Mass-Energy Equivalence
Student Expectation
The student is expected to describe the significance of mass-energy equivalence and apply it in explanations of phenomena such as nuclear stability, fission, and fusion.
Key Concepts
The total amount of mass and the total amount of energy in a closed system are conserved, that is, they remain unchanged. In classical physics, mass and energy are considered to be distinct quantities. In modern physics, mass and energy are seen as two aspects of the same thing. Increasing an object’s energy increases its mass, and vice versa. Thus, the energy content of mass and the change in mass with energy must be considered when applying conservation laws.
The mass-energy equivalence formula, E = mc2, allows us to calculate the amount of energy equivalent to a given amount of mass.
A nucleus has a mass defect because the sum of all the rest masses of the particles that make up the nucleus is greater than the rest mass of the nucleus. This mass defect is equal to the binding energy released when binding the nucleus together. The binding energy is a consequence of the strong nuclear force.
MASS-ENERGY EQUIVALENCE
Classical Physics
Energy and mass are independent concepts. In classical physics, the quantity of energy and mass in a closed system always remains unchanged or conserved. Energy can change forms, but the total amount of energy in a closed system remains constant. Mass is also conserved in classical physics. The total mass of a closed system does not change. Energy and mass are different quantities in classical physics, even though both are conserved.
Modern Physics
Modern physics describes the aspects of the universe—both large- and small-scale—we do not typically observe. The laws of modern physics have many important differences from those of classical physics. Energy is still conserved in modern physics theories. An important difference is that modern physicists think of mass as a type of energy. That is, the sum of the total mass and energy in a system remains unchanged, but mass and energy can change from one form to the other. This result—expressed in the equation E = mc2—is one outcome of Albert Einstein’s theory of relativity. It is known as mass-energy equivalence. This is the point of view in quantum physics.
Formula for mass-energy equivalence
In 1905, Albert Einstein wrote this famous equation in one of his papers. The “c” is the speed of light in the vacuum. It illustrates that an object contains huge energy. In addition, it expands the concept of conservation of mass and energy. The total amount of mass and energy is conserved and they are closely related to each other.
Mass Defect Equals Binding Energy
A nucleus has a mass defect because the sum of all the masses of the particles is not equal to the mass of the nucleus.
This mass defect is equal to the binding energy that holds the particles in the nucleus together. From Einstein’s point of view, the mass defect does not mean the mass is lost. It exists in another form, the binding energy. The greater the binding energy of the nucleus, the more stable the nucleus is.
Nuclear Fission
When an element of great mass undergoes fission, two more stable nuclei of less mass will be generated. The nuclei will lose mass in the form of kinetic or radiant energy. Some heavy elements, such as thorium and uranium, undergo both induced fission and spontaneous fission. The induced fission is a type of nuclear reaction while the spontaneous fission is a form of radioactive decay.
Nuclear Fusion
The most important fusion process in nature is the one that powers stars. The net result is the fusion of four hydrogen nuclei into 1 helium-4 nuclei and energy. The gamma rays produced in the core of the Sun are released as visible light when they leave the surface of the Sun.