Question

(a) Explain nuclear fusion. What are the conditions for fusion?

(b) What are the sources of energy in sun and stars?

Answer 1

(a) Nuclear fusion: Nuclear fusion is process in which lighter nuclei (like ₁H¹) are fused together into heavier atom (like ₂He⁴), with the release of enormous amoun t of energy. The energy thus released is known as thermonuclear energy and the reaction is called thermonuclear reaction. The cause of energy is again mass defect. Nuclear fusion takes place at very high temperature and very high density.

e.g. two deuterons can fuse together to form a medium nucleus and release energy of about 24 MeV.

i.e., ₁H² + ₁H² → ₂He⁴ + 24 MeV.

Some other examples of such reactions are

₁H¹ + ₁H¹ → ₁H² + +₁e⁰ + 0.42 MeV

4 ₁H¹ → ₂He⁴ + 2 +₁e⁰ + 26.7 MeV

₁H³ + ₁H² → ₂He⁴ + ₀n¹ + 17.6 MeV

₃Li⁷ + ₁H¹ → ₂He⁴ + ₂He⁴ + 17.3 MeV.

Nuclear fusion reactions take place in the presence of very high temperature (about 10⁷ K), so they are called thermonuclear reactions. All these reactions are exoergic and release huge energies. The energy comes from conversion of mass of final stage nucleus formed as a result of fusion is always less than that of the individual light nuclei.

The temperature of the order of 10⁷ K is very difficult to obtain. The mechanism involved for producing high temperature is a self-sustained fission explosion. When an atom bomb containing U²³⁵ explodes, very high temperature ( ≃ 10⁷ K) is produced. Thus atomic explosion acts as trigger and release tremendous amount of energy.

Conditions for Fusion

(1) Either the reacting nuclei should be given large amount of kinetic energy to overcome the mutual repulsion between them.

(2) Or a very higher temperature should be produced. Such high temperature exists in the territory of stars. Therefore, thermal fission is possible in stars.

(b) Source of energy in sun and stars.

The temperature of the core of the sun is about 20 million degrees. At this high temperature, proton-proton cycle takes place with the release of large amount of energy. The reactions are:

₁H¹ + ₁H¹ → ₁H² + ₁e⁰ + 0.41 MeV

₁H¹ + ₁H² → ₂He³ + 5.51 MeV

₂He³ + ₂He³ → ₂He⁴ + 2(₁H¹) + 12.98 MeV

Adding these reactions, we get

4(₁H¹) → ₂He⁴ + (₁e⁰) + 24.7 MeV

Hence, fusion of 4 hydrogen nuclei results one helium nucleus, two positrons (₁e⁰) and liberation of 24.7 M eV of energy. Estimation of hydrogen in the sun reveals that it can continue proton-proton cycle going for about 30 billion years. A similar thermonuclear reaction takes place in some stars also.
But in stars, whose core is much more hot than that of sun, another type of thermonuclear reactions take place. These are called carbon nitrogen cycle.

Carbon-Nitrogen Cycle

₆C¹² + ₁H¹ → ₇N¹³ + hv (energy)

₇N¹³ → ₆C¹³ + +1^e (positron)

₆C¹³ + ₁H¹ → ₇N¹⁴ + hv (energy)

₇N¹⁴ + ₁H¹ → ₈O¹⁵ + hv (energy)

₈O¹⁶ → ₇N¹⁵ + 1^e (positron)

₇N¹⁴ + ₁H¹ → ₈O¹⁶ + hv (energy)

₈O¹⁶ → ₆C¹² + ₂He⁴

Adding, 4 ₁H¹ → ₂He⁴ + 2^e+ + 26.7 MeV

Hydrogen bombs make use of fusion reactions. Hence, they are called fusion bombs.