The SI system of Units for Electrical Quantities
In this course we use a standard set of electrical units such as the volt (V), ampere (A), second (s), etc. These are part of the International System of Units (abbreviated SI). The SI is based on six fundamental dimensions, and associated units, and unit symbols. These are shown below:
|
Fundamental
Dimensions
|
Units
(SI)
|
symbol
|
|
length
|
meter
|
m
|
|
mass
|
kilogram
|
kg
|
|
time
|
second
|
s
|
|
electric
current
|
ampere
|
A
|
|
temperature
|
Kelvin
|
K
|
|
luminous
intensity
|
candela
|
cd
|
To these fundamental SI dimensions and units, we add others that are appropriate to electrical circuits and electronics. The list below includes many of those we will use routinely.
|
Electrical
Dimensions
|
Units
(SI)
|
symbol
|
related
quantities
|
|
voltage
|
Volt
|
V
|
work
/ charge
|
|
current
|
Ampere
|
A
|
charge
/ time
|
|
power
|
Watt
|
W
|
energy
/ time
|
|
energy
|
Joule
|
J
|
power
x time; work
|
|
resistance
|
Ohm
|
W
|
voltage
/ current
|
|
reactance
|
Ohm
|
W
|
voltage
/ current
|
|
capacitance
|
Farad
|
F
|
charge
/ voltage
|
|
inductance
|
Henry
|
H
|
flux
linkage / current
|
|
conductance
|
Siemens
(or mhos)
|
S
|
1
/ resistance
|
|
permittivity
|
Farads/meter
|
e
|
capacitance
/ length
|
|
permeability
|
Henrys/meter
|
m
|
inductance
/ length
|
|
charge
|
Coulomb
|
C
|
current
x time
|
|
frequency
|
Hertz
|
Hz
|
cycles
/ second
|
Multiples and submultiples of basic units
For convenience, we also use multiples and submultiples of the basic units. This makes it much easier to express very small or very large quantities. For examples a current of 0.001 A is more easily expressed as 1 mA (1 millii-Ampere). A resistance of 1 million ohms would be expressed as 1 MW . In the first case, we use the prefix "m" to mean a multiplier of 0.001, expressed as "milli". In the second case, the prefix "M" signifies a multiplier of 1 million, expressed as "mega"..
|
Prefix
|
Abbreviation
|
Magnitude
|
Root
word
|
|
tera
|
T
|
10
^ 12
|
Greek
teras, "monster"
|
|
giga
|
G
|
10
^ 9
|
Latin
gigas, "giant"
|
|
mega
|
M
|
10
^ 6
|
Greek
megas, "great"
|
|
kilo
|
k
|
10
^ 3
|
Greek
chilioi "a thousand"
|
|
milli
|
m
|
10
^ -3
|
Latin
mille "a thousand"
|
|
micro
|
m
|
10
^ -6
|
Greek
mikvos "small"
|
|
nano
|
n
|
10
^ -9
|
Greek
nanos "dwarf"
|
|
pico
|
p
|
10
^ -12
|
Spanish
pico "small quantity"
|