quantities and units c

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Electrical &

Physical Quantities and

Their units And

Tables

Lesson #1 CUnits and Measurements


Scientists identify quantities which describe the natural world.In order to decide how much of a particular quantity a given object has, for example, MASS, it is necessary to compare that object to the masses of other objects which have known masses,

A PHYSICAL QUANTITY

It can be defined in terms of the operations necessary to measure it.For example, the length of an object can be determined by comparing it to an object of known length,

Units and Measurements


The International System of Units

(French: Système International d'Unités, SI) is the modern form of the metric system, and is the most widely used system of measurement. It comprises on seven base units.

The system was published in 1960 as the result of an initiative that started in 1948. It is based on the meter-kilogram-second system of units (MKS) rather than any modified of the centimeter–gram–second system (CGS)



The Seven Base SI Units Units and Measurements


Derived SI Units (examples)

Quantity unit Symbol

Volume cubic meter m3

Density kilograms per cubic meter

kg/m3

Speed meter per second m/s

Newton kg m/ s2 N

Energy Joule (kg m2/s2) J

Pressure Pascal (kg/(ms2) Pa



A SI prefix is a name that is added to the name of a basic unit and which indicates whether that unit is a multiple (or a fraction) of that unit. For example, the prefix "kilo" added to "meter" gives "kilometer", which is a unit 1 000 times LARGER than the base unit "meter". Similarly, The prefix "mille" added to "gram" gives "milligram", which is a unit 1 000 times SMALLER than the base unit "gram".

The table shown below lists the names of approved SI prefixes.

SI prefixes

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