Units of Measurement

# Units of Measurement

### System International (SI) units

#### Length: based on the meter (length from your nose to your hand).

• 1 kilometer = 1 km = 103 m.
• 1 meter = 1 m.
• 1 centimeter = 1 cm = 10-2 m.
• 1 millimeter = 1 mm = 10-3 m.
• 1 micrometer = 10-6 m. (abbreviation: Greek mu + m)
• 1 nanometer = 1 nm = 10-9 m.

#### Temperature: based on the Kelvin

• 0 K is "absolute zero."
• water freezes at 273 K.
• 300 K is room temperature.
• water boils at 373 K.
• cakes bake at 450 K.

#### Frequency:

Frequency is number of complete waves passing per unit time. It is measured in Hertz (Hz), the number per second.
• 1 KHz = 103 Hz.
• 1 MHz = 106 Hz.
• For sound, "A" is 440 Hz.

#### Energy and Power:

Energy is measured in Joules, J. The rate at which energy is transfered ("power") is measured in Joules per second. One Joule per second is called a Watt, W.
• a lightbulb typically consumes 100 W of power.
• a typical person consumes about 1000 W = 1 KW for heating, cooking, etc .
• The amount of energy received at the Earth per unit time per unit area from the Sun is about 1 KW/m2.

### Non-System-International (SI) units

#### Energy:

The electron volt eV is commonly used to measure energy of atoms and molecules, or of individual photons. Of course, an electron volt is a ridiculously small unit for measuring macroscopic amounts of energy.

#### Angles:

• A circle is divided into 360 degrees.
• A degree is divided into 60 arc minutes.
• An arc minute is divided into 60 arc seconds.

#### Distance:

• The Earth-Sun distance is the astronomical unit (AU): 1 AU = 150 x 106 km.
• Astronomers like to use parsecs (pc): 1 pc = 3.09 x 1013 km.
• Sometimes you see distances in light-years (ly)
• 1 ly = the distance light travels in one year.
• Light travels at a speed c = 3 x 108 m/s.
• One year is about 3 x 107 s.
60 x 60 x 24 x 365.25 = 31557600.
• So 1 ly = 3 x 108 m/s x 3 x 107 s ~ 1016 m.
• The relation to parsecs is 1 ly ~ 0.3 pc.

Updated 26 September 2007

Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA