📚 IGCSE WJEC Physics: Particle Physics Key Points | IGCSE WJEC 物理：粒子物理考点精讲
Particle physics in the IGCSE WJEC Physics syllabus focuses on the fundamental constituents of atoms, the nature of radioactivity, and how nuclear processes can be described, measured and applied. Mastering these ideas is essential not only for the exam but also for understanding the physical world at its smallest scale. This article walks you through every key point, from atomic structure and isotopes to fission, fusion and radiation safety.
IGCSE WJEC 物理大纲中的粒子物理部分重点关注原子的基本组成、放射性的本质，以及如何描述、测量和应用核过程。掌握这些概念不仅是为了应对考试，更是为了从最微小的尺度理解物理世界。本文将带你逐一梳理从原子结构和同位素到裂变、聚变及辐射安全的所有核心考点。
1. The Atom and its Particles | 原子及其粒子
An atom consists of a small, dense nucleus surrounded by electrons in shells. The nucleus contains protons and neutrons (collectively called nucleons). Protons carry a positive charge, neutrons are neutral, and electrons carry a negative charge. Almost all the mass of the atom is concentrated in the nucleus, while the electrons occupy most of the volume.
原子由一个微小致密的原子核和在其外层壳中运动的电子组成。原子核包含质子和中子（统称核子）。质子带正电，中子不带电，电子带负电。几乎原子的全部质量都集中在原子核内，而电子则占据了绝大部分体积。
The relative masses and charges of these subatomic particles are fundamental data you must memorise. The proton and neutron each have a relative mass of 1, while the electron has a relative mass of approximately 1/1836 (often taken as negligible). In terms of charge, the proton has a relative charge of +1, the electron –1, and the neutron 0.
你必须牢记这些亚原子粒子的相对质量和电荷。质子和中子的相对质量均为 1，而电子的相对质量约为 1/1836（通常视为可忽略）。电荷方面，质子的相对电荷为 +1，电子为 –1，中子为 0。
| Particle / 粒子 | Relative mass / 相对质量 | Relative charge / 相对电荷 |
|---|---|---|
| Proton / 质子 | 1 | +1 |
| Neutron / 中子 | 1 | 0 |
| Electron / 电子 | ≈ 0 (1/1836) / 约 0 (1/1836) | –1 |
2. Atomic and Mass Numbers | 原子序数与质量数
The atomic number (Z) is the number of protons in the nucleus. It defines the element. The mass number (A) is the total number of protons and neutrons (nucleons) in the nucleus. An atom is represented as ᴬZX, where X is the chemical symbol.
原子序数（Z）是原子核中的质子数，它决定了元素种类。质量数（A）是原子核中质子和中子（核子）的总数。一个原子通常表示为 ᴬZX，其中 X 是元素符号。
For a neutral atom, the number of electrons equals the number of protons. The number of neutrons is given by A – Z. In WJEC IGCSE, you will frequently need to calculate neutron numbers from given data or identify an isotope.
对于中性原子，电子数等于质子数。中子数的计算为 A – Z。在 WJEC IGCSE 考试中，你经常需要根据给定的数据计算中子数，或者识别同位素。
3. Isotopes | 同位素
Isotopes are atoms of the same element (same atomic number Z) but with different mass numbers (A), meaning they have different numbers of neutrons. For example, carbon-12 (¹²₆C) and carbon-14 (¹⁴₆C) are isotopes. They share identical chemical properties because their electron arrangements are the same, but their physical properties such as mass and nuclear stability may differ.
同位素是同一元素（原子序数 Z 相同）但质量数（A）不同的原子，即它们的中子数不同。例如，碳-12（¹²₆C）和碳-14（¹⁴₆C）是同位素。由于它们的电子排布相同，因此具有完全相同的化学性质，但质量、核稳定性等物理性质可能不同。
Some isotopes are stable, while others are radioactive (radioisotopes). Radioisotopes undergo decay to become more stable, emitting radiation in the process. Understanding isotopes is essential for interpreting nuclear equations and applications like carbon dating.
有些同位素是稳定的，有些则是放射性同位素。放射性同位素通过衰变趋向稳定，并在衰变过程中放出辐射。理解同位素对于解读核方程以及碳定年法之类的应用至关重要。
4. Radioactive Decay | 放射性衰变
Radioactive decay is the spontaneous, random emission of radiation from an unstable nucleus. The nucleus may emit an alpha particle, a beta particle, or gamma radiation. Decay is not affected by external conditions such as temperature or pressure. The process changes the nucleus into a different nuclide, often of a different element.
