Monday 10 April 2023

Particles

Matter and AntimatterFundamental Forces

Matter and AntimatterEvery particle has an equivalent antiparticle. An antiparticle is

like the mirror image of the respective particle. So an antiparticle:

• Has the same mass as the particle• Has opposite charge• It spins in the opposite direction

Particle Anti-particle

Particle SymbolsMost antiparticles are represented by the symbol of the particle with a

bar on top, e.g. p is the symbol for an antiproton. However, some have their own symbol and name.

Fill in the table below.

Particle AntiparticleName Symbol Charge Name Symbol Charge

Electron e- -1 Positron e+ +1

Neutron n 0 Antineutron n 0

Proton p +1 Antiproton p -1

Neutrino ne 0 Antineutrino ne 0

The Photon: a very peculiar particleWe’ve always thought of light as a wave, because it behaves like a wave

in many cases (e.g. refraction, reflection, diffraction…). However, Einstein discovered that in some instances light behaves like a particle. He called these “particles” PHOTONS. His observations extend to all electromagnetic waves.

EM wavesOscillations of electric

and magnetic fields

Carry energy

Higher frequency = higher the energy

Photons

Packets of EM waves

Are packets of energy

Energy depends on frequency

What are they made of?

What are they made of?

What do they carry?

What do they carry?

How is the energy carried affected?

On what does their energy depend?

Representing a photonSo, why does a photon behave like a particle?

1) It is a packet of electromagnetic energy gives the idea of an “item” occupying a certain space, and not a continuum like a wave propagating in space

2) It travels in one direction only. So, a light bulb emits photons in all possible directions, with each photon travelling in one direction only.

3) The energy of a single photon is “quantized” and measurable. So, if a single photon hits a surface, it is a bit like a ball hitting a wall.

Photons emitted by filament lamp

Energy of a photonWe can measure the energy of a photon using Einstein’s equation:

h = 6.63 x 10-34 Js Planck constant

f = frequency of photon/electromagnetic radiation

c = 3 x 108 m/s speed of light in a vacuum

l = wavelength of photon/electromagnetic radiation

hc

hfE

Fundamental ForcesWe know that electromagnetic forces are much stronger than

gravitational forces. So, how can the nuclei of atoms stay together when they contain protons (positively charged)? What forces keep the nucleus together?

• Like charges repel, so the nucleus should not be able to hold together.

• There must be another force(s) that keeps the nucleons together.

• These forces must be stronger than electromagnetic forces.

• They must be attractive forces.

• They have a short range of action, or they would win over the repulsive electromagnetic forces of particles relatively far from each other.

Fundamental ForcesAll the forces present in the universe come from four fundamental

forces• Gravitational Force: weakest force, but has infinite range of action.

All matter is affected by it, and it is an attractive force.

• Electromagnetic Force: stronger than gravitational forces. It has infinite range of action and keeps atoms and molecules together. It is responsible for chemical, mechanical and electrical properties of matter.

• Weak Nuclear Force: weaker than EM forces, but stronger than G forces. Its range of action does not extend beyond the nucleus. It is responsible for b-decay and fusion reactions in stars.

• Strong Nuclear Force: strongest force, but very short range (only between neighbouring nucleons). It keeps the nucleons together.

Exchange ParticlesWe can feel and measure these forces, but until recently Scientist

couldn’t explain the nature of these forces (what causes them). Particle Physicists have discovered that particles interact by exchanging particles called EXCHANGE PARTICLES. These particles have the following properties:

• Each type of force has its own exchange particle.

• They can produce an attractive or repulsive force.

Exchange Particles

Force Acts on Relative strength Range (m) Exchange

particle

Strong nuclear Quarks 1 10-15 Gluon (g)

Electro-magnetic

Charged particles 10-2 ∞ Photon (g)

Weak nuclear

Quarks and leptons 10-5 10-17 Z0,W+,W-

particles

Gravity Everything with mass 10-40 ∞ Graviton