1. Particles
Matter and Antimatter
Fundamental Forces
Thursday, 24 November 2011

2. Matter and Antimatter
Every 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

3. Particle Symbols
Most 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

4. The Photon: a very peculiar particle
We’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?

5. Representing a photon
So, 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

6. Energy of a photon
We 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

7. Fundamental Forces
We 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.

8. Fundamental Forces
All 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.

9. Exchange Particles
We 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.

10. Exchange Particles
Relative Exchange
Force Acts on Range (m)
strength particle
Strong
Quarks 1 10-15 Gluon (g)
nuclear
Electro- Charged
magnetic particles
10-2 ∞ Photon ( )
Weak Quarks and Z0,W+,W-
10-5 10-17
nuclear leptons particles
Everything
Gravity
with mass
10-40 ∞ Graviton