Introduction to Physical Quantities and Units
A physical quantity is defined as a quantity that can be used in mathematical equations of science and technology. It is characterized by its numerical value (magnitude) and associated unit. The following rules apply to physical quantities and their units:
Rules for Physical Quantities and Units:
- Symbols for physical quantities are set in italics (sloping type), while symbols for units are set in roman (upright) type (e.g., m = 21 kg; E = 15 MeV; K = 220 Gy).
- Superscripts and subscripts used with physical quantities are set in italics if they represent variables, quantities, or running numbers; they are in roman type if they are descriptive (e.g., Nx, λm but λmax, Eab, μtr).
- Symbols for vector quantities are set in bold italics (e.g., v).
International System of Units (SI)
The currently used metric system of units is known as the International System of Units (SI). This system is founded on base units for seven basic physical quantities. All other quantities and units are derived from these seven base quantities and units. The seven base SI quantities and their units are:
| Physical Quantity | Symbol | SI Unit |
|---|---|---|
| Length | l | metre (m) |
| Mass | m | kilogram (kg) |
| Time | t | second (s) |
| Electric Current | I | ampere (A) |
| Temperature | T | kelvin (K) |
| Amount of Substance | n | mole (mol) |
| Luminous Intensity | Iv | candela (cd) |
Units in Radiation Physics
The following are some units commonly used in radiation physics, with conversions to SI units:
| Physical Quantity | Symbol | SI Unit | Units Commonly Used in Radiation Physics | Conversion |
|---|---|---|---|---|
| Length | l | metre (m) | nm, Å, fm | 1 m = 109 nm = 1010 Å = 1015 fm |
| Mass | m | kilogram (kg) | MeV/c2 | 1 MeV/c2 = 1.78 × 10–30 kg |
| Time | t | second (s) | ms, μs, ns, ps | 1 s = 103 ms = 106 μs = 109 ns = 1012 ps |
| Current | I | ampere (A) | mA, μA, nA, pA | 1 A = 103 mA = 106 μA = 109 nA |
| Temperature | T | kelvin (K) | T (in K) = T (in °C) + 273.16 | |
| Electric Charge | q | coulomb (C) | e | 1 e = 1.602 × 10–19 C |
| Energy | E | joule (J) | eV, keV, MeV | 1 eV = 1.602 × 10–19 J = 10–3 keV |