المبادئ: الكميات والوحدات والبادئات

Principles

Quantity, units and symbols

To avoid misunderstandings it is important to use the correct physical expressions.
 

Physical quantity	Symbol	SI Unit	Symbol	Measurement device
length	l	meter	m	ruler, caliper
mass or weight	m	gram	g	scale
		pounds	lb
current	I	ampere	A	ammeter, clamp meter
voltage	V	volt	V	voltmeter
resistance	R	ohm	Ω	ohmmeter
wattage, power	P	watt	W	wattmeter

Examples:  A current I is 2 A (Ampere).
                The voltage V is 12 V (Volt).
                The resistor R has 10 kΩ (kilo ohm). The resistor R13 in the circuit has
                56 kΩ (kilo ohm).

SI Prefixes

In practice the values of measurements are often too big or too small to manage with the basic units only. The following prefixes are used in electronics:

Prefix	Symbol	Factor	Power of 10
giga	G	1 000 000,000	109
mega	M	1 000 000	106
kilo	k	1 000	103
milli	m	0.001	10-3
micro	µ	0.000 001	10-6
nano	n	0.000 000 001	10-9
pico	p	0.000 000 000 001	10-12

Examples:      1 A = 1 000 mA
                    100 kΩ = 100 000 Ω (or 0.1 MΩ)
                    97.6 MHz = 97 600 kHz (or 97 600 000 Hz)
                    100 nF = 0.1 µF (or 100 000 pF)

Usage

When converting a unit or a formula write down all steps carefully.
Do not do it mental! A 0 is lost and a . is put at a wrong place easily.

-  Learn the sequences. For numbers which are bigger than the units: kilo – mega – giga
    and for number smaller than the unit: milli – micro – nano – pico
-  Remember that the steps between the following or the previous prefix is the
    factor 1 000
-   During converting the value remains the same. When the prefix gets bigger
    (mega instead of kilo) the number must get smaller (1 instead of 1 000)


Examples:  1 A = ? mA
                the difference of A to mA is 1 000
                mA is smaller than A. So the number must get bigger (x 1 000)
                1 A = 1 000 mA

                2.2 KΩ = ? Ω
                the difference of KΩ to Ω is 1 000
                Ω is smaller than KΩ. So the number must get bigger (x 1 000)
                2.2 KΩ = 2 200 Ω

                3 000 V = ? kV
                the difference of V to kV is 1 000
                K is bigger than V. So the number must get smaller (/ 1 000)
                3 000 V = 3 kV

                94.5 MHz = ? Hz
                the difference of M to the Hz is 1 000 000 (kilo is in between)
                Hz is smaller than MHz. So the number must get bigger (x 1 000 000)
                94.5 MHz = 94 500 000 Hz (or 94 500 kHz)

Exercises

Please convert the following values.
To see the answer just  the space in front of the target value.

1 000 V =                  1 KV

1 000 mV =                1 V

2.2 KΩ =              2 200 Ω

3.3 MΩ =             3 300 KΩ

250 mV =               0.25 V

25 mV =               0.025 V

545 V =               0.545 KV

11 KV =              11 000 V

11 KV =        11 000 000 mV

500 µA =                 0.5 mA

50 µA =                 0.05 mA

50 µA =           0.000 05 A

0.230 KV =              230 V

220 nF =               0.22 µF

10 000 µF =             10 mF

10 000 µF =  10 000 000 nF

22 pF =               0.022 nF

Sources and additional information

SI-prefixes:         http://en.wikipedia.org/wiki/SI_prefix
Units:                 http://en.wikipedia.org/wiki/Conversion_of_units
Unit converter:    www.cactus2000.de/uk/unit/mass.shtml