The meaning of volt

伏特是國際單位制中電壓的單位，符號V。
在一根均勻的、溫度和寬度恆定的導線上假如有一安培電流流動，那麼導線的電阻在一定的距離內可以將電能轉化為熱能。這個距離之間的電壓差就被定義為一伏特：


圖片參考：http://upload.wikimedia.org/math/0/2/2/0227a9f97f16bc681b905f117f0736e6.png

在國際單位制中安培是基本電學單位，伏特的SI定義是以安培和力學單位瓦特導出的。 依據國際單位制的基本電學單位安培，伏特定義為:"在載荷1A恆定電流的導線上，當兩點之間導線上的功率耗散為1W(1W=1 J/S)時，這兩點之間的電位差"。
此單位是以發明電源的義大利物理學家亞歷山德羅·伏特的名字命名的。
The volt (symbol: V) is the SI derived unit of electric potential difference or electromotive force [1]. It is named in honor of the Italian physicist Alessandro Volta (1745–1827), who invented the voltaic pile, the first chemical battery.
The volt is defined as the potential difference across a conductor when a current of one ampere dissipates one watt of power. Hence, it is the base SI representation m2 · kg · s-3 · A-1, which can be equally represented as one joule of energy per coulomb of charge, J/C.


圖片參考：http://content.answers.com/main/content/wp/en/math/c/e/7/ce7a54a5296725cb897f775a377e47ec.png
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Since 1990 the volt is maintained internationally for practical measurement using the Josephson effect, where a conventional value is used for the Josephson constant, fixed by the 18th General Conference on Weights and Measures as

K{J-90} = 0.4835979 GHz/µV.