HKDSE Physics
Chapter 3: Nuclear Energy | 核能
Billy Sir’s Smart Notes: Master Mass Defect, Fission, Fusion, and Nuclear Reactors.
由中大物理系碩士 Billy Sir 編寫，助你極速掌握質量虧損、核裂變、核聚變及核反應堆。
1. Mass Defect and Energy Release | 質量虧損與能量釋放
In nuclear reactions, the total mass of the products is always slightly less than the total mass of the reactants. This missing mass is called the mass defect (\( \Delta m \)), and it is converted entirely into energy. 在核反應中，生成物的總質量總是略小於反應物的總質量。這失去的質量稱為質量虧損，它會完全轉化為能量釋放出來。
⚡ Einstein’s Mass-Energy Equivalence | 愛因斯坦質能等價公式
- \( E \): Energy released (in Joules, J) (釋放的能量，單位為焦耳)
- \( \Delta m \): Mass defect (in kg) (質量虧損，單位為公斤)
- \( c \): Speed of light (\( 3 \times 10^8 \text{ m s}^{-1} \)) (光速)
📏 Atomic Mass Unit & Alternative Calculation | 原子質量單位與替代計算法
Definition: One atomic mass unit (\( 1 \text{ u} \)) is defined as exactly one-twelfth (\( \frac{1}{12} \)) of the mass of a carbon-12 (\( ^{12}\text{C} \)) atom. (定義：一個原子質量單位定義為一個碳-12原子質量的十二分之一。)
If the mass defect is given in atomic mass units (\( \text{u} \)), we can directly convert it to energy in Mega-electronvolts (\( \text{MeV} \)):
Therefore, Energy Released \( E = \Delta m \text{ (in u)} \times 931 \text{ MeV} \).
2. Nuclear Fission vs. Nuclear Fusion | 核裂變與核聚變
| Process (過程) | Definition (定義) | Key Characteristics (主要特點) |
|---|---|---|
| Nuclear Fission 核裂變 |
A heavy nucleus (e.g., Uranium-235) splits into two lighter nuclei after absorbing a slow-moving neutron. (一個重原子核（如鈾-235）吸收慢中子後分裂成兩個較輕的原子核) | Produces extra neutrons, which can trigger a continuous chain reaction. Used in current nuclear power plants. (產生額外的中子，可引發持續的連鎖反應。用於現時的核電廠) |
| Nuclear Fusion 核聚變 |
Two light nuclei (e.g., Hydrogen isotopes) combine to form a heavier nucleus. (兩個輕原子核（如氫的同位素）結合成一個較重的原子核) | Releases much more energy per unit mass than fission. Requires extremely high temperature and pressure (occurs naturally in the Sun). (單位質量釋放的能量比裂變多。需要極高溫和高壓（自然發生在太陽中）) |
3. Nuclear Power Plants & Associated Problems | 核電廠與相關問題
Nuclear power plants use controlled nuclear fission to generate heat. This heat boils water to produce high-pressure steam, which then drives turbines to generate electricity. 核電廠利用受控的核裂變產生熱能。熱能將水煮沸產生高壓蒸汽，推動渦輪機發電。
⚠️ Problems with Fission Reactors | 裂變反應堆的問題
- Radioactive Waste (放射性廢料): The spent fuel rods and by-products are highly radioactive and have very long half-lives. They require safe, long-term storage deep underground to prevent environmental contamination. (用過的燃料棒和副產品具有強放射性且半衰期極長，需要深埋地下進行安全的長期儲存，以防污染環境。)
- Risk of Accidents / Meltdown (事故/熔毀風險): If the cooling system fails, the reactor core can overheat and melt (a meltdown). This can potentially release massive amounts of radioactive materials into the environment (e.g., Chernobyl, Fukushima). (如果冷卻系統出現故障，反應堆核心可能會過熱並熔毀，可能向環境釋放大量放射性物質（如切爾諾貝爾、福島核災）。)
- Thermal Pollution (熱污染): Discharging large amounts of hot water used for cooling into nearby rivers or oceans can decrease dissolved oxygen levels and harm aquatic ecosystems. (將大量用於冷卻的熱水排放到附近的河流或海洋中，會降低水中溶氧量並損害水生生態系統。)
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