13.2.10
Particle Interactions
The Four Fundamental Forces
The Four Fundamental Forces
Nature is governed by four fundamental forces:


Gravity
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
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
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
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
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
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
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
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
Feynman Diagrams
We can represent particle interactions using Feynman diagrams.


General rules
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
Electromagnetic repulsion
- Two electrons repel each other because of the electromagnetic force.
- The exchange particle is a virtual photon.


Beta-minus decay
Beta-minus decay
- n → p + e- + νe
- A neutron decays into a proton and W- boson which then decays into an electron and an electron antineutrino.


Beta-plus decay
Beta-plus decay
- p → n + e+ + νe
- A proton decays into a neutron and W+ boson, which then decays into a positron and an electron neutrino.


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


Electron-proton collision
Electron-proton collision
- p + e- → n + νe
- 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.
1Physical Quantities & Units
2Measurement Techniques
3Kinematics
4Dynamics
4.1Momentum & Newton's Laws of Motion
4.2Non-Uniform Motion
4.3Linear Momentum & Conservation
4.4Force, Density & Pressure
4.4.1Fields
4.4.2Force in Uniform Fields
4.4.3Friction
4.4.4Buoyancy
4.4.5Terminal Speed
4.4.6End of Topic Test - Acceleration Due to Gravity
4.4.7Centre of Mass
4.4.8Forces & Equilibrium
4.4.9End of Topic Test - Scalars & Vectors
4.4.10Moments
4.4.11End of Topic Test - Moments & Centre of Mass
4.4.12Density
4.4.13Pressure
4.5Work, Energy & Power
5Gravitational Fields
5.1Gravitational Fields (A2 only)
6Deformation of Solids
7Thermal Physics
7.1Thermal Physics
7.1.1Temperature
7.1.2Measuring Temperature
7.1.3Ideal Gas Law
7.1.4Ideal Gases
7.1.5Boyle's Law & Charles' Law
7.1.6Molecular Kinetic Theory Model
7.1.7Molecular Kinetic Theory Model 2
7.1.8Thermal Energy Transfer
7.1.9Thermal Energy Transfer Experiments
7.1.10End of Topic Test - Thermal Energy & Ideal Gases
7.1.11First Law of Thermodynamics
8Oscillations
8.1Simple Harmonic Motion
8.2Waves
8.2.1Progressive Waves
8.2.2Intensity of Waves
8.2.3Wave Speed & Phase Difference
8.2.4Longitudinal & Transverse Waves
8.2.5End of Topic Test - Progressive Waves
8.2.6Electromagnetic Waves
8.2.7Doppler Effect
8.2.8Sound Waves
8.2.9Measuring Sound Waves
8.2.10End of Topic Test - Waves
8.2.11Ultrasound Imaging
8.2.12Ultrasound Imaging 2
9Communication
9.1Communication Channels
9.2Digital Communication
10Electric Fields
10.1Electric Fields
11Current Electricity
11.1Current Electricity
11.1.1Basics of Electricity
11.1.2Mean Drift Velocity
11.1.3Current-Voltage Characteristics
11.1.4End of Topic Test - Basics of Electricity
11.1.5Resistivity
11.1.6End of Topic Test - Resistivity & Superconductors
11.1.7Power and Conservation
11.1.8Microphones
11.1.9Components
11.1.10Relays
11.1.11Strain Gauges
12Magnetic Fields
12.1Magnetic Fields
13Modern Physics
13.1Quantum Physics
13.1.1The Photoelectric Effect
13.1.2The Photoelectric Effect Explanation
13.1.3End of Topic Test - The Photoelectric Effect
13.1.4Collisions of Electrons with Atoms
13.1.5Energy Levels & Photon Emission
13.1.6Wave-Particle Duality
