# E=MC²

## The Einstein formula, a relation between mass and energy

Mass-energy equivalence: E=mc2
This pair of scales would show that an alpha particle is lighter than the two neutrons and two protons that make it up. This ‘mass defect’ can be translated into an energy difference by multiplying by c2. The resultant energy is known as the binding energy: in the case of two protons and two neutrons, 28 million electronvolts. This substantial value of binding energy represents less than 1% of the internal energy (or mass) of the alpha particle.

The energy released by a chemical reaction is generated by variations of  ‘internal’ energies. A good example is the combustion of carbon: C + O² => CO²

The calories released by logs burning in a chimney are the result of an internal energy loss between the initial state of carbon and oxygen and the final state of carbon dioxide. Where do these ‘internal’ energies come from?

The E in Einstein’s famous equation E=MC² represents this internal energy (sometimes referred to as the mass energy) :

This energy is equivalent to the mass M of an object multiplied by the square of the speed of light c. The value of c, some 300 million meters per second, is one of the largest and most impressive natural constants. The multiplying factor of c to the square is therefore enormous. It needs only takes minuscule changes in the value of M to generate the energies we see in our chimneys, the engines of our cars, or even in our factories.

Bound nucleons in nuclei  lose weight
The English physicist Francis Aston was the first to measure the mass of nuclei. Dividing this mass by the number of nucleons present allows for a calculation of the average mass of a bound nucleon. Aston’s curve shows that belonging to a nucleus makes a nucleon lose just under 1% of its mass. This loss, when multiplied by the speed of light squared, comes out an energy loss which can surpass 8 million electronvolts. This is the energy that on average needs to be spent to rip a nucleon away from the nucleus: the binding energy.  In the case of a nucleus, the mass lost is low but detectable, whereas in chemical phenomena involving energies of a few electronvolts the masses defects are too small to measure.