Units. Systems of Units. Fundamental and Derived SI Units Revision Notes

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1.1Units. Systems of Units. Fundamental and Derived SI Units


In these revision notes for Units. Systems of Units. Fundamental and Derived SI Units, we cover the following key points:

  • What is a unit? Why do we need units?
  • Why it is necessary to have commonly accepted systems of units?
  • Which are the main systems of units used in science and daily life?
  • What is the difference between the fundamental and derived units?
  • What are multiples and submultiples of units?

Units. Systems of Units. Fundamental and Derived SI Units Revision Notes

In Physics, a unit is a widely accepted standard of measurement.

A correct result of a measurement must contain two elements: the magnitude of measurement (the numerical value) and the unit of measurement.

There are two main systems of units used in science and daily life today:

  1. The SI (or metric) System of Units
  2. The Imperial (or English) System of Units

The first one (SI System) is recognized in science and technology while the other (Imperial System) is mostly used in daily activities in English-speaking countries.

By definition, units are classified in two main categories: a) Fundamental and b) Derived units. Fundamental units are considered each of a set of unrelated units of measurement, which are arbitrarily defined and from which other units are derived. Thus, it is obvious that all derived units are obtained by the combination of two or more fundamental ones.

There are 7 fundamental units in the SI System of Units. They are:

The meter (symbol: m); it is used to measure length. The kilogram (symbol: kg); it is used to measure mass. The second (symbol: s); it is used to measure time. The ampere (symbol: A); it is used to measure electric current. The kelvin (symbol: K); it is used to measure temperature. The mole (symbol: mol); it is used to measure amount of substance or particles in matter. The candela (symbol: cd); it is used to measure light intensity.

The differences between the Imperial and SI Systems of Units consist in the units of length and mass. The other units are the same. Thus, in the Imperial System of Units, the unit of length is the yard (yd). The conversion factor between yard and metre (which is the corresponding the unit of length in the SI system is 1 yard = 0.9144 m.

The unit of mass in the Imperial system is the pound (lb). The conversion factor is 1 lb = 0.4536 kg.

Multiples and submultiples are the prefixes that we frequently use to express or show quantities in scientific terminology. We use the Greek terms to write the multiples and submultiples in words and standard notation to express them in numbers.

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