# Why doesn't fusion contradict the 1st law of thermodynamics?

I was reading up on the 1st law of thermodynamics for my Chemistry exam and I was wondering why doesn't fusion contradict the 1st law of thermodynamics?

The 1st law states that

The energy of an isolated system is constant

or that whatever is put into the system, you get out of it, but in fusion you get more out of the initial reactions than you put in

Hydrogen + Hydrogen = Helium etc.

I am still a bit confused... Thanks for any help!

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## migrated from skeptics.stackexchange.comApr 1 '13 at 12:33

This question came from our site for scientific skepticism.

Imagine that you are converting atoms of deuterium (hydrogen with one proton and one neutron per nucleus) into the most common isotope of helium, called helium-4 (two protons, two neutrons). Deuterium isn't the most common form of hydrogen in nature, but let's just keep the math simple. Two atoms of deuterium produce one atom of helium-4.

Deuterium's molar weight can be found here to be 2.0141017778. Twice that would be 4.0282035556; but helium-4 has, as per the same source, molar weight only 4.00260325415.

Where did the remaining mass of 0.02560030145 go? Photons, radiated away. If you did not allow them to escape, you could convert them back into matter of your preference and be able to verify that the mass of such matter is still 0.02560030145, assuming no other energy has escaped your lab.

Using the famous equation mentioned in sister answers, you can even use this math to convert familiar units of mass (such as grams) to units of energy (such as megawatt hours), if you prefer to keep the energy as energy, sell some of it on the power market and waste the rest during the conversion to electricity.

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By that logic, a battery violates the law also.

"Look, all I did was put in enough energy to flip a switch, and now an LED keeps shining and shining! I got more energy out than I put in!"

In nuclear fusion, we are releasing some latent energy which is present in the materials, by changing the nuclear structure into other materials that contain less energy.

The energy we are getting out was "put in" to those nuclear reagents.

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To answer the question simply, $E=mc^2$.

Energy is a manifestation of mass, and mass is a manifestation of energy. In a fusion or fission process, the total "energy" of the system remains constant, it just changes shape.

By "energy" I mean the totality of the already present energy, and the bound energy of the mass that takes part in the reaction.

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In fact, also in comubstion mass (a really small quantity) disappears. –  Fabio F. Apr 1 '13 at 20:09
May I add that mass or energy are never "destroyed". It is just that matter may be destroyed, and all you have is a photon, or gamma, etc..that is a form of immense energy, and has mass=energy/c^2 –  Saurabh Raje Apr 2 '13 at 12:46
@FabioF - Yes, but only if you allow energy to escape the system (light, infrared, thermal dissipation). Not suspecting you of allowing an exhaust pipe! –  Jirka Hanika Apr 2 '13 at 13:53