HKDSE Physics Elective
Chapter 1: Electrostatics | 靜電學
Billy Sir’s Smart Notes: Master Charges, Electric Fields, Coulomb’s Law, and Applications.
由中大物理系碩士 Billy Sir 編寫，助你極速掌握電荷、電場、庫侖定律及靜電應用。
1. Basics of Charges, Attraction & Repulsion | 電荷基礎與引力斥力
There are two types of electric charges: positive (+) and negative (-). The SI unit of charge is the Coulomb (C). 電荷分為正電荷(+)和負電荷(-)。電荷的國際單位是庫侖(C)。
🧲 Fundamental Law of Electrostatics | 靜電學基本定律
- Like charges repel: Positive repels positive, negative repels negative. (同性電荷相斥)
- Unlike charges attract: Positive attracts negative. (異性電荷相吸)
2. Charging Processes | 起電過程
| Process (過程) | Mechanism (機制) |
|---|---|
| Friction 摩擦起電 |
Rubbing two different insulators together transfers electrons from one to the other. (摩擦兩種不同的絕緣體，使電子從一個轉移到另一個) |
| Induction 感應起電 |
Bringing a charged object near a neutral conductor causes charge separation. Grounding it leaves it with a net charge. (將帶電體靠近中性導體導致電荷分離，接地後使其帶淨電荷) |
| Sharing / Contact 接觸起電 |
Touching a charged conductor to a neutral one shares the charge between them. (帶電導體接觸中性導體，使電荷在兩者間分配) |
3. Breakdown of Air Insulation & Lightning | 空氣絕緣擊穿與閃電
Air is normally an insulator. However, under a very strong electric field, air molecules can be ionized, turning air into a conductor. 空氣通常是絕緣體。但在極強的電場下，空氣分子會被電離，使空氣變成導體。
⚡ Lightning & Point Action | 閃電與尖端放電
- Spark Discharge: The sudden flow of charges through ionized air, producing light and heat. (火花放電：電荷突然流過電離的空氣，產生光和熱。)
- Lightning: A massive natural spark discharge between clouds or between a cloud and the ground. (閃電：雲層之間或雲層與地面之間巨大的自然火花放電。)
- Lightning Rods: Use “point action” (charges concentrate at sharp points) to safely guide lightning to the ground. (避雷針：利用「尖端放電」安全地將閃電引導至地面。)
4. Applications of Electrostatics | 靜電的應用
Electrostatics is widely used in daily life and industry: 靜電在日常生活和工業中廣泛應用：
- Photocopiers & Laser Printers: Use static electricity to attract toner to specific areas on a drum. (影印機和激光打印機：利用靜電將碳粉吸引到滾筒的特定區域。)
- Electrostatic Precipitators: Remove dust and smoke particles from factory chimneys by charging them and attracting them to collecting plates. (靜電除塵器：使工廠煙囪中的灰塵和煙霧微粒帶電，並將其吸引到收集板上。)
- Spray Painting: Paint droplets are given the same charge so they repel each other (fine mist) and are attracted to the grounded object being painted. (靜電噴漆：漆滴帶同種電荷互相排斥形成細霧，並被吸引到接地的待漆物體上。)
5. Coulomb’s Law | 庫侖定律
Calculates the electric force between two point charges. 計算兩個點電荷之間的電場力。
📐 Formula | 公式
\( F \): Force (N), \( Q_1, Q_2 \): Charges (C), \( r \): Distance (m), \( \varepsilon_0 \): Permittivity of free space. (力、電荷、距離、真空電容率)
6. Electric Field & Electric Force | 電場與電場力
An electric field is a region where a charge experiences an electric force. 電場是電荷受到電場力的區域。
🌐 Electric Field Intensity (E) | 電場強度
Defined as the electric force per unit positive charge. (定義為單位正電荷所受的電場力。)
Direction: The direction of the force acting on a positive test charge. (方向：正測試電荷所受力的方向。)
7. Electric Field of a Point Charge & Parallel Plates | 點電荷與平行板的電場
| Field Source (電場來源) | Characteristics (特徵) | Formula (公式) |
|---|---|---|
| Point Charge 點電荷 |
Radial field. Field strength decreases with the square of the distance. (輻射狀電場。電場強度隨距離的平方遞減。) | $$ E = \frac{Q}{4\pi\varepsilon_0 r^2} $$ |
| Parallel Plates 平行板 |
Uniform field between the plates (except at the edges). Field lines are parallel and equally spaced. (板間為均勻電場。電場線平行且等距。) | $$ E = \frac{V}{d} $$ |
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