According to Albert Einstein, mass and energy are interconvertible quantities. That is, mass of a system can be converted into energy and the energy of the system can be converted into mass. But sum of mass and energy of a system remains constant.
The energy (E) equivalent to mass (m) is given by the relation,
E =mc2, where e=3 x 108 ms-1 is the speed of light in air or vaccum.
Example: When 1 g of coal is completely burnt, then energy produced is given by.
E = mc2 = (1 g) x (3 x 108 ms-1)2
= 10-3 kg x 9 x 1016 m2 s-2
= 9 x 1013 m2 s-2 = 9 x 1013 joule.
