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Units & Prefixes

Introduction

It is important to know the standard units used in electronics. In many cases quantities are very small or very large and prefixes or standard form are used to make the numbers more manageable. Using prefixes, and converting between prefixes and standard form, is a key skill when undertaking calculations for component values etc.

Units

The standard quantities and units used in electronics are, with their corresponding symbols:


Prefixes & Standard Form

Quantities in electronics are quite often very large or very small. Writing out the quantity with lots of zeros is inconvenient and confusing, leading to mistakes. To represent large and small quantities prefixes are used to modify the unit or standard form is used in place of the extra zeros. It does not matter which system is adopted but it is often easier to talk about quantities using prefixes and to do calculations using standard form.

For example a current of 0.005 A is more conveniently talked about as 5 mA and more readily used in calculations as 5×10−3 A


Working with small numbers

For example, a voltage of 8 µV in standard units is 8÷1000,000 = 0.000008 V

For example, 0.0035 V can be more conveniently written as 3.5 mV because 1 mV = 0.001 V


Working with large numbers

For example, a resistance of 4.7 kΩ in standard units is 4.7×1000 = 4700 Ω

For example, a voltage of 25,000 V is more conveniently written as 25 kV because 1 kV = 1000 V


Examples

Example 1: What is 20 mA in amperes? 20 mA = 20÷1000 A = 0.02 A

Example 2: What is 3.9 MΩ in ohms? 3.9 MΩ = 3.9 × 1000,000 Ω = 3,900,000 Ω

Example 3: What is 0.1 µF in nanofarads? All of the prefixes are different by a factor of 1000. In this example, nano is 1000 times smaller than micro meaning 1 µF = 1000 nF. Therefore 0.1 µF = 0.1 × 1000 nF = 100 nF