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What Is Nuclear Fusion? - Definition & Process

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  • 0:02 What Powers the Sun?
  • 0:58 Nuclear Fusion in Stars
  • 3:39 Nuclear Fusion in our Sun
  • 5:33 Nuclear Fusion on Earth
  • 6:26 Nuclear Fusion Reactors
  • 7:54 Lesson Summary
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Lesson Transcript
Instructor: Damien Howard

Damien has a master's degree in physics and has taught physics lab to college students.

Learn about the process of nuclear fusion and where it occurs naturally in our universe. Then go further by learning how we have harnessed its power and about some future possible uses for nuclear fusion.

What Powers the Sun?

Without our sun, life on Earth wouldn't exist. It's the light and heat given off by it that allows us to thrive on our planet. Yet the sun itself, our nearest star, is nearly 93 million miles away from the earth. To be able to provide light and heat all the way out to our planet and even beyond it requires a massive amount of energy. The process that fuels our sun and allows it to give off that much energy is called nuclear fusion.

Nuclear fusion is a reaction where two atomic nuclei fuse together to create a larger nucleus and in the process release energy. You can view a fusion reaction like a car collision you might see in a movie. Two cars slam into each other and get stuck permanently together while little pieces of them go flying off in every direction. Nuclear fusion is much the same where the cars are atomic nuclei and the little pieces are various particles and waves given off.

Nuclear Fusion in Stars

The main sources of nuclear fusion in our universe are stars like our sun. Every star in the sky is powered by some form of nuclear fusion, including the carbon fusion cycle, the triple-alpha process, and the type of fusion that commonly occurs in our sun, proton-proton fusion. The various forms of nuclear fusion are differentiated by the initial atoms undergoing the process and the atom resulting from the fusion.

Nuclear fusion occurs in stars due to the extremely high pressure and temperature in their cores. At the core of the star, the gas has been heated to the point of it becoming a plasma. In a plasma, Electrons may leave their associated atoms, creating a gas filled with positive ions and free electrons. Pressure is needed to bring the atoms close enough together to fuse. High temperature is needed to overcome the Coulomb force between the atomic nuclei. The Coulomb force is the force given off by objects due to electric charge. Objects with the same charge repel and objects with opposite charges attract.

The atomic nuclei being fused consist of protons and neutrons. Since protons are positively charged and neutrons have no charge, atomic nuclei all have a net positive charge. This means they repel each other, and under normal circumstances, would not fuse together. Have you ever tried to touch two magnets together with the same poles? They try to push each other apart much like the two positively charged nuclei want to in atomic fusion. It's extremely high temperatures that give them the energy needed to overcome the Coulomb force and fuse together.

The type of nuclear fusion occurring in a star is dependent on its mass and age. The high temperature and pressure in a star are caused by its own gravity pushing inwards on it. The more massive the star, the higher the pressure and temperature inside it. The types of atoms that fuse together in a star can change depending on how high the temperature and pressure are in the star.

Regarding age, the atoms in a star being fused together are not infinite. As the star grows older, there will be less and less of these atoms until it can no longer support large scale fusion of them. The fusion in stars releases a great amount energy that creates an outwards pushing force, which is kept in balance by the star's gravity pushing inwards. When the atoms being fused are running out, fusion slows down. Now, the inwards pushing force of gravity will be greater than the lesser outwards force created by fusion, and the star will begin to collapse. When this collapse happens, the environment inside the star can change in such a way that a new type of fusion is ignited, re-expanding the star.

Nuclear Fusion in our Sun

The major type of nuclear fusion happening in our sun is known as proton-proton fusion. In proton-proton fusion, hydrogen atoms are fused together through several steps to eventually become a helium-4 atom. Initially, two hydrogen atoms collide forming a single deuterium atom, a positron, and a neutrino.

Deuterium is an isotope of hydrogen. An isotope has the same number of protons in its nucleus as the standard form of its element but a different number of neutrons. A positron is a particle with the same mass and magnitude of charge as an electron but has a positive charge instead of a negative one. A neutrino is a charge-less particle with a mass of nearly zero.

In the next step, the deuterium atom fuses with a proton to form a helium-3 atom and gives off a gamma ray as a result. Finally, the helium-3 atom fuses with another helium-3 atom that was created in the same process from another two hydrogen atoms. This creates an end result of a single helium-4 atom and two protons.

It should be noted that the single helium-4 atom is lighter than the four hydrogen atoms that started this process to form it. So what happened to all that extra mass from the hydrogen atoms that we started out with? Well, that's where we get the massive amount of energy given off by nuclear fusion.

From Einstein's famous formula, we know that E = mc^2. In this formula, E is energy, m is mass, and c is the speed of light. The speed of light in a vacuum is 3 x 10^8 meters per second. That's a huge number, and in Einstein's equation for energy, it's squared. Knowing that and looking at the right side of the equation, we can see that it doesn't take that much mass to produce a lot of energy. This is why nuclear fusion produces such a vast quantity of energy.

Nuclear Fusion on Earth

While stars are still the main source of nuclear fusion, we too on Earth have been able to produce nuclear fusion artificially. The first instance of artificially induced nuclear fusion was in the hydrogen bomb. The hydrogen bomb gets its name from the hydrogen isotopes, deuterium and tritium, it uses in the fusion process.

Much like the stars we talked about in the previous section, high pressure, and high temperature are required for the nuclear fusion process. In order to get a high enough temperature and pressure inside the bomb, another nuclear fission explosion must be triggered first. Nuclear fission differs from fusion in that It's caused by splitting a single atomic nucleus apart, in contrast to combining two together. The fission explosion is contained within the bomb long enough to raise the temperature and pressure, triggering the fusion process responsible for the bomb's explosion.

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