Understanding Mass Defect: Energy Calculation in Fusion and Fission

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Discussion Overview

The discussion revolves around the concepts of mass defect and binding energy in the context of nuclear fusion and fission. Participants explore how to calculate energy changes and mass defects in these reactions, questioning the correct order of subtraction for both energy and mass in various scenarios.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether the calculation for energy should be BE(products) - BE(reactants) or the reverse, noting inconsistencies in their solutions.
  • Another participant suggests that mass defect is calculated as mass of reactants minus mass of products when energy is released, and the opposite when energy is absorbed.
  • Some participants assert that both fission and fusion reactions release energy, implying that mass of reactants should always be greater than mass of products.
  • However, others argue that it depends on the specific reaction, indicating that not all fission or fusion processes behave the same way.
  • A participant provides a guideline that fusing nuclei lighter than iron and fissioning nuclei heavier than iron generally releases energy, while the reverse processes require energy, though this is noted as an approximation.

Areas of Agreement / Disagreement

Participants express differing views on the calculations of energy and mass defect, with no consensus reached on the correct approach. Some agree on general trends regarding energy release in fusion and fission, while others emphasize the dependency on specific reactions.

Contextual Notes

There are unresolved questions regarding the definitions of energy calculations and the conditions under which mass defect is determined. The discussion highlights the complexity of nuclear reactions and the need for clarity in definitions and contexts.

Ivore
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Hi, this may be a very trivial question but,

When calculating energy, is it BE(products) - BE(reactants) or is it the other way round?
I have been solving questions and the solutions were products - reactants for some and reactants - products for some.

Also for mass defect , is it mass ( products ) - mass ( rxts ) or again is it the other way round ?
Does it change for fusion/fission ?

Edit: I think I got it, if energy is released then mass defect = (mass reactants) - (mass products) , if it is energy absorbed it is the other way round.
For energy it is BE(Products)-BE(reactants) for energy released and the other way round for energy absorbed.
Am I right?
 
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Ivore said:
When calculating energy, is it BE(products) - BE(reactants) or is it the other way round?
Calculating energy of what? There are two reasonable definitions, those two have equal magnitude but opposite sign.

Ivore said:
Does it change for fusion/fission ?
It is a property of the nucleus, independent of its origin.

Ivore said:
For energy it is BE(Products)-BE(reactants) for energy released and the other way round for energy absorbed.
Sure, this is just energy conservation.
 
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i think since both fission and fusion reactions release energy so mass defect = mas of reactants- mas of product
so,i think it doesn't change
 
heavystray said:
i think since both fission and fusion reactions release energy
It depends on the reaction.
 
mfb said:
It depends on the reaction.

oh really, depends on what reaction exactly?

both fission and fusion releases energy right? isn't that the mass of reactants will always be higher than the mass of products?
maybe my physics syllabus is too basic, would you mind explaining it to me?
 
Fusion of hydrogen to helium releases energy, fission of helium to hydrogen needs energy. The second part is a direct consequence of energy conservation.
Fission of uranium releases energy, fusion of the fission products to uranium needs energy.

As a rough guideline, fusing nuclei lighter than iron and fission of nuclei heavier than iron releases energy, while the opposite directions need energy. This is not always true, but it is a good approximation.
 
mfb said:
Calculating energy of what? There are two reasonable definitions, those two have equal magnitude but opposite sign.

It is a property of the nucleus, independent of its origin.

Sure, this is just energy conservation.

Sorry for the late reply. Thanks a lot for your help again!
 

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