Fusion's Thermodynamics: Energy, Entropy, And The Laws Of Physics

Nuclear fusion is the process of combining two lighter atomic nuclei to form a heavier nucleus, releasing a large amount of energy. This process occurs in stars, like the Sun, where high temperatures and pressure allow fusion to take place. It might seem like nuclear fusion breaks the first law of thermodynamics, which states that the energy of an isolated system remains constant. However, this is not the case. The energy released during fusion comes from the conversion of a small amount of mass into energy, as described by Einstein's famous equation, E=mc². In other words, energy is a manifestation of mass, and mass is a manifestation of energy. Therefore, the total energy of the system remains constant; it just changes shape.

Energy is converted, not created

Nuclear fusion does not break the laws of thermodynamics because energy is converted, not created.

The first law of thermodynamics, also known as the conservation of energy, states that energy cannot be created or destroyed, only converted from one form to another. This means that the energy of an isolated system remains constant. In other words, whatever energy is put into a system is what you get out.

In nuclear fusion, two lighter atomic nuclei are combined to form a heavier nucleus, and a large amount of energy is released in the process. This energy is not created; it comes from the conversion of a small amount of mass into energy, as described by Einstein's famous equation, E=mc².

In essence, energy is a manifestation of mass, and mass is a manifestation of energy. In a fusion or fission process, the total "energy" of the system remains constant; it just changes shape. The energy we get out was "put in" to the nuclear reagents.

For example, in a power plant, fuel goes in and energy comes out. Power plants contain an energy release mechanism that converts stored energy in the fuel into thermal energy (heat), which is then converted into mechanical energy (motion) using a heat engine. A generator then converts the mechanical energy into electrical energy, which is easy to transport long distances.

Similarly, in nuclear fusion, the energy released is dispersed into the surroundings, and the overall entropy of the system increases, adhering to the second law of thermodynamics.

Energy is released from mass conversion

Nuclear fusion is a process that occurs in stars, including our Sun, where high temperatures and pressures allow fusion to take place. It involves combining two lighter atomic nuclei to form a heavier nucleus, releasing a large amount of energy. This energy comes from the conversion of a small amount of mass into energy, as described by Einstein's famous equation, E=mc².

The mass-energy conversion during fusion can be understood through the following example. The principal source of energy in the sun is a net fusion reaction where four hydrogen nuclei fuse to produce one helium nucleus and two positrons. This reaction can be represented as:

> 4^1_1H ⟶ ^4_2He + 2^0_1n

A helium nucleus has a mass that is 0.7% less than that of four hydrogen nuclei. This lost mass is converted into energy during the fusion process, resulting in the release of about 3.6 x 10^11 kJ of energy per mole of ^4_2He produced. This energy output is significantly larger than that produced by the nuclear fission of one mole of U-235 (1.8 x 10^10 kJ) or the combustion of one mole of octane (5471 kJ).

Another example of a fusion reaction is the fusion of deuterium and tritium, two heavy isotopes of hydrogen, at extremely high temperatures. This reaction can be represented as:

> ^2_1H + ^3_1H ⟶ ^4_2He + 2^1_0n

This reaction results in a mass loss of 0.0188 amu, corresponding to the release of 1.69 x 10^9 kilojoules per mole of ^4_2He formed. The high temperature provides the necessary kinetic energy for the nuclei to overcome their mutual electrostatic repulsion and collide.

The mass-energy conversion in fusion reactions, as described by E=mc², demonstrates that energy is released from the conversion of mass. This conversion occurs because energy is a manifestation of mass, and mass is a form of energy. Therefore, while the total energy of the system remains constant, it changes form during the fusion process.

Entropy increases in isolated systems

The second law of thermodynamics states that the entropy of an isolated system will always increase over time. This is because isolated systems always evolve towards thermodynamic equilibrium, a state with maximum entropy. This is also known as a condition of maximum probability.

In the case of an ice cube in a room, the room is the isolated system. The ice will melt, and the total entropy inside the room will increase. The ice will gain entropy as it gains heat, and the room will lose entropy as it loses heat. Since the ice has a lower temperature than the room, the total change in entropy will be positive. This exchange will continue until the temperatures are equal, i.e. equilibrium is reached.

In the context of fusion, the overall entropy of the system increases as the energy released is dispersed into the surroundings.

Energy is conserved in fusion

Fusion involves combining two lighter atomic nuclei to form a heavier nucleus, which releases energy. This energy comes from the conversion of a small amount of mass into energy, as described by Einstein's famous equation, E=mc². In other words, the mass of the reacting matter (e.g. deuterium atoms) is slightly less than the resulting matter product (e.g. helium atoms), and this difference is the amount of matter converted into energy.

Thus, the total "energy" of the system remains constant; it just changes shape. This is true for both chemical and nuclear reactions. In chemical reactions, the overall mass and energy are conserved separately, whereas in nuclear reactions, the combined sum of mass and energy is conserved.

For example, in a gasoline engine, energy output is greater than input when the energy potential of the fuel is not counted. Similarly, in nuclear fusion, energy is extracted from the nuclear bonds of the atoms involved in the reaction. This is the same energy reaction that occurs in stars like our Sun, where fusion takes place due to high temperatures and pressures.

Therefore, fusion does not break the laws of thermodynamics; instead, it is a natural process that follows these fundamental principles.

Mass-energy equivalence

Nuclear fusion does not break the laws of thermodynamics. In fact, it is a natural process that obeys these fundamental principles.

The first law of thermodynamics, also known as the conservation of energy, states that energy cannot be created or destroyed, only converted from one form to another. In fusion, the energy released comes from the conversion of a small amount of mass into energy, as described by Einstein's famous equation, E=mc². This equation is only representative of an object at rest, so this energy is called the "rest energy" of an object. The full equation Einstein wrote down includes the energy of a moving object, but the simplified version is still profound. The "c²" term is a tremendously large quantity, so this means that a small amount of mass corresponds to a large amount of energy.

The mass-energy equivalence formula can be seen as a special case of Einstein's more general energy-momentum equation:

> E_r^2 = (pc)^2 + (m_0c^2)^2

This equation gives the relativistic energy (E_r) of a particle with intrinsic rest mass (m_0), moving with a momentum (p) relative to the reference frame it is being observed in. So we can see the total relativistic energy of a particle is composed of a kinetic component associated with its momentum relative to an observer, as well as a component stemming from the intrinsic rest mass of the particle, which is constant in all frames. If we are in the rest frame of the particle, where p=0, the equation becomes Einstein's mass-energy equivalence relation: E_r^2 = (m_0c^2)^2, which gives E_r = m_0c^2.

The mass of an atom is lower than the sum of the masses of its constituent particles. This is due to the binding energy released when the atom was first formed. This difference in mass is known as the mass defect. The binding energy (ΔE) of an atom can be calculated from the mass defect (Δm) of the atom using the formula: ΔE = Δmc^2.

The energy we are getting out of a fusion reaction was "put in" to the nuclear reagents. In other words, the total "energy" of the system remains constant; it just changes shape.

Frequently asked questions