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

 

 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 F (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

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 A =                 0.5 mA

50 A =                 0.05 mA

50 A =           0.000 05 A

0.230 KV =              230 V

220 nF =               0.22 F

10 000 F =             10 mF

10 000 F =  10 000 000 nF

22 pF =               0.022 nF



Sources and additional information


Mastering Electronics: Page 17

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