放射性衰变是不稳定原子核自发、随机地放出辐射的过程。原子核可以放射出 α 粒子、β 粒子或 γ 射线。衰变不受温度、压力等外部条件影响。这一过程使原子核转变成另一种核素，通常成为不同元素的原子核。
WJEC requires you to describe the nature, penetration and ionising ability of the three types of radiation. Alpha radiation consists of helium nuclei (₂⁴He), beta radiation is high-speed electrons (₋₁⁰e), and gamma radiation is an electromagnetic wave of very short wavelength. All three originate from the nucleus.
WJEC 要求你描述三种辐射的本质、穿透能力和电离能力。α 辐射是氦核（₂⁴He），β 辐射是高速电子（₋₁⁰e），γ 辐射是波长极短的电磁波。三者都来自原子核。
5. Properties of Alpha, Beta and Gamma | α、β 和 γ 辐射的性质
Alpha particles are highly ionising but have low penetrating power; they can be stopped by a sheet of paper or a few centimetres of air. Beta particles are moderately ionising and can travel through air for about one metre; they are stopped by a few millimetres of aluminium. Gamma rays are weakly ionising but extremely penetrating; thick lead or several metres of concrete are needed to reduce them significantly.
α 粒子电离能力很强，但穿透能力很弱；一张纸或几厘米的空气就能阻挡它们。β 粒子电离能力中等，在空气中可穿行约一米；几毫米厚的铝就能将其阻挡。γ 射线电离能力极弱，但穿透能力非常强；需要厚铅板或几米厚的混凝土才能显著减弱它们。
In electric and magnetic fields, alpha (positive) and beta (negative) particles are deflected in opposite directions; gamma rays are not deflected. Alpha particles show the least deflection because of their larger mass. These deflection experiments provide evidence for the charges and masses of the emissions.
在电场和磁场中，带正电的 α 粒子和带负电的 β 粒子会向相反方向偏转；γ 射线则不会偏转。由于 α 粒子质量较大，其偏转幅度最小。这些偏转实验为射线的电荷和质量提供了证据。
6. Detecting Radiation | 探测辐射
Common detectors include the Geiger–Müller (GM) tube, photographic film, and cloud chambers. A GM tube connected to a counter gives a count rate, usually in counts per second. When investigating absorption or half-life, the background count must be subtracted to obtain the corrected count rate.
常见的探测器包括盖革-米勒计数管（GM 管）、照相胶片和云室。GM 管与计数器相连可得出计数率，通常以每秒计数为单位。在研究吸收或半衰期时，必须扣除背景计数以得到修正后的计数率。
For thickness monitoring in industry, a radiation source and detector are placed on opposite sides of a material. The amount of radiation passing through indicates the thickness. Beta emitters are often used for thin metal foils, while gamma emitters are used for thicker materials.
在工业厚度监测中，辐射源和探测器被放置在材料的两侧。从材料穿透过来的辐射量可指示厚度。β 源常用于金属薄箔，而 γ 源则用于较厚的材料。
7. Background Radiation | 背景辐射
Background radiation is the low-level radiation that is always present in the environment. It comes from natural sources such as cosmic rays, radon gas from rocks, and radioactive isotopes in the ground and food, as well as from artificial sources like medical X‑rays and fallout from nuclear weapons testing.
背景辐射是环境中始终存在的低强度辐射。它来自宇宙射线、岩石释放的氡气、土壤和食物中的放射性同位素等天然源，也来自医疗 X 光、核武器试验沉降物等人工源。
When measuring the activity of a radioactive sample, background radiation must be accounted for. The corrected count rate = total count rate − background count rate. The unit of radioactive activity is the becquerel (Bq), where 1 Bq = 1 decay per second.
在测量放射性样品的活度时，必须考虑背景辐射。修正后的计数率 = 总计数率 − 背景计数率。放射性活度的单位是贝克勒尔（Bq），1 Bq = 每秒 1 次衰变。
8. Half-life | 半衰期
Half-life is the time taken for half the radioactive nuclei in a sample to decay, or equivalently, the time taken for the activity (or count rate) to fall to half its initial value. It is a constant for a given isotope and cannot be changed by physical or chemical means.
半衰期是指样品中一半放射性原子核发生衰变所需的时间，或者等效地，活度（或计数率）下降到初始值一半所需的时间。对于给定的同位素，半衰期是一个常数，不随物理或化学条件而改变。
You may be asked to calculate half-life from a decay graph, or predict the activity remaining after several half-lives. For example, after n half-lives, the fraction remaining is (½)ⁿ. This is used in carbon‑14 dating to estimate the age of archaeological samples.
