August 6, 1945, the day remembered in history for all the wrong reasons, depicted the devastation science, without humanitarian concern, can cause; A nuclear bomb was dropped on Japanese city Hiroshima. Three days later on August 9, 1945 another bomb was dropped on Nagasaki. Around 130,000 people were killed in the bombings. Lets keep aside the political facet of the incident and discuss the physics behind. Both the atomic bombs dropped work based on nuclear fission. What is nuclear fission and how does it result in release of such large amounts of energy? And how is it connected to Einstein’s famous equation E=mc2?
In the equation E=mc2, E represents energy, m is mass while c is speed of light in air or vacuum. The equation says that mass is also a form of energy and as a result mass and energy are inter-convertible under right circumstances. And since the value of the square of speed of light is of the order of 1016 the amount of the energy released even from a very small quantity of mass is very high (around 9 x 1013 kilo joules per kilogram). The atomic bomb is a result of chain reaction in which nuclear fission takes place. Nuclear fission is a reaction in which a large unstable nucleus of an atom breaks down and results in the formation of smaller atoms and this process some of the mass is converted into energy. When free neutrons hit an isotope and initiate fission it results in the formation of daughter nuclei and free neutrons which in turn initiate fission of more (parent) nuclei. Each nucleus, generally, requires one neutron to undergo fission and results in the formation of more than one neutron resulting in an exponential increase in the number of neutrons within few seconds of the initiation of the chain reaction. The similar process takes place in a nuclear reactor where the number of neutrons (therefore the number of fissions) is controlled by neutron absorbing chemical rods.
In typical uranium nuclear fission, 0.1% of the mass of the nucleus is converted into energy. This is about 200 million electron volts of energy. The initiation of such a fission reaction requires only 6 to 7 million electron volts of energy. We have also compared this with that of nuclear fusion during our Physics Tuition classes. The release of such enormous amounts of energy from very small quantities of the explosive material and the rate at which the reaction proceeds due to the chain reaction nature of the events makes it an explosive of very high impact.