Fusion And The Forces

April 17, 2014 at 2:53 pm

Before I begin to talk about efficiency and the particle energies present inside the Fusor, I will give an introduction to fusion for those who do not have much knowledge on the subject.  There is a previous post here in which I explain how fusion occurs inside the Fusor, but it is not very specific to how actual fusion occurs.

According to the Oxford Dictionary, nuclear fusion is “A nuclear reaction in which atomic nuclei of low atomic number fuse to form a heavier nucleus with the release of energy.”  This means that fusion is where the nuclei of two light atoms – light in this case is up to Fe-56 – stick together to form a nucleus heavier than both.  However, the nucleus formed is always slightly lighter than the combined masses of the two nuclei which fused together.  Sometimes, the loss of mass is because neutrons and protons get emitted as part of the reaction, but even if these particles are accounted for, there is still a deficit in mass compared to the two original nuclei.

The reason for this is that energy is released as part of the fusion reaction, whether it be kinetic, light, or thermal.  The energy released is able to account for the mass lost because of Einstein’s famous equation “e=mc2,” which shows that mass is in fact a form of energy and thus, can be converted into other forms of energy, such as the aforementioned kinetic, heat, or light. This is essentially the basis of how our sun provides the Earth with heat and light; Two hydrogen nuclei fuse and begin a chain reaction which results in a helium nucleus and the shining sun that we can see in the sky, as long as the weather is permitting. Some of you may be thinking that there are the same number of protons and neutrons left after the reaction as before and energy has been released, so where is the mass lost to make up for the released energy?

The answer to this question involves the fact that free nucleons – protons and neutrons – are more massive than those bound together inside nuclei.  Also, nucleons inside smaller nuclei are more massive than those found inside larger nuclei.  The greater mass of free nucleons can be explained with two of the four fundamental forces, the strong nuclear force and the electromagnetic force.

The strong nuclear force is responsible for binding particles together.  It is stronger than the electromagnetic force by a factor of about 1038 times.  However, the strong force has a very short range, much smaller than that of the electromagnetic force and can only work its magic inside nuclei, but this is good news.  If the strong force had a large range, everything would simply stick together at the atomic level and we would have none of the chemistry present today – no stars, no animals, no humans.  Anyways, the electromagnetic force does nearly the opposite of the strong force, it causes like charges to repel each other (positive and positive etc.) and opposite charges to attract.  This gives rise to electrons and protons attracting each other and nuclei (which are very positive with all their protons) to repel each other.  Even though the strong force is…stronger than the electromagnetic force, the electromagnetic force still “works” because it can work over a distance much larger than the strong force.  But how does this give free particles more mass?

When free particles are brought together to form a nucleus, energy must be expended to “push” them together against the electromagnetic force which repels protons from each other.  As this energy has mass, when it is used up, it is taken away from the mass of the total nucleus compared to its constituents and so, the nucleus is lighter than the free particles.  In the case of smaller nuclei’s components being more massive than larger nuclei’s, the larger combined mass is because in a larger nucleus, more particles have been brought together and more energy has been used up.  If we take carbon-12 as an example, three helium nuclei (two protons and two neutrons each) make up six protons and six neutron, which is the same as what makes up a C-12 nucleus.  The three helium nuclei, if measured individually and added together are heavier than the C-12 nucleus.  This goes to show that as you combine more particles and fuse more nuclei, you release more and more energy.

And that’ll be all for today folks.