Conservation laws

Conservation of charge

One of the fundamental laws of Physics is that charge can never be created or destroyed. Charge is always conserved in any reaction. A simple example of this is the rubbing of a polythene strip with a duster. Initially the strip and the dusted were uncharged but after rubbing the strip gains a net negative charge and the duster gains an equal amount of positive charge – the total charge in the process has been conserved.

Charge is also conserved in a nuclear reaction the. Consider the example shown below:

The total charge on the left hand side of the reaction is zero (88 electrons and 88 protons in the radium atom). On the right hand side of the equation the total charge is still zero (86 protons and electrons in the radon atom and 2 protons and 2 electrons within the helium atom)

The same rules can be applied to the charge at a junction. There is no build up of charge at a point in an electrical circuit. The charge flowing into the junction is exactly balanced by the charge flowing out from it.

Conservation of lepton number

A further conservation law is that of lepton number. This states that the lepton number is unchanged throughout a reaction.

Leptons (electrons and muons with their associated neutrinos) have a lepton number of +1 while antileptons (positrons, muon plus and their associated antineutrinos) have a lepton number of – 1.

In the decay of a muon (lepton number +1) we have:
Muon (+1) decays to an electron (+1) + neutrino (+1) + antineutrino (-1)

Another example is beta decay, the decay of a neutron to a proton, and electron and an antineutrino.


The lepton number in each reaction has been conserved.