International System of Units

The International System of Units defines a common system for expressing physical quantities. Its base units include the kelvin for thermodynamic temperature; derived units combine them to describe quantities such as force and energy.^[ref]

Table of contents

Sievert ( $Sv = J k g^{- 1}$ )

The SI unit used for equivalent dose and effective dose. Sharing dimensions with the gray does not make the quantities interchangeable.^[ref]

Gray ( $Gy = J k g^{- 1}$ )

The SI unit of absorbed dose.^[ref]

Mole (unit, $mol$ )

The mole is the SI base unit of amount of substance. One mole contains exactly

6.02214076 \times 1 0^{23}

specified entities, as fixed by the Avogadro constant. It counts entities rather than a fixed mass or volume. ^[ref]

Newton (unit, $N$ )

The newton is the SI unit of force:

1 N = 1 kg m s^{- 2} .

This combines the units of mass and acceleration in Newton's second law. ^[ref].

Pascal (unit, $Pa$ )

The pascal is the SI unit of pressure:

1 Pa = 1 N m^{- 2} = 1 kg m^{- 1} s^{- 2} .

For a uniform normal force of

10 N

over area

0.02 m^{2}

, the pressure is

500 Pa

. The unit expresses a force per area, rather than the total force alone. ^[ref].

Joule ( $J = N m = kg m^{2} s^{- 2}$ )

The joule is the SI unit of energy, work, and heat. One newton of force acting along one metre of displacement does one joule of work. The relation

1 J = 1 N m

makes mechanical and electrical transfers comparable in the same energy unit. ^[ref]

Coulomb ( $C = A s$ )

The coulomb is the SI unit of electric charge. A constant electric current of

1 A

1 C

in a time of

1 s

; a constant current of

2 A

for

3 s

6 C

. The prefix in

1 nC = 1 0^{- 9} C

explains the much smaller charge scale used in the worked Coulomb's law example. ^[ref]

Volt ( $V = J/C$ )

The volt is the SI unit of electric potential and electric potential difference:

1 V = 1 J/C

, one joule per coulomb. A positive electric charge of

2 C

moved through a potential increase of

3 V

gains

Δ U = q ΔΦ = 6 J

of potential energy; the electric field does

- 6 J

of work. ^[ref].

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International System of Units by

Wikipedia Bot  1

The International System of Units (SI) is the standard system of measurement used globally in science, industry, and commerce. It provides a consistent framework for measurement based on a set of base units and derived units, which are used to express physical quantities. ### Base Units The SI is built upon seven base units, each defined by a specific physical phenomenon: 1. **Meter (m)** - The unit of length. 2. **Kilogram (kg)** - The unit of mass.

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International System of Units by

Ciro Santilli  40 Updated 2025-07-16

The key is to define only the minimum number of measures: if you define more definitions become over constrained and could be inconsistent.

Learning the modern SI is also a great way to learn some interesting Physics.

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