# Anti-matter

Not only have we found sub atomic particles but we also now know there is 'anti-matter'. Anti-matter was predicted by Paul Dirac (English physicist - despite the name!) in 1928.

He solved the equations for electron orbitals in the atom and proposed negative energy states for electrons.

A simple example of how he came to this conclusion is by asking ourselves,

'What is the root of 4?'

A root can produce two possible solutions +2 or -2

In 1932 Carl Anderson found experimental evidence for one in a cloud chamber (we study those next year).

Anti-matter is the complement to matter - it has the opposite characteristics.

Characteristics of particles are given by quantum numbers (for charge, baryon number, strangeness etc.) - anti-particles have the opposite sign for each one when compared to the matter particle's characteristic quantum numbers.

Remember:

You can have anti-matter - but there is no such thing as anti-energy!

The rest mass of a particle is the same as the rest mass of an anti-particle - you don't have negative mass..

The charge, baryon number and strangeness of a particle is the opposite of the charge, baryon number and strangeness of an anti-particle.

### Annihilation

An anti-electron is a positron β+.

This anti-particle is positive and has the same mass as an electron.

If a positron and electron meet they annihilate one another - poof! and their combined mass (2me) is converted to electromagnetic energy (in accordance with the Einstein equation (E = mc2) ).

The energies are calculated in terms of electron volts (eV) The eV is a very small unit of energy - used a lot in nuclear calculations.

You need to be able to convert from one to the other.

Remember at GCSE you learned that:

E     =     Q         V

(joule) = (coulomb) (volt)

So, a joule could be thought of as a 'coulomb volt'

Similarly, if we don't use a full coulomb of charge, but only the charge on an electron 'e'

eV          =             e                       V

(electron volt) (charge on an electron) (Volt)

The charge on an electron is only 1.6 x 10-19 C

Therefore the eV is only 1.6 x 10-19 J