 10-12 picometer
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.
MassUnits 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.
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
Volume (m3)
The space occupied by an objectm3 represented by a cube whose sides are all
one meter in length.We will use cm3 for solids with regular
dimensions.Metric unit we are used to is the liter 1 (L) = 1 cubic decimeter (dm3) 1 liter ≈ 1 quartWe will normally use milliliters (mL)
Density (g/cm3 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
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.
Temperature
Water’s boiling pt:212º F, 100º C,
373.15K
Water’s freezing pt:32º F , 0º C, 273.15
K
Dry ice’s freezing pt:-108º F , -78º C, 200
K
Absolute zero:-459º F, -273º C, 0K
Converting between temperature scales
ºF= 9/5 ºC + 32 OR ºC=5/9 (ºF - 32) K= ºC + 273 OR ºC=K - 273
What is the relationship between K and ºF?ºF= 9/5 (K-273) + 32ºF= 9/5 K – 491 + 32 ºF= 9/5 K – 459 OR K=5/9 (ºF + 459)