INTRODUCTION
A measurement result consists of two things: a number and a unit. Units are defined in terms of physical standards. Standards come in many sizes and shapes, and some are better than others. All standards, however, are related through measurements to seven International System (SI) primary standards.

 

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Exercise 1-1: What is the Name that the following Quantity and Symbol describe?

Quantity: power, radiant flux
Symbol: W

Solution: The SI unit of power is one joule per second (1 J/s), which is called one watt (1 W).

Breakdown to primary SI standards:
The second in joule per second is the base unit for time.

A joule, J, can be converted to primary standards as follows:
1 J = 1 newton x 1 meter = 1 N x 1 m; the meter being the base unit of length.

1 N = mass x acceleration
      = 1 kilogram (the primary standard for mass) x 1 meter per square second
      = 1 kg x 1 m/s²

 

Exercise 1-2: What is the Name that the following Quantity and Symbol describe?

Quantity: inductance
Symbol: H

Solution: The SI unit of inductance is one weber per ampere (1 Wb/A), which is called one henry (1 H).

A weber, W, can be converted to primary standards as follows:

1 Wb = 1 volt-second per ampere = 1 Vs/A; the ampere being the base unit of electric current.

1 V = 1 joule per coulomb = 1 J/C

1 C = 1 ampere X second = 1 As

In conclusion, in the International System (SI), all compound, or derived, units can be expressed as a combination of some of the base units.