MASS ENERGY

On earth, the greatest supply of useful energy ultimately comes from the sun, mainly as sunlight, which is a form of radiant energy. The energy we obtain from fossil fuel, such>é” coal and wood, and the energy we get from hydroelectric dams came originally from the sun. On a clear day, the sun delivers as much energy to half a square mile of tropical land as was released by the first atomic bomb. In about 1 millionth of a second, the
sun radiates out into space an amount of energy equal to that used by all of mankind during an entire year.
The sun emits radiant energy at such an enormous rate that if it burned like a huge lump of coal, it would last about 5000 years before burning out. Yet the sun has been burning at nearly its present rate for over 5 billion years and should continue burning for another 5 billion years. How the sun could emit all of this energy was explained in 1905 when Einstein discovered that mass and energy are related through the well-known equation
E =mc² .......(1)
where E is energy, m mass, and c the speed of light.
The sun's source of energy is the tiny fraction of its mass that is being converted continually to radiant energy through nuclear reactions. Similar processes occur when the hydrogen bomb is exploded. To indicate the amount of energy that is in principle available as mass energy, imagine that the mass of a 5–cent piece (5 gm) could be converted entirely into electrical energy. This electrical energy would be worth several million dollars.
The problem is that we do not have the means available to convert mass completely into a useful form of energy. Even in the nuclear reactions in the sun or in the atomic or hydrogen bombs, only a few tenths of 1% of the mass is converted to energy.
Since most of the energy in the universe is in the form of mass energy, we shall begin to develop our equation for energy with Einstein's formula E = mc². As we mentioned, we will add terms to this equation as we discover formulas for other forms of energy.