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Quantity | Usual Symbol |
Unit | Unit Symbol |
Voltage | V | volt | V |
Current | I | amp* | A |
Charge | Q | coulomb | C |
Resistance | R | ohm | |
Capacitance | C | farad | F |
Inductance | L | henry | H |
Reactance | X | ohm | |
Impedance | Z | ohm | |
Power | P | watt | W |
Energy | E | joule | J |
Time | t | second | s |
Frequency | f | hertz | Hz |
* strictly the unit is ampere, but this is
almost always shortened to amp. |
The relationship between quantities can be written using words or symbols (letters), but symbols are normally used because they are much shorter; for example V is used for voltage, I for current and R for resistance:
As a word equation:
voltage = current × resistance
The same equation using symbols: V = I × R
To prevent confusion we normally use the same symbol (letter) for each quantity and these symbols are shown in the second column of the table.
Please click on the quantities in the table for further information.
Prefix | Prefix Symbol |
Value | |
milli | m | 10-3 | = 0.001 |
micro | µ | 10-6 | = 0.000 001 |
nano | n | 10-9 | = 0.000 000 001 |
pico | p | 10-12 | = 0.000 000 000 001 |
kilo | k | 103 | = 1000 |
mega | M | 106 | = 1000 000 |
giga | G | 109 | = 1000 000 000 |
tera | T | 1012 | = 1000 000 000 000 |
Some of the units have a convenient size for electronics, but most are either too large or too small to be used directly so they are used with the prefixes shown in the second table. The prefixes make the unit larger or smaller by the value shown.
Some examples:
25 mA = 25 × 10-3 A = 25 × 0.001 A = 0.025 A
47µF = 47 × 10-6 F = 47 × 0.000 001 F = 0.000 047 F
270k
= 270 × 103
= 270 × 1000
= 270 000
In fact if you use an equation frequently you can use special sets of prefixed units which are more convenient...
For example: Ohm's Law, V = I × R
the standard units are volt (V), amp (A) and ohm (),
but you could use volt (V), milliamp (mA) and kilo-ohm (k)
if you prefer.
Take care though, you must never mix sets of units: using V, A and
k
in Ohm's Law would give you wrong values.