考试可能会要求你根据衰变曲线计算半衰期，或预测若干半衰期后的剩余活度。例如，经过 n 个半衰期后，剩余的比例为 (½)ⁿ。碳‑14 定年法正是利用这一点来估计考古样品的年龄。
Activity after n half-lives = initial activity × (½)ⁿ
9. Nuclear Equations | 核方程
Nuclear equations show the changes in atomic and mass numbers during decay. In alpha decay, the mass number decreases by 4 and the atomic number decreases by 2. For example, uranium-238 undergoes alpha decay to thorium-234: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He.
核方程表示了衰变过程中原子序数和质量数的变化。在 α 衰变中，质量数减少 4，原子序数减少 2。例如，铀-238 经过 α 衰变生成钍-234： ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He。
In beta-minus decay, a neutron turns into a proton and an electron (the beta particle) is emitted. The mass number stays the same while the atomic number increases by 1. An antineutrino is also emitted but is not required in IGCSE equations. Example: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e.
在 β⁻ 衰变中，一个中子转变为质子，同时放出一个电子（β 粒子）。质量数保持不变，而原子序数增加 1。同时还会放出一个反中微子，但在 IGCSE 方程中不作要求。例如： ¹⁴₆C → ¹⁴₇N + ⁰₋₁e。
Gamma emission usually accompanies alpha or beta decay, representing the loss of excess energy. It does not change the atomic or mass number. Nuclear equations must balance in terms of both total mass number and total atomic number on each side.
γ 辐射通常伴随 α 或 β 衰变，表示剩余能量的释放。它不改变原子序数和质量数。核方程必须确保两侧的总质量数和总原子序数分别相等。
10. Nuclear Fission | 核裂变
Nuclear fission is the splitting of a large, unstable nucleus (such as uranium-235 or plutonium-239) into two smaller, more stable daughter nuclei, accompanied by the release of two or three neutrons and a huge amount of energy. It can occur spontaneously or be induced by bombarding the nucleus with a neutron.
核裂变是一个大的不稳定原子核（如铀-235 或钚-239）分裂成两个较小的、较稳定的子核，同时释放出两到三个中子以及巨大能量的过程。裂变可以自发发生，也可以通过中子轰击诱发。
The neutrons released can go on to cause further fissions, leading to a chain reaction. In a nuclear reactor, control rods (e.g., boron) absorb excess neutrons to keep the reaction controlled. The energy released is used to heat water into steam, which drives turbines to generate electricity.
裂变释放出的中子可以引发进一步裂变，形成链式反应。在核反应堆中，控制棒（如硼）吸收多余的中子以保持反应可控。释放的能量用于将水加热成蒸汽，驱动汽轮机发电。
11. Nuclear Fusion | 核聚变
Nuclear fusion is the joining of two light nuclei to form a heavier nucleus, releasing even more energy per unit mass than fission. Fusion powers the Sun and other stars, where hydrogen nuclei fuse to form helium under extremely high temperature and pressure.
核聚变是两个轻原子核结合成一个较重的原子核，单位质量释放的能量比裂变更多。聚变是太阳及其他恒星的能源，在极高的温度和压力下，氢核聚变成氦核。
On Earth, achieving controlled fusion is extremely difficult because positively charged nuclei repel each other. Scientists are developing fusion reactors that use magnetic confinement or laser-driven implosion to achieve the necessary conditions. Fusion produces far less long-lived radioactive waste than fission.
在地球上实现可控聚变极其困难，因为带正电的原子核彼此排斥。科学家们正在研发使用磁约束或激光驱动内爆来实现必要条件的聚变反应堆。聚变产生的长寿命放射性废物远少于裂变。
12. Uses and Dangers of Radiation | 辐射的应用与危险
Radiation has many beneficial uses: medical imaging (gamma cameras), cancer treatment (radiotherapy), sterilising medical equipment (gamma rays), and tracers in industry and medicine (beta/gamma emitters). The choice of isotope depends on half-life and penetrating ability needed for the job.
辐射有很多有益的应用：医学成像（伽马照相机）、癌症治疗（放疗）、医疗设备灭菌（伽马射线）以及工业和医学示踪（β/γ 源）。同位素的选择取决于所需任务的半衰期和穿透能力。
However, ionising radiation can damage living cells, causing mutations or killing them. High doses can lead to radiation sickness and cancer. Safety precautions include using shielding, minimising exposure time, keeping distance from sources, and wearing protective clothing. Radioactive waste must be stored safely for thousands of years.
然而，电离辐射会损伤活细胞，引起突变或将其杀死。高剂量可能导致放射病和癌症。安全措施包括使用屏蔽、减少暴露时间、与放射源保持距离以及穿戴防护服。放射性废物必须安全储存数千年。
Always remember: Time, Distance, Shielding — the three pillars of radiation protection.
时刻牢记：时间、距离、屏蔽——辐射防护三原则。
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