Quantum physics is applicable to systems at the smallest scales, such as atoms, molecules, and subatomic particles. It governs the behavior of particles and their interactions, providing a framework to understand phenomena that occur on the quantum level.
The transition between classical physics and quantum physics is not strictly defined by a specific size. Rather, it is determined by the characteristics and properties of the system being considered. As a general guideline, when the characteristic properties or behaviors of a system are influenced by quantum effects, quantum physics becomes relevant.
For example, at the atomic scale, quantum effects become significant. Atomic systems exhibit discrete energy levels, electron configurations, and quantized angular momentum. These phenomena cannot be explained solely by classical physics but require the principles of quantum mechanics.
Furthermore, when dealing with particles such as electrons, photons, or individual atoms, their behavior is inherently quantum mechanical. Interactions between these particles are described by wave-particle duality, superposition, and quantum entanglement, which are fundamental aspects of quantum physics.
It's worth noting that while quantum effects are most prominent at the microscopic level, they can also manifest in macroscopic systems under specific conditions. Examples include superconductivity, where electrical resistance disappears at very low temperatures, or superfluidity, where a liquid flows with zero viscosity. These macroscopic phenomena emerge from the quantum behavior of a large number of particles working together coherently.
In summary, quantum physics begins to dominate at the scale where the characteristic properties and behaviors of a system are influenced by quantum effects, typically at the atomic and subatomic levels.