# AQA A-Level Physics/Particles and Anti-particles/Conserving and illustrating interactions

## Fundamental rules of particle conservation

4 Things must always be conserved in a particle interaction;

Charge, Baryon Number, Lepton number & Mass/Energy.

The first three are the ones which will most commonly come up in exam questions or be expected when asked to determine if an interaction could occur.

There is also 1 thing which is almost always conserved, being Strangeness.

### Conservation of Charge, Baryon Number & Lepton Number

Conservation of these factors is calculated using the values of these given in the formula book. For an interaction to occur, charge must be completely equal on both sides of the 'equation'.

It is important to note that the data given in the formula booklet regards quarks, so you must have knowledge of how quarks come together to form hadrons to answer these questions.

### Conservation of Mass/Energy

These conservations are mainly studied in the mechanics section, however, it is important to know that these will also come up in the 'Radiation' subtopic in particles. In the context of Particles and Radiation, the mass of a particle is equal to its energy at rest. This, and its applications will come up in the Radiation subtopic.

### Conservation of strangeness

Strangeness must be conserved in all interactions, except for weak interactions.

A weak interaction can be easily noticed in a few ways;

Weak interactions can change the flavour of quarks and leptons, while other interactions cannot.

In weak interactions, strangeness can vary by 1 either positively or negatively. So, for example, if there is a strangeness of 1 on one side, in a weak interaction, this can become either 0, or 1. It can also stay the same.

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