13.7.2. Nuclear fusion – energy generation in stars

When two light nuclei fuse to form a larger nucleus, energy is released, since the larger nucleus is more tightly bound, as seen from the binding energy curve in Fig. 13.1.

Nuclear fusion occurs when two small atomic nuclei combine to create a larger nucleus. This process releases energy because the larger nucleus has a more favorable binding energy, meaning it is more stable than the two smaller ones alone. The binding energy curve illustrates how energy levels change as nuclei combine, showing that heavy nuclei have more binding energy per nucleon than light ones.

Think about a crowded room (two light nuclei) where people (nucleons) want to form a larger group (a larger nucleus). When they all come together, they feel safer and more secure (more stable), which is akin to the energy released during fusion.

Some examples of such energy liberating nuclear fusion reactions are:

• 1H+1H→2H+e+ + n + 0.42 MeV
• 2H+2H→3He+n + 3.27 MeV
• 2H+2H→3H+1H + 4.03 MeV

The text presents specific nuclear fusion reactions, showing how combining hydrogen nuclei (1H) can create deuterium (2H), helium (3He), and tritium (3H). Each of these reactions releases a specific amount of energy, measured in MeV. This energy release is due to the difference in binding energies between reactants and products, which is a fundamental concept in nuclear physics.

Imagine combining LEGO blocks (1H) to build a bigger and more complex structure (2H, 3He). The process of assembling the blocks releases excitement (energy), just as fusion generates energy due to higher stability as compared to individual pieces.

However, since they are both positively charged particles, they experience coulomb repulsion. They, therefore, must have enough energy to overcome this coulomb barrier. The height of the barrier depends on the charges and radii of the two interacting nuclei.

Coulomb repulsion is the force that pushes two positively charged particles away from each other. For fusion to occur, the nuclei need enough energy to overcome this repulsive force, which is called the Coulomb barrier. The height of this barrier is influenced by the charges and sizes of the nuclei involved, making it a significant challenge in achieving fusion.

Consider trying to push two magnets with the same poles together. They resist coming close, just like positively charged nuclei do. You need to apply considerable effort (increase temperature) to get them to touch, representing the energy required to overcome the Coulomb barrier.

Thermonuclear fusion is the source of energy output in the interior of stars. The interior of the sun has a temperature of 1.5×10^7 K, which is considerably less than the estimated temperature required for fusion of particles of average energy. Clearly, fusion in the sun involves protons whose energies are much above the average energy.

Thermonuclear fusion refers to the process where the high temperatures inside stars provide the necessary energy for particles to fuse. In the sun, for instance, the extremely high temperature enables protons to overcome the Coulomb barrier and fuse, releasing a significant amount of energy in the process. This explains why stars can emit immense amounts of energy over billions of years.

Think of a pressure cooker: the high temperature and pressure inside provide an environment for the food to cook faster. Similarly, the conditions in stars, particularly the sun, allow particles to fuse efficiently, producing energy akin to cooking a meal quickly under pressure.

The fusion reaction in the sun is a multi-step process in which the hydrogen is burned into helium. Thus, the fuel in the sun is the hydrogen in its core.

The sun generates energy through a multi-step fusion process where hydrogen nuclei (protons) combine in several reactions to form helium. This process releases energy, contributing to the sun's heat and light. Understanding this series of reactions helps explain how stars sustain their energy over long periods.

Imagine a factory assembly line, where raw materials (hydrogen) go through several steps to become final products (helium). Each stage in this line not only creates a product but also releases energy, similar to how the sun converts hydrogen into helium while generating a significant amount of energy.

In controlled fusion reactors, the aim is to generate steady power by heating the nuclear fuel to a temperature in the range of 10^8 K.

Controlled fusion aims to replicate the natural fusion processes of stars in a controlled environment on Earth. By achieving the high temperatures necessary for fusion and maintaining the conditions needed for stability, scientists hope to create a new and sustainable energy source that could provide almost unlimited power.

Think of trying to tame fire in a fireplace: you need to control the temperature and airflow to keep it burning steadily. Similarly, controlled fusion is about managing extreme conditions to harness the energy efficiently, much like using fire safely for cooking.

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