Mass and Energy

The fundamental relation that is used to discuss properties of nuclei and nuclear reactions is the very well-known formula of Einstein:

  

which associates an energy E to a mass m, with c = 3.0 x 10⁸ m/s being the speed of light (in a vacuum). The amount of energy contained in a 1 kg mass is therefore enormous - of the order of 9 billion Joules. However, somewhat analogous to potential energy, this mass energy is most useful to us when it gets converted to other forms of energy, especially the kinetic energy of moving particles. This transformation of energy occurs mostly in nuclei, where the masses (and energy equivalents) involved are smaller, as seen in the table below.

 

Table 13.1: Rest masses and energy equivalents of various particles

Particle	Mass (kg)	Energy equivalent (MeV)
Proton	1.6726 x 10-27	938.28
Neutron	1.6750 x 10-27	939.57
Electron	9.109 x 10-31	0.511

 

Recall that the unit of energy used here, the MeV (mega electron volt), is one million electron volts, where one electron volt is the kinetic energy gained by an electron when accelerated through a potential difference of one Volt ( 1.6 x 10^-19 J).