ShortVersion20.ElectricCharge,Force,Fields短版20.pptVIP

Exploded view of a TN (Twisted Nematic) liquid crystal cell showing the states in an OFF state (left), and an ON state with voltage applied (right) 超扭轉向列型液晶單元的放大圖：關閉狀態(左圖) ，開放(加電壓後)狀態(右圖) 。 Conductors, Insulators, Dielectrics導體，絕緣體，和電介體 Bulk matter consists of point charges: e p. 大塊的物質由點電荷組成 Conductors: charges free to move ( ? electric currents ), 導體: 電荷自由移動 (?電流 ) e.g., e (metal), ion ( electrolytes ), e+ion (plasma). 例： e(金屬) ，離子(電解質) ，e+離子(電漿) Insulators: charges are bounded. 絕緣體: 電荷受縛 Dielectrics: insulators with intrinsic / induced dipoles. 電介體: 有內建或感應雙極的絕緣體 Induced dipole 感應雙極 Alignment of intrinsic dipoles. 內建雙極的對齊 internal field from dipoles 雙極所生內場 Short Version : 20. Electric Charge, Force, Fields 短版: 20. 電荷，力，和場 20.1. Electric Charge 電荷 2 kinds of charges: + ?. 兩種電荷: 正和負 Total charge = algebraic sum of all charges. 總電荷 = 全部電荷的和 Like charges repel. Opposite charges attract. 同性相斥，異性相吸。 All electrons have charge ?e. 所有電子都帶 ?e 電荷. All protons have charge +e. 所有電子都帶 +e 電荷. = elementary charge 基本電荷 Theory (standard model) : basic unit of charge (carried by quark) = 1/3 e. 理論 (標準模型) ： 基本電荷單位 (由夸克帶著) = 1/3 e. Quark confinement ? no free quark can be observed. 夸克禁閉 ? 無法量得自由夸克 ? Smallest observable charge is e. ? 量得到的最小電荷是 e。 1st measured by Millikan on oil drops. 首先由密立根在油滴上量得。 Conservation of charge: total charge in a closed region is always the same. 電荷守恆：在一個密閉系统中總電荷永遠不變。 Coulomb’s law (force between 2 point charges) : 庫倫定律 (兩點電荷間的力) : [q] = Coulomb = C Conceptual Example 20.1. Gravity Electric Force 重力和電力 The electric force is far stronger than the gravitational force, 電力比重力強很多， yet gravity is much more obvious in everyday life. 可是重力在日常生活中卻比較明顯。 Why? 為甚麼? Only 1 kind of gravitational “charge” 祗有一種重力 “荷” forces from different parts of a source tend to reinforce. 一個質源各部份產生的力傾向於相增。 2 kinds of electric charges 有兩種電荷 forces from different parts of a neutral source tend to cancel