A quick question about fission and binding energy

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#1
I'm currently going through Radioactivity, A2 Physics, and I'm reading through mass defects and how it pertains to the concepts of fission and fusion. Specifically I don't get how fission releases energy.

When nucleons combine, their total mass decreases and this mass defect is then released as energy, equivalent to the energy to bind them. As a larger nucleus, the binding energy is higher, so more energy is released. This is fusion, and makes sense to me.

When nucleons split, though, the constituents of the original particle increase in mass, which, logically, requires energy to be converted into matter, right? The binding energies of the two resultant particles increase, yes, but how is energy released if they increase in weight which requires energy to convert to matter?
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2 years ago
#2
(Original post by Dggj_19)
I'm currently going through Radioactivity, A2 Physics, and I'm reading through mass defects and how it pertains to the concepts of fission and fusion. Specifically I don't get how fission releases energy.

When nucleons combine, their total mass decreases and this mass defect is then released as energy, equivalent to the energy to bind them. As a larger nucleus, the binding energy is higher, so more energy is released. This is fusion, and makes sense to me.

When nucleons split, though, the constituents of the original particle increase in mass, which, logically, requires energy to be converted into matter, right? The binding energies of the two resultant particles increase, yes, but how is energy released if they increase in weight which requires energy to convert to matter?
Who says the fission products increase in mass?

Typical fusion would be something like
1n + U 235 -> Ba 139+Kr 94+3n

masses
U 235 = 235.04 amu
n = 1.01 amu
total 236.05 amu

Ba 139 = 138.91 amu
Kr 94 = 93.93 amu
3n = 3.03 amu
total 235.87 amu

difference
-0.18 amu
0
2 years ago
#3
(Original post by Dggj_19)
I'm currently going through Radioactivity, A2 Physics, and I'm reading through mass defects and how it pertains to the concepts of fission and fusion. Specifically I don't get how fission releases energy.

When nucleons combine, their total mass decreases and this mass defect is then released as energy, equivalent to the energy to bind them. As a larger nucleus, the binding energy is higher, so more energy is released. This is fusion, and makes sense to me.

When nucleons split, though, the constituents of the original particle increase in mass, which, logically, requires energy to be converted into matter, right? The binding energies of the two resultant particles increase, yes, but how is energy released if they increase in weight which requires energy to convert to matter?

Even though there is a decrease in rest-mass of the products in the nuclear fission reaction, the total binding energy of the products is still greater than that of the reactant.

It may sound contradicting – it has to do with the sign convention of binding energy. You may want to think about it.

It seems that students tend to have the following thought:
“The change in mass (mass defect in nuclear physics) is converted into energy. OR Energy can be converted to mass.”

This is not really correct.
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