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Physics > SI Units


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The term SI comes from the French 'Le Système international d'unités' which translates as 'The International System of units'. It represents a metric standard of units for use in science and mathematics superseding older units such as 'yards' and 'inches' etc. Much of the developed world now uses these new units as standard although older non-SI units are still commonplace in day to day life. We sometimes still weigh groceries in pounds and ounces even though schools no longer teach in these old units.

It is important when conducting scientific enquiry to use SI units throughout. It can be confusing to students who have been brought up on SI units to use old equipment which measured in non-SI units. Old balances and rulers are examples of equipment that schools may still use which may measure in SI and non-SI units.

The seven basic SI units are listed below along with their official symbol in brackets. Please be aware that when using SI units, these symbols are case sensitive. The units beyond the first seven were derived from these basic units to serve a variety of different physical and mathematical applications.

Second [s]

The second is the basic unit of time. It is the length of time taken for 9192631770 periods of vibration of the caesium-133 atom to occur.

Metre [m]

The metre is the basic unit of length. It is the distance that light travels in a vacuum in 1/299792458th of a second.

Kilogram [kg]

The kilogram is the basic unit of mass. It is the mass of an international prototype in the form of a platinum-iridium cylinder kept at Sevres in France.

Ampere [A]

The ampere is the basic unit of electric current. It is named after the French physicist Andre Ampere (1775-1836).

Candela [cd]

The candela is the basic unit of luminous intensity. It is the intensity of a source of light of a specified frequency, which gives a specified amount of power in a given direction.

Kelvin [K]

The kelvin is the basic unit of temperature. It is 1/273.16th of the thermodynamic temperature of the triple point of water. It is named after the Scottish physicist Lord Kelvin (1824-1907).

Mole [mol]

The mole is the basic unit of substance. It is the amount of substance that contains as many elementary units as there are atoms in 0.012 kg of carbon-12.

Farad [F]

The farad is the SI unit of electrical capacitance. (the capacity to store electricity.) It is a large unit and therefore is more often used as a microfarad. Named after the English chemist and physicist Michael Faraday (1791-1867).

Watt [W]

The watt is used to measure power or the rate of doing work. One watt is a power of 1 joule per second. Named after the Scottish engineer James Watt (1736-1819).

Hertz [Hz]

The hertz is the SI unit of frequency. One hertz indicates that 1 cycle occurs every second. For most work much higher frequencies are needed such as the kilohertz [kHz] and megahertz [MHz]. Named after the German physicist Heinrich Rudolph Hertz (1857-94).

Ohm [Ω]

The ohm is the SI unit of electrical resistance. Its symbol is the capital Greek letter 'omega'. Named after the German physicist Georg Simon Ohm (1789-1854).

Pascal [Pa]

The pascal is the SI unit of pressure. One pascal is the pressure generated by a force of 1 newton acting on an area of 1 square metre. It is a small unit and therefore is more often used as a kilopascal [kPa]. Named after the French mathematician, physicist and philosopher Blaise Pascal (1623-62).

Volt [V]

The volt is the SI unit of electric potential. One volt is the difference of potential between two points of an electical conductor when a current of 1 ampere flowing between those points dissipates a power of 1 watt. Named after the Italian physicist Count Alessandro Giuseppe Anastasio Volta (1745-1827).

Joule [J]

The joule is the SI unit of work or energy. One joule is the amount of work done when a force of 1 newton moves through a distance of 1 metre in the direction of the force. Named after the English physicist James Prescott Joule (1818-89).

Newton [N]

The newton is the SI unit of force. One newton is the force required to give a mass of 1 kilogram an acceleration of 1 metre per second per second. Named after the English mathematician and physicist Sir Isaac Newton (1642-1727).

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