Understanding Mass Defect: Energy Calculation in Fusion and Fission

In summary, the conversation discusses the methods for calculating energy and mass defect in nuclear reactions, specifically in fusion and fission. It is mentioned that the direction of the calculations depends on whether energy is released or absorbed. The expert also explains that the release or absorption of energy in nuclear reactions depends on the nuclei involved and is not always consistent. A graphical representation of this concept can be seen in the "binding energy curve".
  • #1
Ivore
10
0
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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  • #2
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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Likes Ivore
  • #3
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
 
  • #4
heavystray said:
i think since both fission and fusion reactions release energy
It depends on the reaction.
 
  • #5
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?
 
  • #6
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.
 
  • #8
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!
 

Related to Understanding Mass Defect: Energy Calculation in Fusion and Fission

1. What is mass defect?

Mass defect is the difference between the mass of a nucleus and the sum of the masses of its individual protons and neutrons. This difference is due to the conversion of mass into energy during nuclear reactions.

2. How is mass defect related to energy calculation in fusion and fission?

In fusion reactions, when two light nuclei combine to form a heavier nucleus, the resulting nucleus has a lower mass than the sum of the two original nuclei. This difference in mass is converted into energy in accordance with Einstein's famous equation, E=mc². In fission reactions, a heavy nucleus splits into two lighter nuclei, releasing energy due to the difference in mass between the original nucleus and the two resulting nuclei.

3. How is mass defect measured?

Mass defect is measured using a mass spectrometer, which separates and measures the masses of different particles. By comparing the mass of a nucleus with the sum of the masses of its individual particles, the mass defect can be calculated.

4. What is the significance of mass defect in nuclear reactions?

The release of energy through the conversion of mass in nuclear reactions has significant practical applications, such as in nuclear power plants and nuclear weapons. Understanding mass defect is crucial for predicting and controlling the energy output of these reactions.

5. Can mass defect be explained by the conservation of mass and energy?

Yes, mass defect is a result of the conservation of mass and energy. In nuclear reactions, the total mass and energy before and after the reaction must remain constant. The mass lost during the reaction is converted into energy, following the law of conservation of mass-energy.

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