Why Does Fusion Release Energy?

[Potaire]

Brand new member--am I at the right place??

Greetings one and all. I was reading some history of the Manhattan Project the other day, and my mind started to wander/wonder. I wish to better understand the concepts of fussion, in particular, and other aspects of nuclear physics. I already have done a little research, and have many questions! For example, I do not understand why fussion GIVES OFF energy when binding the two hydrogen atoms into one helium atom--it seems the process would require energy to be GIVEN TO the process. Actually, I can see where no energy need be added--mass is converted to the binding energy that holds the helium atom together (mass defect). But, I still do not see where the extra energy produced comes from, other than a small amount from the extra neutron being ejected from the new helium atom. Am I at the right place to seek answers to my questions?

 

[chroot]

Welcome to PF, Potaire. You're in the right place.

Take a look at the plot of nuclear binding energy vs. atomic number.

http://en.wikipedia.org/wiki/Image:Binding_energy_curve_-_common_isotopes.svg

Notice that the curve is like an inverted U. The highest binding energy per nucleon occurs for iron. That means iron is the most tightly-bound nucleus.

For elements lighter than iron, fusing them makes the more tightly bound, and thus releases energy. For elements heavier than iron, fissioning them makes them more tightly bound, and thus releases energy.

- Warren

 

[Potaire]

Warren, thanks for the welcome and reply. I am still missing the boat somewhere. I still don't see where the energy for us comes from in fusion. I understand that fusing the two hydrogen isotopes together results in a helium atom which weighs less than the two hydrogen isotope weights added together. I understand that this "missing" mass is converted to energy. Here is where I lose it---is this not the energy that is used to bind the 4 nucleons (2 protons and 2 neutrons) together?? If it is indeed the energy used to bind, where does the energy for us come from?

 

[jtbell]

is this not the energy that is used to bind the 4 nucleons (2 protons and 2 neutrons) together??

No, in this situation the fusion process gives you a net release of energy corresponding to the difference between the total mass of the four original nucleons and the mass of the final helium nucleus.

You do have to supply some energy initially in order to overcome the electrical repulsion of the protons in bringing the nucleons close enough together for the strong nuclear interaction to take effect. You have to supply a lot of energy, in fact. (This is why it's so difficult to build a hydrogen bomb.) But when the strong interaction does take effect, you get that initial "activation energy" back, plus a lot more.

An analogy is rolling a ball into a hole that has a raised lip around it. You have to push the ball uphill initially to get it over the lip, but once the ball is past the top of the lip, it rolls downhill of its own accord. When it reaches the bottom, it has more kinetic energy than the work that you did initially.

And a friendly tip: It helps to put the subject of your posting in the subject line of your posting.

 

[Potaire]

OK, last one, then I'll start a new thread--with appropriate title-- when time comes for next question. Do I have this right now?? The energy released to us, coming from the conversion of "missing mass" to energy, is ALL FOR US--none is needed to bind the nucleons (they are already bound by that time?). However, WE must FIRST supply the energy (a pigload) to bind the nucleons initially. The only means we have of doing so at this time is by using the heat from a fission blast.

 

[LURCH]

As a further response to this:

I understand that this "missing" mass is converted to energy. Here is where I lose it---is this not the energy that is used to bind the 4 nucleons (2 protons and 2 neutrons) together?? If it is indeed the energy used to bind, where does the energy for us come from?

Think of it this way; the energy used to bind the nucleus together is still contained within the atom (it's in their binding the nucleus). Mass that the atom looses cannot be accounted for by anything that is still within the atom. It's like if I got liposuction and then carried the extracted fat around in a bucket, I would not loose any weight. I'd step on a scale and it would read the same as if I'd never had the proceedure (assuming a zero-mass bucket, of course).

On the other hand, if I got liposuction and then used the extracted tissue as fuel in a lamp, then the "fuel" would be converted to heat and light until it was all used up, and then I'd be lighter. I realize this is a somewhat disgusting annalogy, but I may have just had a great new idea for an alternative energy thread!

 

[mgb_phys]

Nice analogy, will this become known as the Lurch-liposuction model of nuclear binding energy ?

 

[Potaire]

OK, my last shot or I'm going to drink a whole bottle of Tritrium! We have 2 hydrogen isotope atoms. As the nucleus of each has a positive charge, they repel each other. Us humans find that unacceptable, and apply a pigload of energy (heat from fission) to the two brats, finally getting them to sit still and behave---right next to each other! Lo and behold, they find they are actually in love. They and their respective friends (neutrons) all shed some weight (mass) in order to look good for each other. This "missing mass" is converted to energy---some of which is used to bind all of these ex-holigans together. However, as there are only a small number of members (nucleons) in this party, there is a LOT of the "missing mass" energy left over, which is released to us humans as a thankyou from the now-happy-and-loving party members, who have themselves changed into helium atoms.

Us humans think that we should try this same thing with bigger, more complex atoms. This is where the Energy Binding Curve comes into play. We find that the bigger the atom, the more party members (nucleons) involved. Therefore, more of the "missing mass" energy is required to bind these folks together, leaving less "missing mass" energy left over for us humans. In fact, the higher we go up the atomic chart, the more energy is kept for the binding, and the less is left over for us. When Iron is reached, the whole thing becomes a wash--ALL of the "missing mass" energy is used to bind the party members (nucleons), and none is left for us! Past Iron we find that even all of the "missing mass" energy is not enough to bind all of this large group of nucleons, and even MORE must be supplied by us if we wish to create fusion. This "more" is above and beyond what we ALREADY supplied to get these jerks to sit still and behave in the first place! Obviously we will never again invite Iron, or anyone higher on the list than Iron, to a "Fusion Party"!

OK, do I finally have a grasp of this??

 

[mgb_phys]

Pretty much.
You can get heavy elements to fuse that is how we have anything beyond Iron but it does need ,as you say, serious s***-loads of energy = a supernova.

 

[Potaire]

Excellent! Thanks to everyone who helped me get a grip on this. Now I am ready to go to work. Give me a couple weeks, and I'll figure out an alternative to heat from fusion for the initial energy source---possibly a super SuperGlue! Seriously, thanks again, and I am glad to have found this place--lots of learning to do!

Potaire