Everyone knows Albert Einstein's famous formula: $E = m * c^2$
In nuclear fission, we make use of it by converting mass into energy.
But what is the mass that gets converted? Of course, when nuclei are formed the specific bond energy is realeased but what does this energy have to do with mass? The higher the bond energy, the less mass the newly formed nucleus has, so this bond energy has an impact on the mass (mass defect). But why does this mass defect exist at all?
Because if you weighed every single proton, neutron, and electron in the nucleus, their mass would stay the same during the formation of the new nucleus.
I've been thinking about this for days after I stumbled across this question while reading a book about physics recently, but although I was researching a lot during the past days, I just couldn't find an answer to my question.
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1 Answers
If you weigh all constituent particle then yes the mass is the same however for this you would need to separate these particle which takes energy and thus adds mass to the system.
Maybe the other way around makes more sense: Take a collection of protons and neutrons. You can now weigh them if you like. However we know that a nucleus formed of these particles is stable and thus energetically favorable to the configuration consisting of separate particles. (Yes there is a potential barrier in-between but we ignore that for now). So because the bound configuration is favourable energy is released in the binding process (which is why fusion works) .
Now we take einsteins formula: From the point of the system the change in energy has to be negative (otherwise the bound configuration would not be stable). Thus the change of mass is also negative and energy is transferred to the surrounding. So what is this energy? It is the potential energy stored in no being a fully bound state.
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