HKDSE Physics Core
Chapter 3: Domestic Electricity | 家居用電
Billy Sir’s Smart Notes: Master Household Wiring, Safety Devices, Power Rating, and Electricity Cost.
由中大物理系碩士 Billy Sir 編寫，助你極速掌握家居電路、安全裝置、額定功率及電費計算。
1. Basics of Domestic Electricity | 家居用電基礎
In Hong Kong, the mains supply is Alternating Current (AC) with a voltage of 220 V and a frequency of 50 Hz. 在香港，市電供應為交流電 (AC)，電壓為 220 V，頻率為 50 Hz。
⚡ Rated Voltage & Rated Power | 額定電壓與額定功率
- Rated Voltage (額定電壓): The normal operating voltage of an appliance. (電器正常運作所需的電壓)
- Rated Power (額定功率): The power consumed by the appliance when it operates at its rated voltage. (電器在額定電壓下運作時所消耗的功率)
Example: A heater labeled “220 V, 2000 W” will output 2000 W of power only if connected to a 220 V supply.
2. Wires in a Mains Cable | 電源線中的導線
| Wire (導線) | Color Code (顏色) | Function (功能) |
|---|---|---|
| Live Wire (L) 活線 |
Brown (棕色) | Carries current from the supply to the appliance. It is at high voltage (220V). (將電流從電源傳送到電器，處於高電壓 220V) |
| Neutral Wire (N) 中線 |
Blue (藍色) | Provides a return path for the current. It is maintained at around 0V. (提供電流的返回路徑，電壓維持在約 0V) |
| Earth Wire (E) 接地線 |
Yellow & Green (黃綠相間) | A safety wire connected to the metal case of the appliance. It carries no current under normal conditions. (連接電器金屬外殼的安全線，正常情況下沒有電流通過) |
3. Safety Devices | 安全裝置
⚠️ Golden Rule of Wiring | 接線黃金法則
Fuses, Circuit Breakers, and Switches MUST ALWAYS be connected to the LIVE WIRE.
保險絲、斷路器和開關必須安裝在活線 (Live Wire) 上。
Reason: If installed on the neutral wire, even if the switch is off or the fuse blows, the appliance remains at a high voltage (220V), posing a severe shock hazard! (原因：若安裝在中線上，即使開關關閉或保險絲熔斷，電器仍處於高電壓，有觸電危險！)
| Device (裝置) | Mechanism (運作原理) |
|---|---|
| Fuse 保險絲 |
Contains a thin wire that melts and breaks the circuit when current exceeds its rating due to a short circuit or overload. Needs replacement after blowing. (內有幼線，當電流因短路或超載超過額定值時會熔斷並切斷電路。熔斷後需更換。) |
| Circuit Breaker (MCB) 微型斷路器 |
An electromagnetic switch that automatically “trips” (turns off) when current exceeds the safe limit. Can be easily reset without replacement. (電磁開關，當電流超過安全限制時會自動「跳掣」。可輕鬆重置，無需更換。) |
4. Consumer Unit & Ring Circuit | 消費者電路與環形電路
Electricity enters the house through the mains supply, passes through the kWh meter, and goes to the Consumer Unit (配電箱), which divides the electricity into different parallel circuits (e.g., lighting, sockets, air-conditioners).
⭕ Ring Circuit | 環形電路
A special parallel circuit used for mains sockets. The Live, Neutral, and Earth wires form complete loops (rings) starting and ending at the consumer unit. (用於插座的特殊並聯電路。活線、中線和接地線形成完整的環，起點和終點均在配電箱。)
✅ Advantages of a Ring Circuit | 環形電路的優點：
- Thinner wires can be used: Current can flow to a socket via two different paths, reducing the current in each path. (可使用較幼的導線：電流可經兩條路徑到達插座，減少每條路徑的電流負載。)
- Less voltage drop: The equivalent resistance of the cables is lower. (電壓降較小：電纜的等效電阻較低。)
- Easy to add sockets: New sockets can be easily added anywhere along the ring. (容易增加插座：可在環形電路任何位置輕易加裝新插座。)
5. Power Adjustment & Cost of Electricity | 功率調整與電費
⚙️ Adjustment of Power Rating | 功率調整
If an appliance is operated at a voltage (\( V_{actual} \)) different from its rated voltage (\( V_{rated} \)), its actual power output changes. Assuming resistance (\( R \)) remains constant: (若電器在非額定電壓下運作，其實際輸出功率會改變。假設電阻不變：)
💰 Cost of Electricity | 電費計算
Electricity companies charge based on electrical energy consumed, measured in kilowatt-hours (kWh), commonly known as a “Unit” (度). (電力公司按消耗的電能收費，單位為千瓦小時 (kWh)，俗稱「度」。)
Note: \( 1 \text{ kWh} = 1000 \text{ W} \times 3600 \text{ s} = 3.6 \times 10^6 \text{ Joules (J)} \).
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