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Nuclear Physics: Nuclear Force & Building Energy

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  • 0:01 What Is Nuclear Physics?
  • 1:15 Binding Energy &…
  • 3:08 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After watching this video, you should be able to explain what nuclear physics is, including the concept of binding energy. You should also be able to explain how fusion and fission work. A short quiz will follow.

What is Nuclear Physics?

Nuclear physics is a field of physics focusing on the interactions of atomic nuclei and the particles that make them up. Nuclear physics helps explain nuclear reactions, including fusion in the Sun, fission in nuclear power plants, and radioactivity. Nuclear physics also has many applications, including nuclear power, nuclear medicine, nuclear weapons, and radiocarbon dating. The field even led to the development of particle physics. The nuclear force (otherwise known as the strong force) is the force that holds the nucleus together and is explained by nuclear physics.

An important piece of prerequisite knowledge to understand nuclear physics is that mass and energy are equivalent. The famous equation that describes this, derived by Albert Einstein, is E = mc-squared. It shows how mass has an equivalent amount of energy and vice-versa. In fact, more than that, energy and mass are really the same quantity viewed from a different perspective.

Many nuclear reactions involve a release of energy. For example, fusion, in the Sun, involves combining small nuclei into larger ones, and fission, in power plants, occurs when large nuclei are broken down into smaller ones. Whenever this happens, mass is converted into energy.

Binding Energy and Nuclear Reactions

These days, we know the masses of lots of particles. We know the masses of protons, neutrons and electrons. And we know the masses of most known elements in the periodic table.

But sometimes those numbers might seem to be hard to explain. It turns out that the total mass of the things an atom is made from, for example, 2 protons, 2 electrons and 2 neutrons, isn't the same as the mass of the atom itself. The mass of an atom isn't equal to the mass of its constituent parts. But how is that possible? On the face of it, this would seem to defy logic.

The reason is that when you put an atom together, energy is released. And conversely, when you take an atom apart, you have to expend energy to make it happen. Since energy and mass are really the same thing, this means that an assembled atom will have less mass than a disassembled atom. The difference between these two figures is called the mass defect, or in energy units, it's instead called the binding energy. It is defined as the energy required to disassemble a whole system into separate parts.

When nuclear reactions release energy, it is because there was a difference between the binding energy of the things you started with and the binding energy of the things you ended up with. If the things you end up with have less total binding energy, then energy will be released in the nuclear reaction.

It turns out that iron is the most stable atom in the periodic table - or in other words, it's the atom with the lowest energy state. Atoms lighter than iron release energy when they're fused together; this is called nuclear fusion. It happens in the Sun and other stars. Atoms heavier than iron release energy when they're broken apart, and this is called nuclear fission. It happens in nuclear power plants. This graph showing binding energy illustrates this. Any reaction that moves you closer to iron will release energy.

Graph of binding energy
Graph of binding energy

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