This topic will cover the fundamental concepts of measurement, including units of measurement, the importance of standard units, and the difference between accuracy and precision.
Introduction to Measurement
Fundamentals, Units, Tools, and Best Practices
| Aspect | Description |
|---|---|
| Definition | The process of assigning a numerical value to a physical quantity by comparing it with a standard. |
| Purpose | Quantify observations, enable communication, support calculations, and facilitate scientific/engineering decisions. |
| Key Elements | • Quantity (what is being measured) • Unit (reference standard) • Instrument (tool used) • Result (numerical value + unit). |
| Category | Examples |
|---|---|
| Fundamental | Length, Mass, Time, Electric current, Thermodynamic temperature, Amount of substance, Luminous intensity. |
| Derived | Speed (m · s⁻¹), Force (N), Pressure (Pa), Energy (J), Power (W), Voltage (V). |
| Auxiliary | Angle, Plane angle (rad), Solid angle (sr). |
Derived quantities are expressed as algebraic combinations of fundamental units.
| System | Characteristics | Common Use |
|---|---|---|
| International System of Units (SI) | 7 base units, coherent derived units, universal acceptance. | Science, industry, education worldwide. |
| CGS (centimeter‑gram‑second) | Based on cm, g, s; older system. | Some fields of physics, astrophysics. |
| Imperial / US Customary | Inches, feet, pounds, seconds, etc. | United States (construction, everyday life). |
| Specialized | e.g., atomic units, natural units (Planck). | High‑energy physics, quantum chemistry. |
When working internationally, always state the system and, if needed, provide conversion factors.
| Quantity | Symbol | Unit | Symbol | Definition (2024) |
|---|---|---|---|---|
| Length | L | meter | m | Distance light travels in vacuum in 1/299 792 458 s. |
| Mass | M | kilogram | kg | Defined by the Planck constant (h = 6.626 070 15×10⁻³⁴) J·s. |
| Time | T | second | s | Duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of cesium‑133. |
| Electric current | I | ampere | A | Flow of 1 C s⁻¹ (based on elementary charge (e)). |
| Thermodynamic temperature | Θ | kelvin | K | 1/273.16 of the thermodynamic temperature of the triple point of water. |
| Amount of substance | N | mole | mol | Amount containing exactly (6.022 140 76×10²³) elementary entities. |
| Luminous intensity | J | candela | cd | Luminous power per steradian of a source emitting monochromatic radiation of frequency 540 × 10¹² Hz with radiant intensity 1/683 W·sr⁻¹. |
Derived units (e.g., newton, joule) are built from these base units.