Tiny bits move in a fast way. They fill up small spots. They cannot stay in the same spot. Even when cold, they zoom around. This helps stars stay big. It is a busy world! Do you like to zoom?
Tiny bits like electrons move in fast ways. These bits must fill up spots. Two bits cannot stay in the same spot. They fill the lowest spots first. They go from low to high energy. Even when it is very cold, they zoom. The fastest bits have a special energy. This energy helps white dwarf stars stay big. It also helps the center of an atom stay strong. It is a busy world of moving bits!
Tiny bits like electrons move in fast ways. These bits are called fermions. This group includes protons and neutrons too. Fermions follow a special rule. This rule says two fermions cannot stay in the same spot. We call these spots quantum states. To find the lowest energy, we fill these spots one by one. We start with the lowest energy spots first. We add particles until all spots are full. The energy of the very last spot is the Fermi energy. This energy is measured at a temperature called absolute zero. At this cold state, the bits still move fast. The fastest bits have a speed called Fermi velocity. This energy helps many things in space. It helps white dwarf stars stay big. It also helps the center of an atom stay strong. In metals, this energy is quite small. In a white dwarf star, the energy is much larger. The energy in an atom's nucleus is also very large.
Science helps us understand how tiny bits of matter work. Some particles, like electrons, protons, and neutrons, are called fermions. These fermions follow a rule called the Pauli exclusion principle. This rule says two fermions cannot stay in the same quantum state. You can think of states like seats in a theater. Each seat can only hold one person at a time. Because of this, particles must stack up into different energy levels.
To find the lowest energy, we imagine a system at absolute zero. This is the coldest temperature possible. We start with an empty system and add particles one by one. We fill the lowest energy states first. We keep going until all the particles are placed. The energy of the very last particle is the Fermi energy. This is the difference between the highest and lowest occupied states.
Scientists use these ideas to study many different things. This concept is important in the study of metals and superconductors. It also helps us understand quantum liquids like helium. Even the center of an atom uses these rules. In space, Fermi energy helps explain white dwarf stars. These stars stay stable because of the energy of their electrons.
Different things have different amounts of Fermi energy. In metals, the energy is usually between 2 and 10 electronvolts. The density of electrons in these metals is very high. It is about 10 to the 28th or 29th power per cubic meter. In a white dwarf star, the energy is much higher. It is about 0.3 MeV. The nucleons inside an atom's nucleus have even more energy. That value is usually about 38 MeV.
We can also measure how fast these particles move. Even at absolute zero, fermions are still moving quite fast. The fastest particles move at the Fermi velocity. This speed matches the kinetic energy of the Fermi energy. There is also a thing called the Fermi temperature. This is the temperature where heat starts to change how particles act. For a metal, this temperature is much higher than room temperature.
Fermi energy is a fundamental concept in quantum mechanics. It describes a specific energy difference in a system of fermions. Fermions are a group of particles that include electrons, protons, and neutrons. This concept is vital for understanding how matter behaves at very low temperatures. It helps scientists study metals, superconductors, and even the stars in deep space.
To understand this, we must look at the Pauli exclusion principle. This principle states that two fermions cannot occupy the same quantum state at once. In a system of non-interacting fermions, we can analyze these as stationary states. These states are usually separated by different levels of energy. To find the ground state, we imagine an empty system. We add particles one by one, filling the lowest energy states first. Once all particles are placed, the energy of the highest occupied state is the Fermi energy.
It is important to distinguish Fermi energy from the Fermi level. In semiconductor physics, many people use these terms interchangeably. However, they have distinct scientific meanings. Fermi energy is only defined at absolute zero temperature. It represents an energy difference, usually corresponding to kinetic energy. In contrast, the Fermi level, or electrochemical potential, is defined at any temperature. The Fermi level includes both kinetic energy and potential energy. It remains well-defined even in complex, interacting systems at equilibrium.
Even at absolute zero, fermions are not still. Because they must occupy different states, they continue to move. This movement happens even if we extract all possible heat from the system. The fastest particles move at a speed called the Fermi velocity. This velocity corresponds to the kinetic energy of the Fermi energy. There is also a related concept called the Fermi temperature. This is the temperature where thermal effects become comparable to quantum effects. For most metals, this temperature is much higher than room temperature.
We can see different values for Fermi energy in various environments. In metals, electrons act like a Fermi gas. The number density of conduction electrons is very high. It ranges between 10^28 and 10^29 electrons per cubic meter. This density results in a Fermi energy of 2 to 10 electronvolts. This is a relatively small amount compared to other systems. However, it is essential for the physics of solid matter.
Other systems show much higher energy levels. In a white dwarf star, the density is extreme. These stars have a mass similar to our Sun but a much smaller radius. Their electrons form a degenerate electron gas. The Fermi energy in these stars is about 0.3 MeV. Inside an atom, the nucleons also show high energy. The Fermi energy for nucleons in a nucleus is typically 38 MeV. This high energy helps explain why white dwarf stars do not collapse under gravity.
Scientists use mathematical formulas to calculate these values for specific systems. For a three-dimensional, non-relativistic, non-interacting group of identical spin-fermions, the formula uses several variables. It involves the number of particles, the mass of each fermion, and the volume of the system. It also uses the reduced Planck constant. Other related quantities include Fermi momentum and the Fermi wavevector. These describe the momentum and velocity of a fermion at the Fermi surface. These measurements are crucial for the study of quantum liquids, such as superfluid helium-3.
Where will your curiosity go next?
More to explore
✨ What else?
Related topics you might enjoy
🔬 Go deeper
More advanced topics to explore
🪜 Step back
Simpler topics to build understanding
What is Nepedia?
A free, ad-free encyclopedia for children. Every article is written at five reading levels, so the same page works for a five-year-old and a fifteen-year-old — use the level switcher above to see this one change. No account needed to read.