13.1.7End of Topic Test - Absorption & Emission
13.1.8Band Theory
13.1.9Diagnostic X-Rays
13.1.10X-Ray Image Processing
13.1.11Absorption of X-Rays
13.1.12CT Scanners
13.2Nuclear Physics
13.2.1Rutherford Scattering
13.2.2Atomic Model
13.2.3Isotopes
13.2.4Stable & Unstable Nuclei
13.2.5A-A* (AO3/4) - Stable & Unstable Nuclei
13.2.6Alpha & Beta Radiation
13.2.7Gamma Radiation
13.2.8Particles, Antiparticles & Photons
13.2.9Quarks & Antiquarks
13.2.10Particle Interactions
13.2.11Radioactive Decay
13.2.12Half Life
13.2.13End of Topic Test - Radioactivity
13.2.14Nuclear Instability
13.2.15Mass & Energy
13.2.16Binding Energy
13.2.17A-A* (AO3/4) - Nuclear Fusion
Jump to other topics
1Physical Quantities & Units
2Measurement Techniques
3Kinematics
4Dynamics
4.1Momentum & Newton's Laws of Motion
4.2Non-Uniform Motion
4.3Linear Momentum & Conservation
4.4Force, Density & Pressure
4.4.1Fields
4.4.2Force in Uniform Fields
4.4.3Friction
4.4.4Buoyancy
4.4.5Terminal Speed
4.4.6End of Topic Test - Acceleration Due to Gravity
4.4.7Centre of Mass
4.4.8Forces & Equilibrium
4.4.9End of Topic Test - Scalars & Vectors
4.4.10Moments
4.4.11End of Topic Test - Moments & Centre of Mass
4.4.12Density
4.4.13Pressure
4.5Work, Energy & Power
5Gravitational Fields
5.1Gravitational Fields (A2 only)
6Deformation of Solids
7Thermal Physics
7.1Thermal Physics
7.1.1Temperature
7.1.2Measuring Temperature
7.1.3Ideal Gas Law
7.1.4Ideal Gases
7.1.5Boyle's Law & Charles' Law
7.1.6Molecular Kinetic Theory Model
7.1.7Molecular Kinetic Theory Model 2
7.1.8Thermal Energy Transfer
7.1.9Thermal Energy Transfer Experiments
7.1.10End of Topic Test - Thermal Energy & Ideal Gases
7.1.11First Law of Thermodynamics
8Oscillations
8.1Simple Harmonic Motion
8.2Waves
8.2.1Progressive Waves
8.2.2Intensity of Waves
8.2.3Wave Speed & Phase Difference
8.2.4Longitudinal & Transverse Waves
8.2.5End of Topic Test - Progressive Waves
8.2.6Electromagnetic Waves
8.2.7Doppler Effect
8.2.8Sound Waves
8.2.9Measuring Sound Waves
8.2.10End of Topic Test - Waves
8.2.11Ultrasound Imaging
8.2.12Ultrasound Imaging 2
9Communication
9.1Communication Channels
9.2Digital Communication
10Electric Fields
10.1Electric Fields
11Current Electricity
11.1Current Electricity
11.1.1Basics of Electricity
11.1.2Mean Drift Velocity
11.1.3Current-Voltage Characteristics
11.1.4End of Topic Test - Basics of Electricity
11.1.5Resistivity
11.1.6End of Topic Test - Resistivity & Superconductors
11.1.7Power and Conservation
11.1.8Microphones
11.1.9Components
11.1.10Relays
11.1.11Strain Gauges
12Magnetic Fields
12.1Magnetic Fields
13Modern Physics
13.1Quantum Physics
13.1.1The Photoelectric Effect
13.1.2The Photoelectric Effect Explanation
13.1.3End of Topic Test - The Photoelectric Effect
13.1.4Collisions of Electrons with Atoms
13.1.5Energy Levels & Photon Emission
13.1.6Wave-Particle Duality
13.1.7End of Topic Test - Absorption & Emission
13.1.8Band Theory
13.1.9Diagnostic X-Rays
13.1.10X-Ray Image Processing
13.1.11Absorption of X-Rays
13.1.12CT Scanners
13.2Nuclear Physics
13.2.1Rutherford Scattering
13.2.2Atomic Model
13.2.3Isotopes
13.2.4Stable & Unstable Nuclei
13.2.5A-A* (AO3/4) - Stable & Unstable Nuclei
13.2.6Alpha & Beta Radiation
13.2.7Gamma Radiation
13.2.8Particles, Antiparticles & Photons
13.2.9Quarks & Antiquarks
13.2.10Particle Interactions
13.2.11Radioactive Decay
13.2.12Half Life
13.2.13End of Topic Test - Radioactivity
13.2.14Nuclear Instability
13.2.15Mass & Energy
13.2.16Binding Energy
13.2.17A-A* (AO3/4) - Nuclear Fusion
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