Fundamental Units of Measurement
by Ron Kurtus
You measure things by defining a standard unit and then stating the measurement in terms of multiples of that unit. A fundamental unit of measurement is a defined unit that cannot be described as a function of other units.
The International System of Units (SI) defines seven fundamental units of measurement. They can be applied to the various Physical Science areas of study. However, there are some questions about the definitions.
Distance, time and mass are the fundamental units.
- What is a unit of measurement?
- What are the seven fundamental units?
- what are some problem areas with these definitions?
This lesson will answer those questions. Useful tool: Units Conversion
Measuring in units
In order to measure something, you need to define a unit of measurement. "Unit" refers to 1. In this way, all measurements are multiples of that unit. For example, the unit of mass is the kilogram. Thus, the measurement of mass is in multiples—or fractions—of 1 kilogram.
Originally, the English unit foot was the length of the King's foot. Thus, a distance of 25 feet was 25 time the foot unit of measurement.
Unfortunately, each King had a different sized foot, so that brought about some confusion. Finally, they agreed on a standard length for a foot that would not vary.
The units of measurement are defined as standard and do not vary.
SI base units
The International System of Units (SI) defined seven basic units of measure from which all other SI units are derived.
These SI base units or commonly called metric units are:
| Measure | Unit | Symbol | Area of Science |
|---|---|---|---|
| Time | Second | s | All |
| Length or distance | Meter or Metre | m | All |
| Mass | Kilogram | kg | Physics | Electric Current | Ampere | A | Physics | Temperature | Kelvin | K | Physics |
| Luminous Intensity | Candela | cd | Optics |
| Amount of Substance | Mole | mol | Chemistry |
Although these SI base quantities are supposed to be a set of mutually independent dimensions, some may well be interdependent.
Problem areas
There are some variations or problem areas in these fundamental units.
Length
Although multiples or fractions of a meter are useful in most sciences, the unit is impractical in Astronomy. Instead, the fundamental unit of length in Astronomy is the light-year, which is the distance light travels in kilometers in one year.
Mass
It would seem more intuitive to define the fundamental of mass as a gram. However, the SI decision was to say that 1000 grams or a kilogram was fundamental.
Electric current
Since electric current is man-made and depends on a number of factors, the ampere does not seem appropriate as a fundamental unit. The ampere is defined as:
"The constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in a vacuum, would produce between these conductors a force equal to 2 × 10−7 newton per meter of length."
That does not seem too fundamental.
Summary
Units of measurement that are considered fundamental, such that they cannot be described as a function of other units. They often related specifically to the various sciences. However, there are some problems with the definitions.
Look to the source of things
Resources
Websites
Units of measurement - Wikipedia
General Tables of Units of Measurement - National Institute of Standards and Technology
Units of Measurement and SI Base Units - Includes conversions
Fundamental unit - Wikipedia
Base Units of the International System (SI) - University of North Carolina at Chapel Hill
SI base unit - Wikipedia
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Fundamental Units of Measurement