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Flip to reveal answersWhat is the 'mass defect' of a nucleus?
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All 15 Flashcards — Mass-energy equivalence and binding energy
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Question
What is the 'mass defect' of a nucleus?
Answer
(Mass of the separate protons + neutrons) − (mass of the bound nucleus). The nucleus is the **lighter** one.
Question
What is the 'binding energy' of a nucleus?
Answer
The energy equivalent of the mass defect (E = mc²) — the energy needed to **pull the nucleus apart** into separate nucleons.
Question
What is 'binding energy per nucleon'?
Answer
Binding energy ÷ number of nucleons (A). It lets you **compare the stability** of different nuclei fairly.
Question
Which equation links the mass defect to the binding energy?
Answer
$E = mc^{2}$ — mass-energy equivalence (given in the data booklet). Here m is the mass defect.
Question
Fast way to convert a mass defect in u into energy in MeV?
Answer
Multiply Δm (in u) by **931.5**, because 1 u = 931.5 MeV c⁻².
Question
On the binding-energy-per-nucleon curve, what does 'higher' mean?
Answer
**More tightly bound = more stable.** The curve peaks near iron (A ≈ 56), the most stable nuclei.
Question
Why does fusion of light nuclei release energy?
Answer
It moves **up** the steep left side of the curve — the product is more tightly bound — so energy is released.
Question
Why does fission of heavy nuclei release energy?
Answer
It moves **up** the gentle right side of the curve toward iron — the products are more tightly bound — so energy is released.
Question
Which nucleus sits at the peak of the curve?
Answer
Iron (around **A ≈ 56**) — the most tightly bound, most stable nucleus.
Question
Fusion vs fission — which releases more energy per unit mass of fuel?
Answer
**Fusion** — it climbs the steep light-nuclei side, giving several times more MeV per nucleon than fission.
Question
A nucleus has Δm = 0.030 u and 4 nucleons. Binding energy per nucleon?
Answer
E = 0.030 × 931.5 ≈ 28 MeV total, then 28 ÷ 4 ≈ **7 MeV per nucleon**.
Question
What is the strong nuclear force?
Answer
An **attractive**, **short-range** (≈ 1–3 fm) force between nucleons that holds the nucleus together against the protons' electric repulsion.
Question
Why does the strong force not pull nucleons into one point?
Answer
Below about 0.5 fm it becomes strongly **repulsive**.
Question
How do you turn a mass given in MeV c⁻² into an energy?
Answer
Drop the c⁻²: a mass defect of 2.0 MeV c⁻² is a binding energy of **2.0 MeV**.
Question
Why is the binding energy per nucleon roughly constant above A ≈ 60?
Answer
The strong force is **short-range**: each nucleon is bound only by its nearest neighbours, however big the nucleus.
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Topic 5.4 hub
Fission
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