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  • SI 基地单位

    How long is a yard? 
    ::一个院子要多久?

    It depends on who , and when, you asked the question. Today, we have a standard definition of the yard, which you can see marked on every football field. If you move the ball ten yards, you get a first down and it doesn’t matter whether you are playing in Los Angeles, Dallas, or Green Bay. But at one time that yard was arbitrarily defined as the distance from the tip of the king’s nose to the end of his outstretched hand. Of course, the problem there is simple: new king, new distance (and then you have to remark all those football fields).
    ::这取决于谁,以及你何时提出这个问题。 今天,我们有一个院子的标准定义,您可以在每个足球场上看到这个定义。 如果你移动10码球,你就会得到第一个球位,而不管你是在洛杉矶、达拉斯还是格林湾比赛。 但有一次,那个院子被武断地定义为从国王鼻子的顶端到他伸展的手的尽头的距离。 当然,问题很简单:新国王,新的距离(然后你必须说出所有的足球场 ) 。

    SI Base Units
    ::SI 基地单位

    All measurements depend on the use of units that are well known and understood. The English system of measurement units (inches, feet, ounces, etc.) are not used in science because of the difficulty in converting from one unit to another. The metric system is used because all metric units are based on multiples of 10, making conversions very simple. The metric system was originally established in France in 1795. The International System of Units is a system of measurement based on the metric system. The acronym SI is commonly used to refer to this system and stands for the French term, Le Système International d’Unités . The SI was adopted by international agreement in 1960 and is composed of seven units, as shown in Table .
    ::由于难以从一个单位转换为另一个单位,因此在科学中不使用英语测量单位系统(英寸、脚、盎司等),使用衡量系统是因为所有计量单位都以10倍的倍数为基础,使转换非常简单。衡量系统最初于1795年在法国建立。国际单位系统是一种基于衡量系统的计量系统。

    SI Base Units of Measurement
    Quantity SI Base Unit Symbol
    Length meter m
    Mass kilogram kg
    Temperature kelvin K
    Time second s
    Amount of a Substance mole mol
    Electric Current ampere A
    Luminous Intensity candela cd

    The first five units are frequently encountered in chemistry . All other measurement quantities, such as volume , force, and energy , can be derived from these seven base units.
    ::前五个单元在化学中经常遇到,所有其他测量数量,如体积、力和能量,都可以从这七个基单元中得出。

    Metal bar with a grooved surface, possibly used for precision measurements.
    Meter standard.
    A glass bell jar covering a cylindrical weight on a base.
    Kilogram standard

      

     

    The map in Figure shows the adoption of the SI units in countries around the world. The United States has legally adopted the metric system for measurements, but does not use it in everyday practice. Great Britain and much of Canada use a combination of metric and imperial units.
    ::图中图示了世界各国采用SI单元的情况,美国合法采用了衡量标准系统,但并未在日常实践中使用。 英国和加拿大大部分地区使用衡量标准和帝国单位的组合。

    Map showing countries that have adopted SI units, predominantly in green.
    Areas of world using metric system.

     

    Summary
    ::摘要

    • The SI system is based on multiples of ten.
      ::SI系统以10倍的倍数为基础。
    • There are seven basic units in the SI system.
      ::SI系统有7个基本单元。
    • Five of these units are commonly used in chemistry.
      ::其中五个装置通常用于化学。

    Review
    ::回顾

    1. What does SI stand for?
      ::SI代表什么?
    2. When was this system adopted by the international community?
      ::国际社会何时采用了这一制度?
    3. Which of the units are commonly used in chemistry?
      ::哪些单位通常用于化学?