Cambridge A Levels Physics A2 Physics Chapter 23 Nuclear Physics
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Cambridge A Levels A2 Physics
Institution
Chapter 23 Nuclear Physics: 22 PAGES
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Chapter 23 Nuclear Physics
23.1 Mass Defect and Nuclear Binding Energy
Atomic Mass Unit
One unified atomic mass unit (1u) is defined as being equal to one-twelfth of the mass of a carbon-12
atom.
1u = 1.66 × 10-27 kg
Proton mass, mp = 1.007276 u
Neutron mass, mn = 1.008665 u
Electron mass, me = 0.000549 u
Mass Defect and Mass-Energy Equivalence
When the 126𝐶 nucleus is dismantled,
→ Mass:
▪ The mass of a nucleus is less than the total mass of its component protons and neutrons. (The
mass decreases.)
→ Energy:
▪ The potential energy of the nucleus increases,
When dismantling the 126𝐶 nucleus,
▪ Work has to be done against the strong nuclear force which attracts the nucleons to one another.
▪ Energy is put into the nucleus to pull it apart.
▪ This energy increases the potential energy of the individual nucleons.
Due to the equivalence of mass and energy, this decrease in mass implies that energy is released in the
process.
,Mass Defect, Δm
The mass defects of a nucleus is equal to the difference between the total mass of the
individual separate nucleons which are separated to infinity and the mass of the nucleus.
Mass defect = Total mass of individual separate nucleons – Mass of nucleus
∆𝑚 = [𝑍𝑚𝑃 + (𝐴 − 𝑍)𝑚𝐴 ] − 𝑚𝑡𝑜𝑡𝑜𝑙
Z = proton number
A = nucleon number
mp = mass of proton
mn = mass of neutron
mtotal = measured mass of the nucleus
Mass – Energy Equivalence
Matter can be considered as a form of energy.
▪ Mass can be converted into energy.
▪ Energy can be controlled into mass.
Einstein’s Mass – Energy Equation
ΔE = change in energy / J
2
∆𝐸 = 𝑚𝑐 Δm = change in mass / kg
c = speed of light in vacuum, 3.00 × 108 ms-1
According to Einstein’s Equation,
▪ When energy is supplied to a system, ▪ When energy is released from a system,
the mass of a system increases. the mass of a system decreases.
Radioactive Decay (Refresh)
Parent Nucleus → Daughter Nucleus + Radiation
Both nucleon number A and proton number Z are conserved during radioactive decay.
𝐴
𝑍𝑋
▪ decay ▪ + decay
A: Decreases by 4 A: Unchanged
Z: Decreases by 2 Z: Decreases by 1
There is a mass excess, energy must be absorbed. The change in mass is equivalent to 1.2 MeV.
For this reaction to take place, by conservation of mass-energy, the He nucleus must have kinetic energy of
at least 1.2 MeV when it bombards the N2 nucleus.
For a reaction to occur spontaneously, there must be a mass defect, so that the products of the reaction
have some kinetic energy, thus mass-energy is conserved.
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