Nuclear Physics

[rktpro]

Why elements lighter than iron release energy on fusion and heavier elements absorb energy on fusion?

[timo]

The obvious answer is that it has to do with the masses of the reactants and the product. But you probably wanted to know if and how one can predict the masses. There is no working ground-up method to calculate the masses of nuclei, I think; even the calculation of a proton mass from quantum-chromodynamics (QCD) is numerical and with a few sophisticated approximations, I think. I am pretty sure loads of approximate methods exist (the theory guys have to spend their time on something, after all). The most famous, possibly oldest, and the one that is routinely taught to physics students, is the Bethe-Weizsäcker formula, which should be relatively easy to understand and apply if you bother to work through the individual terms.

[lemur]

Could this have to do with the conservation of energy/mass laws? After all, if elements would continue to fuse exothermically past iron, wouldn't stars be able to grow and release increasing energy infinitely?

 

edit: please not this is not an attempt to hijack the thread with a side-topic. It's an approach to exploring the OP question by considering what reality would be like if elements heavier than iron would fuse exothermically instead of endothermically. I.e. would it violate energy/mass conservation?

Edited June 3, 2011 by lemur

[timo]

Could this have to do with the conservation of energy/mass laws?

It has. That's what I meant by

The obvious answer is that it has to do with the masses of the reactants and the product.

I merely considered the statement "the sign of E in [math]m_1 + m_2 = M + E/c^2[/math] depends on the value of the masses [math]m_1, m_2, M[/math]" so trivial that I don't think it was rktpro's actual question.

[lemur]

It has. That's what I meant by

 

I merely considered the statement "the sign of E in [math]m_1 + m_2 = M + E/c^2[/math] depends on the value of the masses [math]m_1, m_2, M[/math]" so trivial that I don't think it was rktpro's actual question.

Yes, I know you mentioned that. What I was meaning more, though, is that if elements would continue to fuse exothermically and lose mass beyond iron, for example, that stars would burn out more quickly maybe. Or maybe they would continue fusing until all matter was fused into a single atom, and this atom would immediate fuse with any incoming matter immediately due to gravity. I suppose none of this would violate conservation laws but it would make for a very different universe, I think.

 

 

 

[swansont]

Nuclear force is short ranged, and the binding energy per nucleon saturates at about 60 nucleons. That is, the binding from the nuclear force stops getting bigger, since you can only create a set number of pairs of particles that are within range of each other. So the nuclear binding gets stronger and then plateaus. The electrostatic repulsion gets stronger as you add protons, though it is weaker than the nuclear attraction between any pair on protons, but it grows without bound. So the net amount of binding any nucleon feels goes up until you hit iron or thereabouts and down for as you get heavier.

 

 

edit: please not this is not an attempt to hijack the thread with a side-topic. It's an approach to exploring the OP question by considering what reality would be like if elements heavier than iron would fuse exothermically instead of endothermically. I.e. would it violate energy/mass conservation?

 

!

Moderator Note

Which is a hijack of the OP. i.e. it asks a question/investigates a scenario rather than answers the question, when the question has not been answered. That's what hijacking is.

[rktpro]

The obvious answer is that it has to do with the masses of the reactants and the product. But you probably wanted to know if and how one can predict the masses. There is no working ground-up method to calculate the masses of nuclei, I think; even the calculation of a proton mass from quantum-chromodynamics (QCD) is numerical and with a few sophisticated approximations, I think. I am pretty sure loads of approximate methods exist (the theory guys have to spend their time on something, after all). The most famous, possibly oldest, and the one that is routinely taught to physics students, is the Bethe-Weizsäcker formula, which should be relatively easy to understand and apply if you bother to work through the individual terms.

 

I didn't wanted to know if an how one can predict the masses. I wanted to know how masses of reactants and products affect the type of reaction. Swansont gave me an answer. If you could elaborate in a simple language.