Why is energy relesed during the nuclear fusion process?

Answer 1

This is only true when you're under about 63 nucleons.

Picture the nucleus as a ball of a certain radius. As you add nucleons, the radius gets larger.

There are two forces of interest here: the Coulombic force pushing protons away from other protons and the strong nuclear force that is pulling the nucleons of the nucleus together. A nucleus is in a stable configuration when the strong force "overpowers" the Coulombic force and the nucleus gets held together.

The strong force decays very quickly with small distance. When you only have a couple of nucleons, the "radius" of the nucleus is so small that the strong nuclear force is very strong. It is so strong that it completely overpowers the Coulombic force pushing the protons away from each other. Thus, while it may be difficult to push two protons together, once you get them close enough, the strong force takes it the rest of the way and all of that energy you put in gets released plus additional energy. This is the energy that is released during fusion.

However, once you get around 63 nucleons and above, the radius of the nucleus is too large for the strong force to greatly overpower the Coulombic repulsion. Thus, a very small amount of energy (by means of an energetic neutron, for example) has to be added to destabilize the nucleus. Once a few nucleons get outside of the distance where the strong force dominates, they fly away from the nucleus due to the Coulombic repulsion. In fact, as they fly away they release more energy than was put in to destabilize them. (this is the exothermic fission process)

So it's all about the balance between the strong force and the Coulombic force.

(and, of course, if you knew the mass of nuclei before and after fusion or fission processes, you could use the difference in mass to figure out how much energy was released. Similarly, if you were fissioning nuclei below 63 nucleons or fusing nuclei above 63 nucleons, you could use the difference in mass to figure out how much energy was added; this might be interesting, but it is simply a *consequence* of the fundamental reasons why fusion is exothermic below 63 nucleons)

Answer 2

Great answers so far. What I summarize so far is, fusion is difficult to achieve because it takes a lot of energy to overcome the coulombic force holding the protons apart. With enough heat and pressure, as in the conditions inside our sun, the coulombic energy is overcome and the protons get close enough for the strong force to take hold. I assume that the strong force does 'work' on the two protons, and this work must come from somewhere. E=mc^2. Also 1 helium atom is slightly lighter than 2 hydrogen atoms, and that is evidence of the release of fusion energy, so all the pieces fit together.

Answer 3

EVERYBODY GOT IT RIGHT. So far............!

Steve is right that the energy is released because some mass is converted to energy (e=mc^2, and all that).

But the others are also right when they say that the energy is released because the newly created nuclei are more stable than in their previous form.

Or maybe it's the other way around: the nuclei are more stable because energy is released (or because the mass is reduced).

Everything works together, and you want to understand how it works. But which phenomenon causes which other phenomenon that occurs in the process? It's like a simultaneous chicken-egg situation (does the chicken cause the egg, or the egg cause the chicken?).

Bottom line: Everybody's right.

Answer 4

hi,

basically without getting to complicated about it, the centre of an atom contains neutrons and protons. the protons are positive and the neutrons dont have a charge. the electrons that orbit the electrons are negativly charged therefore having an attraction to the nucleus.

so your asking how does the neutrons attract with protons when neutrons have no charge and if there is no charge therefore there is no attraction so under all that repulsion how the nucleus stays in contact and just doesnt explode, well it does and thats called nuclear power the energy released from the repulsion between the neutron and protons.

its much more complicated than that but thats simply as i can put it.

please refer to this animation for an illustration http://www.atomicarchive.com/Movies/Movies/fission.mov

take care.x

Answer 5

that is only true for nuclei smaller than the Iron56 nucleus. any fusion product to be formed that is bigger than the Fe56 nucleus is formed via an endothermic process. ie, you lose energy.

the product is unstable and is prone to fission until all the daughter nuclei are smaller than the Fe56 nucleus.

this is why we call Hydrogen isotope nuclear reactions FUSION reactions and uranium/plutonium nuclear reactions FISSION. unstable nuclei tend to stabilise towards Fe56. the movement towards stability will release energy be it fusion or fission.

energy is released when 2 nuclei are forced together so close that strong nuclear force takes over and binds them. the first step is extremely hard but when it happens, alot of energy is released because energy is released when bonds are formed.

Answer 6

NOBODY GOT IT RIGHT. So far............!

Energy is released during fusion (OR fission!) because the net mass after the reaction is less than the mass prior to the reaction. The energy released is given by Einstein's famous equation:

E = (delta mass)*c^2

All this talk about neutrons, protons etc simply obscures the underlying principle.

Answer 7

In a nutshell it's because the nuclei are more stable fused together than separated. More stable in physics means lower energy. The extra energy has to go somewhere.

Answer 8

Listen to Greg. Everybody else just likes to see themselves talk.