Particle Interactions

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The Four Fundamental Forces

Nature is governed by four fundamental forces:

Gravity

Gravity is the weakest of the four forces.
- It is so weak that its effects are only noticeable for huge masses like stars and planets.
Gravity is a purely attractive force.
Gravity is experienced by all matter.

Strong nuclear force

The strong nuclear force is the strongest of the four fundamental forces.
But, it can only be felt over a very short range (a few fm).
The strong nuclear force is only experienced by hadrons (e.g. protons and neutrons).
The strong nuclear force is attractive for separations above about 0.5 fm, but strongly repulsive for separations less than about 0.5 fm.

Weak nuclear force

The weak nuclear force affects all types of particles.
The weak nuclear force is responsible for beta-plus and beta-minus decay, as well as electron capture interactions.
The weak nuclear force is a very weak, very short range force.

Electromagnetic force

The electromagnetic force is very strong and has a very large range.
The electromagnetic force is responsible for interaction between charged objects like electrons and protons.
Most everyday forces we experience are because of the electromagnetic force.
- When you touch an object, the force you feel is because of the repulsion between the outer electrons on the object and your hand.

Exchange Particles

Two objects cannot interact instantaneously over a finite distance. For the objects to feel a force, an exchange particle must travel from one to the other.

Types of exchange particle

Each fundamental force has its own exchange particle.
The gluon is the exchange particle for the strong nuclear force.
The exchange particle of the electromagnetic force is called a 'virtual photon' (virtual because they only exist for a very short time).
The weak nuclear force has three exchange particles: the W⁺, W^- and Z⁰ bosons.
Exchange particles are sometimes called gauge bosons.

Repulsion

Think of two particles as skaters on an ice rink (with no friction).
If one skater throws a ball straight at the other, both skaters will be pushed further apart as momentum is exchanged.

Attraction

Think again about our two skaters.
Imagine this time that the boomerang is thrown away from himself.
The boomerang circles round to the other skater and pushes him towards the first skater, bringing them closer.

Feynman Diagrams

We can represent particle interactions using Feynman diagrams.

General rules

Particles start at the bottom and move upwards.
Particles have straight lines. Exchange particles have wiggly lines.
Hadrons must stay on the left, leptons on the right.
Particles cannot cross paths, they can only interact via an exchange particle.
The charge entering a node must equal the charge leaving the node.
A W⁺ boson travelling from left to right is the same as a W^- boson travelling from right to left.

Electromagnetic repulsion

Two electrons repel each other because of the electromagnetic force.
The exchange particle is a virtual photon.

Beta-minus decay

n → p + e^- + ν_e
A neutron decays into a proton and W^- boson which then decays into an electron and an electron antineutrino.

Beta-plus decay

p → n + e⁺ + ν_e
A proton decays into a neutron and W⁺ boson, which then decays into a positron and an electron neutrino.

Electron capture

p + e^- → n + ν_e
A proton interacts with an electron via a W⁺ boson, producing a neutron and an electron neutrino.

Electron-proton collision

p + e^- → n + ν_e
The equation is identical to electron capture but the diagram is different.
- The diagram shows an electron colliding with a proton via the W^- boson (travelling the other way to a W⁺ boson), producing a neutron and an electron neutrino.

