SI Units

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Aug 29, 2023
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- Prefixes
- SI Units - A Summary

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The International System of Units (SI) is system of units of measurements that is widely used all over the world. This modern form of the Metric system is based around the number 10 for convenience. A set unit of prefixes have been established and are known as the SI prefixes or the metric prefixes (or units). The prefixes indicate whether the unit is a multiple or a fraction of the base ten. It allows the reduction of zeros of a very small number or a very larger number such as 0.000000001 meter and 7,500,000 Joules into 1 nanometer and 7.5 Megajoules respectively. These SI prefixes also have a set of symbols that precede unit symbol.

However countries such as the United States, Liberia, and Berma have not officially adopted the International System of Units as their primary system of measurements. Since the SI Units are nearly globally though, the scientific and mathematical field will use these SI units in order to provide ease between the sharing data with one another because of a common set of measurements.

Base Units

The SI contains seven BASE UNITS that each represent a different kind of physical quantity. These are commonly used as a convention.

PHYSICAL QUANTITY	NAME OF UNIT	ABBREVIATION
Mass	Kilogram	kg
Length	Meter	m
Time	Second	s
Temperature	Kelvin	K
Amount of Substance	Mole	mol
Electric Current	Ampere	A
Luminous Intensity	Candela	cd

Derived Units

Derived Units are created by mathematical relationships between other Base Units and are expressed in a combination of fundamental and base quantities.

DERIVED QUANTITY	NAME	ABBREVIATION
Area	Square Meter	m²
Volume	Cubic Meter	m³
Mass Density	Kilogram Per Cubic Meter	kg/m³
Specific Volume	Cubic Meter Per Kilogram	m³/kg
Celsius Temperature	degree Celsius	^oC

Prefixes

Metric units use a prefix, used for conversion from or to an SI unit. Below is a chart illustrating how prefixes are labeled in metric measurements.

SYMBOL	PREFIX	MULTIPLICATION FACTOR
T	Tera	10¹²
G	Giga	10⁹
M	Mega	10⁶
k	Kilo	10³
h	Hecto	10²
da	Deka	10¹
d	Deci	10^-1
c	Centi	10^-2
m	Milli	10^-3
µ	Micro	10^-6
n	Nano	10^-9
p	Pico	10^-12

Temperature

Temperature is usually measured in Celsius (although the U.S. still uses Fahrenheit), but is often converted to for the absolute Kelvin scale for many chemistry problems.

For Fahrenheit to Celsius: $F= \dfrac{9}{5} \times C+32 \nonumber$
For Celsius to Fahrenheit: $C= \dfrac{5}{9} \times F - 32 \nonumber$
For Celsius to Kelvin: $K=C+273.15 \nonumber$

Reference Points:

Melting Point of ice is 0°C = 32°F
Boiling Point of water is 100°C = 212°F

The Kelvin scale does not use the degree symbol (°) and only K, which can only be positive since it is an absolute scale

Mass

Mass is usually measured by a sensitive balance machine

1 kilograms = 2.205 lbs.
(Remember that 1 kg = 1000 grams)

Length

The U.S. usually makes measurements in inches and feet, but the SI system prefers meters as the unit for length.

1 meter = 3.281 feet.
1 inches = 2.54 centimeters

Volume

SI units commonly uses derived units for Volume such as meters cubed to liters.

1 cm³ (ce ntimeter cubed) = 1 mL (mililiter)
1000 cm³ = 1 L = 1 dm³

Energy

1 calorie = 4.184 Joules

Amount of Substance

1 mole = 6.022 x 10²³ molecules/atoms
(Avogadro's number)

Problems

Convert to the appropriate SI Units:

1 Day 4 Hours and 20 Minutes
10.8 Lbs.
58.8 Ft.
10,288 grams
128,968,888 mL
1.4 Degrees Celcius
16.13 Cal
18,888,888 km

Answers

#1-4

#5-8

References

Petrucci, Ralph H. General Chemistry. 9th ed. New Jersey: Pearson Prentice Hall, 2005.
Ryamond, Kenneth W. General, Organic, & Biological Chemistry. 2nd Ed. Danvers, MA: John Wiley & Sons Inc., 2008.

Contributors and Attributions

Christina Doan (UCD), Ryan Cheung (UCD)

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