Units and Constants
Measurement in science and electronics depends on a coherent system of units so that quantities can be stated and compared without ambiguity. The International System of Units (SI) is the modern metric framework used worldwide. It is built on a small set of base units, including the metre for length, the kilogram for mass, the second for time and the ampere for electric current, from which all other units are derived. Electrical quantities use derived units such as the volt, ohm, watt, farad and henry, each defined in terms of the base units.
Because the values encountered in electronics range over many orders of magnitude, the metric system uses standard prefixes to scale units by powers of ten. Prefixes such as kilo, mega and giga denote successively larger multiples, while milli, micro, nano and pico denote successively smaller fractions. A resistance might therefore be quoted in kilohms or megohms and a capacitance in microfarads, nanofarads or picofarads, keeping numbers conveniently sized.
Physical constants are fixed quantities that appear throughout physics and engineering calculations. Familiar examples include the speed of light in vacuum, the elementary charge, and the constant of proportionality in electrostatics, among others. Reference tables collect these constants together with unit definitions, conversion factors between SI and older or non-metric units, and the prefix list, providing a consistent foundation for computation and for reading specifications across the field.
Frequently asked questions
- What are the SI base units?
- They are the small set of fundamental units, such as the metre, kilogram, second and ampere, from which all other SI units, including electrical units like the volt and ohm, are derived.
- Why are metric prefixes used?
- Prefixes such as kilo, milli, micro and nano scale a unit by powers of ten, keeping numbers a convenient size when quantities span a very wide range of magnitudes.
- What is a physical constant?
- It is a fixed quantity, such as the speed of light or the elementary charge, that recurs in physical laws and calculations and has the same value wherever it is applied.
