SI_quantities&units

SI quantities and units

Table 1 below has been reproduced using the same conventions and notation as used in the text “Electrical Principles for the Electrical Trades” or in class.

Table 1: SI quantities and units

Name of quantity Quantity

symbol Name of unit Unit

symbol

Angle of rotation θ Degrees °

Angle of rotation (supplementary

unit) α Radian

rad

Area A

Square

metre

Capacitance C

Farad

F Density -

Kilogram/cubic

metre

kg/m3

Diameter d

Metre

m Efficiency η Precent % Electric current I Ampere A

Electric potential V Volt V

Electrochemical equivalent z Kilograms/coulomb kg/C

Energy E

Joule

J Force F

Newton

N Frequency f

Hertz

Hz Frequency (angular) ω Radians/second

rad/s Heat capacity C Joules/Kelvin J/K

Illuminance E Lux

lx Inductance (mutual) M Henry H

Inductance (self) L Henry H

Impedance Z

Ohm

Length l

Metre

m Luminance L

cd/sq.

metre

cd/m2

Luminous flux F Lumen lm

Luminous intensity I Candela cd

Magnetic field strength H Amperes/meter A/m

Magnetic flux Φ Weber

Wb Magnetic flux density Β Tesla T Magnetic reluctance R m Ampere turns /

weber

/H Magnetomotive force F m Ampere A

Name of quantity

Quantity symbol

Name of unit

Unit symbol

Mass m Kilogram kg Number of poles p - - Number of turns N

Permeability actual μ - - Permeability absolute μ0 Henry/metre H/m

(7104?×π) Permeability relative

μr - (ratio)

Permittivity ε Farads/metre F/m Power apparent S Volt ampere VA Power reactive Q Volt ampere reactive

var Power true P

Watt

Power factor λ Ratio cos φ Pressure P Pascal Pa

Quantity of charge

Coulomb

Radius r Metre m Reactance X Ohm ? Resistance R Ohm ? Resistivity ρ Ohm metre ?m Rotational speed n

rpm

rps

Solid angle ω Steradian sr Slip s Percent %

Specific heat capacity C Jouls/kg.kelvin

J/kg.K Temperature absolute T Kelvin K Temperature customary

Degrees Celsius

°C

Time t Second s

Time constant τ Second s

Torque T Newton metre Nm

Transformation ratio

(instrument transformer)

K - -

Turns t - -

Turns ratio (transformer)

n - -

Work W Joule J

SI Units

What does SI mean? It stands for Systeme International d’Unites (the International

System of Units) which is abbreviated in many languages to SI units.

This system is international and uses common units for mechanical, thermal, and

electrical values. Table 2 shows the basic SI units.

Table 2: Base SI units

Quantity Units

Physical Quantity Quantity Symbol Unit Name Unit Symbol

length l metre m mass m kilogram

s time t second

electric current I ampere A

temperature t kelvin K

Luminous intensity I candela cd

rad Angle of rotation α radian

Definitions

Table 3: Unit definitions

Unit Definition

Metre The length of the path travelled by light in a vacuum

during a time interval of 1/299,792,458 second.

Kilogram The mass of the international Prototype of the Kilogram

(Platinum - Iridium)

Second The time interval corresponding with 9,192,631,770

oscillations of a caesium 133 atom at 0°K

Ampere The current required to flow in 2 parallel conductors,

placed 1 metre apart in a vacuum to produce a force of 2

x 10-7 Newton metres between the conductors.

Kelvin 1/273.16 of the (Triple point) of water (Ice Point)

Candela The luminous intensity, in a given direction of a source

that emits monochromatic radiation of 54,012 Hz and has

a radian intensity of 1/683 watts per steradian.

Radian The angle between 2 radii of the same circle, which mark

off, on the circumference of the circle, an arc equal in

length to the radius of the circle.