A-level 物理之Particles & Radiation知识点

在A-level学习中，A-level物理是一门比较综合性的学科，涉及很多知识点，比如光电、辐射等，下面，锦秋小编要为大家介绍的是Particles & Radiation的知识点。

1. 原子的构成

原子由质子(protons)和中子(neutrons) (统称为核子(nucleons))和电子(electrons)组成。核子在原子核(nucleus)中, 电子在轨道(orbitals)中。质子和电子具有相等和相反的电荷(charges)。

涉及的定义

Specific charge isthe ratio of charge to mass.

可以求一个原子核(nucleus)或一个离子(ion)的specific charge。

An isotope is a form of an element's atomwith the same proton number but a different number of neutrons.

各种particles的基本数据

2.Forces in the nucleus

强核力(strongnuclear force)在很短的范围内在核子之间起作用。其引力(attractive force)范围在 3 fm和 0.5 fm 之间。在比这更小的距离时, 其表现为斥力(repulsive force)。它通过克服带正电荷的质子(protons)之间的静电排斥(electrostatic repulsion)来将核子(nucleons)连接在一起。

用图片表示为：

3. Types of decay

Betaminus decay is when a neutron turns into a proton. It then emits an electronand an electron antineutrino. 公式表示为：

Betaplus decay is when a proton turns into a neutron. It also emits a positron (theantiparticle of an electron) and an electron neutrino. 公式表示为：

AlphaDecay is when a helium nucleus is emitted, often referred to as an alphaparticle. 公式表示为：

注：中微子(neutrino)的存在是假设在β衰变中能量守恒(conservation of energy)。

4.Photon

首先，光子(photons)的能量具有类似波(wave)的特性。光子的能量可以用以下公式来求：

5. Antimatter

定义：Antiparticleshave the same mass/rest energy but opposite charges and other quantum numberscompared to their normal matter counterparts.

粒子(particles)和相应的反粒子(antiparticles)在消灭反应(annihilationreaction)中相互消灭, 释放两个光子(photon)。光子必须向不同的方向移动, 以保持动量守恒(conservationof momentum)。

6. Pairproduction

定义：Aphoton interacts with a nucleus and its energy is converted into the mass of aparticle and its corresponding antiparticle.

公式表示为：

7. Particleinteractions

Exchangeparticles are the force carriers for the fundamental forces. They transferenergy, momentum, force and sometimes charge. The size of the exchange particledetermines the range of the force; the bigger the particle, the shorter therange.

注：粒子的相互作用用Feynman diagrams来表示

物理学中的相互作用力可分为：

Feynman diagram of Betaminus decay:

Feynman diagram of Betaplus decay:

Electron Capture A proton captures aninner shell electron and turns into a neutron and electron neutrino. A W+ Bosonis exchanged in the process.

Electron-Proton Collision A proton and electron collide and a W- Boson particle is exchanged. The electron turns into an electron neutrino and the proton turns into an eutron.

Electromagnetic repulsion

8. Classificationof particles

Hadronsinteract by the strong nuclearforce and are made of quarks.

Baryons are made from 3 quarks(antibaryons from 3 antiquarks) and all decay into a proton, the only stablebaryon. The two baryons which need to be known are protons and neutrons.

Mesonsare made from a quark and antiquark pair. They include pions (the exchangeparticle in the strong nuclear force) and heavier kaons which decay into pions.

Leptons are fundamental particles thatdon’t interact by the strong interaction, only the weak interaction.

Electrons are stable leptons; muons are heavier leptons which decayinto electrons. There are alsoneutrinos:the electron neutrino and the muon neutrino, which have almost no mass and nocharge.

9.Quarks

注：Quarks只存在三个一组(如baryons) 或者一个quark 和一个antiquark(mesons).

10. Conservation

当考虑一个变化时，小伙伴们需要考虑以下几点：

Charge: Always indicated by theparticle.

Baryon number: 0 except for baryonswhich are +1

Lµ:0 except for a muon and muon neutrino which are +1

Le:0 except for an electron and electron neutrino which are +1

Strangeness: K+ and Koare +1, K- and anti-Ko are -1.

注：

1. 对于antiparticle来说，以上数值均变为原来的相反数，如+1变-1，但是0还是0.

2. 只有strange quarks 有strangeness number (-1).

3. 在强相互作用力中Strangeness守恒，但在弱相互作用力中它会变为0, +1或-1.

4. 当判断相互作用是否发生时，要判断quantum numbers，energy 和momentum 是否守恒，守恒即会发生。

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