Particles Matter and Antimatter Fundamental ForcesThursday, 24 November 2011
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 Antiparticle Name Symbol Charge Name Symbol Charge Electron e- -1 Positron e+ +1 Neutron n 0 Antineutron n 0 Proton p +1 Antiproton p -1 Neutrino e 0 Antineutrino e 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 waves Photons Oscillations of they What are electric What are they Packets of EM waves and magnetic fields made of? made of? What do they What do they Carry energy Are packets of energy carry? carry? How is frequency = Higher the energy On what does their Energy depends on carriedthe energy higher affected? frequency 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 space2) 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: hc E hf 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 = wavelength of photon/electromagnetic radiation
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 -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 Relative Exchange Force Acts on Range (m) strength particleStrong Quarks 1 10-15 Gluon (g)nuclearElectro- Chargedmagnetic particles 10-2 ∞ Photon ( ) Weak Quarks and Z0,W+,W- 10-5 10-17nuclear leptons particles EverythingGravity with mass 10-40 ∞ Graviton