1Space, Time & Motion

1.1Motion

1.1.1Motion Basics

1.1.2Graphs of Motion

1.1.3Uniform Acceleration Equations

1.1.4Projectile Motion

1.1.5Bouncing Ball Example

1.2Forces

1.2.1Friction

1.2.2Terminal Speed

1.2.3Newton's Laws

1.2.4Exam-Style Question - Forces

1.2.5End of Topic Test - Newton's Third Law

1.3Momentum & Impulse

1.3.1Momentum

1.3.2Momentum 2

1.3.3End of Topic Test - Newton's Laws & Momentum

1.4Work, Energy & Power

1.4.1Work & Energy

1.4.2Conservation of Energy

1.4.3Power & Efficiency

1.4.4IB Multiple Choice- Work, Energy & Power

2The Particulate Nature of Matter

2.1Thermal Concepts

2.1.1Molecular Kinetic Theory Model

2.1.2Molecular Kinetic Theory Model 2

2.1.3Thermal Energy Transfer

2.1.4Thermal Energy Transfer Experiments

2.1.5Thermal Equilibrium

2.1.6Thermal Conductivity

2.1.7Heat Energy Transfer

2.2Modelling a Gas

2.2.1Ideal Gases

2.2.2Ideal Gases 2

2.2.3End of Topic Test - Thermal Energy & Ideal Gases

2.2.4Exam-Style Question - Ideal Gases

3Wave Behaviour

3.1Oscillations

3.1.1Progressive Waves

3.1.2Simple Harmonic Motion

3.2Travelling Waves

3.2.1Wave Speed & Phase Difference

3.2.2Longitudinal & Transverse Waves

3.2.3Electromagnetic Waves

3.2.4Electromagnetic Waves 2

3.2.5Sound Waves

3.3Wave Characteristics

3.3.1Polarisation

3.4Wave Behaviour

3.4.1Reflection & Absorption

3.4.2Reflection of Light

3.4.3Refraction

3.4.4Refractive index

3.5Standing Waves

3.5.1Standing Waves

3.5.2Standing Waves 2

3.5.3End of Topic Test - Standing Waves

3.5.4IB Multiple Choice - Waves

3.6Simple Harmonic Motion

3.6.1Simple Harmonic Motion

3.6.2Simple Harmonic Systems

3.6.3Energy in Simple Harmonic Motion

3.6.4End of Topic Test - Simple Harmonic Motion

3.6.5End of Topic Test - Simple Harmonic Motion

3.7Single Slit Diffraction

3.7.1Diffraction

3.8Interference

3.8.1Interference

3.8.2Interference 2

3.8.3Diffraction Gratings

3.8.4End of Topic Test - Diffraction

3.9Doppler Effect

3.9.1Doppler Effect

4Fields

4.1Circular Motion

4.1.1Circular Motion

4.1.2Centripetal Force

4.1.3IB Multiple Choice - Circular Motion

4.2Newton's Law of Gravitation

4.2.1Newton's Law

4.2.2Orbits of Planets & Satellites

4.2.3Escape Velocity & Synchronous Orbits

4.2.4End of Topic Test - Gravitational Fields

4.3Fields

4.3.1Fields

4.3.2Gravitational Field Strength

4.3.3Gravitational Potential

4.3.4Electric Field Strength

4.3.5Electrostatic & Gravitational Forces

4.3.6Electric Potential

4.3.7IB Multiple Choice - Gravitational Fields

4.4Fields at Work

4.4.1Radial Field Strength

4.4.2Orbits of Planets & Satellites

4.4.3Escape Velocity & Synchronous Orbits

4.4.4End of Topic Test - Gravitational Fields

4.4.5Uniform Electric Fields

4.4.6IB Multiple Choice - Electric Field & Potential

4.5Electric Fields

4.5.1Electric Charge

4.5.2Conservation of Charge

4.5.3Coulomb's Law

4.5.4Current

4.5.5Potential Difference

4.5.6IB Multiple Choice - Electric Charge

4.6Magnetic Effect of Electric Currents

4.6.1Magnetism

4.6.2Magnetic Field

4.6.3Magnetic Force

4.6.4Moving Charges in a Magnetic Field

4.6.5IB Multiple Choice - Magnetism

4.7Heating Effect of Currents

4.7.1Circuits

4.7.2Resistance

4.7.3Power and Conservation

4.7.4IB Multiple Choice - DC Circuits

4.8Electromagnetic Induction

4.8.1Electromagnetic Induction

4.8.2Electromagnetic Induction 2

4.8.3Magnetic Flux & Flux Linkage

4.8.4End of Topic Test - Electromagnetic Induction

4.9Power Generation & Transmission

4.9.1Alternating Currents

4.9.2Operation of a Transformer

4.10Capacitance

4.10.1Capacitance

4.10.2Parallel Plate Capacitor

4.10.3Energy Stored by a Capacitor

4.10.4Capacitor Discharge

4.10.5Capacitor Charge

4.10.6Magnetic Flux Density

4.10.7End of Topic Test - Capacitance & Flux Density

5Nuclear & Quantum Physics

5.1Discrete Energy & Radioactivity

5.1.1Atomic Spectra

5.1.2Atomic Structure

5.1.3Photons

5.1.4Discrete Energy Levels

5.1.5Alpha, Beta & Gamma Radiation

5.1.6Scientific Practices - Unstable Nuclei

5.1.7Radioactive Decay

5.1.8IB Multiple Choice - Atomic Physics

5.2Nuclear Reactions

5.2.1Mass & Energy

5.2.2Nuclear Fission

5.3The Interaction of Matter with Radiation

5.3.1Particles, Antiparticles & Photons

5.3.2Particle Interactions

5.3.3Classification of Particles

5.3.4Wave-Particle Duality

5.3.5Wave Functions & Probability

5.3.6IB Multiple Choice - Quantum Physics

5.4Nuclear Physics

5.4.1Nuclear Instability & Radioactive Decay

5.4.2Nuclear Radius

5.4.3IB Multiple Choice - Nuclear Physics

6Measurements

6.1Measurements & Errors

6.1.1Fundamental Units

6.1.2SI Prefixes, Standard Form & Converting Units

6.1.3End of Topic Test - Units & Prefixes

6.2Uncertainties & Errors

6.2.1Limitation of Physical Measurements

6.2.2Uncertainty

6.2.3Estimation

6.2.4End of Topic Test - Measurements & Errors

6.3Vectors & Scalars

6.3.1Scalars & Vectors

6.3.2Vector Problems

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