Practical Electronics/Units and Quantities
There are many technical terms and quantities associated with the study of electronics, each with a specific, well-defined meaning and units. It is important to understand the meanings of and relationships between these.
Time is considered one of the few fundamental quantities of the universe, and is used to define many other quantities in science. Thus, these quantities cannot be used to define time, as this would mean it would be circularly defined. Time is operationally defined (i.e. we assume that there is a quantity "time" that we can measure) in terms of the SI second (symbol: s), which is defined as:
|“||the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.||”|
Length is a quantity of distance and is defined using the above definition for length and the speed of light, which is held to be constant in our current understanding of the universe. The base unit of length is the metre (symbol: m), which is defined to be:
|“||the length traversed by light in a vacuum in 1/299 792 458 s.||”|
Mass is the only measure with an SI unit still defined according to a synthetic artifact rather than a fundamnetal constant of nature. The base unit of mass is the kilogram (symbol: kg), which is defined as:
|“||a mass equal to that of the International Prototype Kilogram (IPK)||”|
The IPK is a platinum-iridium cylinder stored in a vault at the BIPM in Sèvres, France. Mass is unique in having the base unit having a prefix - the unprefixed "gram" is not considered to be a base unit in the SI system. The kilogram is approximately equal to the mass of one litre of water.
Area is a quantity expressing the two-dimensional size of a defined part of a surface. The base unit of area is the square metre (symbol: m2). This is:
|“||the area of a square that is one metre on a side||”|
Volume is the quantity expressing the three-dimesional size of a given space. The base unit of volume is the cubic metre (symbol: m3). This is:
|“||the volume of a cube that is one metre on a side||”|
Another measure of volume is the litre, equal to the volume of a cube 10 cm on a side, or 0.001 m3.
Force is a measure of how much acceleration a mass is provided. The SI unit of force is the newton (symbol: N), which is:
|“||the force required to accelerate a one kilogram mass at a rate of one meter per second per second, or 1 kg·m·s−2.||”|
A force of one newton is approximately the gravitational force exerted on a mass of 0.1 kg on Earth's surface.
Current is a measure of how much electric charge is flowing in object. It is defined in terms of the attractive force between two wires caused by current flowing in the wires. The SI unit is the ampere (symbol: A), and it is defined as:
|“||the constant current which will produce an attractive force of 2×10–7 newton per metre of length between two straight, parallel conductors of infinite length and negligible circular cross-section placed one metre apart in a vacuum.||”|
The ampere is an SI base unit: it is defined without reference to the quantity of electric charge, though this could change in future with a proposed redefinition of the unit of electric charge.
Electric charge is the most fundamental of electrical properties. It is a quantity carried by many subatomic particles and is the whole reason electronics as a science exists. The base unit of charge is the coulomb (symbol: C), which is defined as:
|“||the charge transferred by a steady current of one ampere in one second||”|
According to conventional values for some other constants, the Coulomb could also be defined as the charge carried by 6.241 509 629 152 65 × 1018 electrons, however, this is not (yet) the formal definition of the unit.