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CBSE NOTES CLASS 12 PHYSICS

CHAPTER 13 NUCLEUS

Subatomic particles

Nucleus

Nucleons

Atomic mass unit

Mass energy

Mass defect

Nuclear force

Nuclear binding energy

Binding energy per nucleon

Properties of binding force and binding energy

Relation of nuclear size and atomic mass number

Nuclear density

Isotopes

Isobars

Isotones

Isomers

Radioactivity

α-rays

β-rays

γ-rays

Law of radioactive decay

Rate of decay of nucleus

Half-life of a radioactive element

Average life or mean life of a radioactive element (τ)

Nuclear fission

Structure of nuclear reactor

Nuclear fuel

Moderator

Control rods

Coolant in nuclear reactor

Pressure vessel or pressure tubes in nuclear reactor

Steam generator in nuclear reactor

Nuclear fusion

The proton-proton (p, p) cycle in the sun

CBSE NOTES CLASS 12 PHYSICS

CHAPTER 13 NUCLEUS

The proton-proton (p, p) cycle in the sun

H11+H11  H12+e++ν+0.42 MeV                        (i)

e++e-  γ+γ+1.02 MeV                              (ii)

H12+H11  H23e+γ+5.49 MeV                        (iii) 

H23e+H23e  H24e+H11+H11+12.86 MeV      (iv)

The final balanced reaction after combining (i) to (iv)

4 H11+2e-  H24e+2ν+6γ+26.7 MeV  or

4 H11+4e-  H24e+2e-+2ν+6γ+26.7 MeV

As the hydrogen in the core gets depleted and becomes helium, the core starts to cool. The star begins to collapse under its own gravity which increases the temperature of the core. If this temperature increases to about 108 K, fusion takes place again, this time of helium nuclei into carbon. This process can generate through fusion higher and higher mass number elements. But elements more massive than those near the peak of the binding energy curve in cannot be produced.

The age of the sun is about 5×109 y and it is estimated that there is enough hydrogen in the sun to keep it going for another 5 billion years.

After that, the hydrogen burning will stop and the sun will begin to cool and will start to collapse under gravity, which will raise the core temperature. The outer envelope of the sun will expand, turning it into the so called red giant.

Controlled thermonuclear reaction may be possible in future.

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