Physical quantity
lastmod 2014-04-03created 2014-04-031 edits
A physical quantity (物理量) is a quantity that is a subject of physics: a measurable quantity of physics, or the result of a measurement often expressed as the product of a numerical value and a physical unit (usually an SI unit).
Physical quantities include length, area, volume, mass (actually weight), brightness (luminous intensity or illuminance), the loudness and pitch of sound, temperature, and electric current and voltage. To measure these various physical quantities accurately, their magnitude is often converted into a length for display.
For example, force is shown by the elasticity of a spring, temperature by the thermal expansion of a liquid or similar, and current and voltage by the movement of a needle using the magnetic field (磁氣場) produced by the current. Atmospheric pressure and the like can also be read directly as the height, that is, the length, of mercury in a vacuum tube. The volume of an object can be measured by filling a container with water, sinking the object in it, and measuring the volume of the overflowing water with a graduated cylinder. This method was discovered by Archimedes before the Common Era, and in the end it measures the volume of the object by substituting length for it. The weight of an object floating on water can also be measured by the weight of the overflowing water.