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Femtometer

A femtometer (fm) is a unit of length in the metric system, equal to one-quadrillionth of a meter (1 fm = 10⁻¹⁵ meters). It is also known as a fermi, named after the physicist Enrico Fermi. Femtometers are used to measure extremely small distances at the subatomic level, such as the size of protons, neutrons, and atomic nuclei. For example, the radius of a proton is about 0.84 femtometers. This unit is commonly used in nuclear physics and particle physics, but it's far too small for everyday measurements.

Classical Electron Radius

The classical electron radius is a theoretical length scale associated with the electron, derived from classical electromagnetism. It represents the size of a hypothetical sphere where the electron’s electrostatic energy equals its rest mass energy.

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  Its value is approximately 2.818 × 10⁻¹⁵ meters (about 2.8 femtometers).

  
  
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  Calculated using fundamental constants: the electron charge, mass, and the speed of light.

  
  
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  It is not the actual physical size of the electron (which is considered point-like in quantum mechanics), but a useful scale in classical physics.

  
  
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  Used in scattering theory and classical electron models.

  
  

Key facts:

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  Classical electron radius ≈ 2.818 × 10⁻¹⁵ m

  
  
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  Represents an electromagnetic length scale, not a physical size

  
  
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  Important in classical models of electron behavior

  
  

This radius helps bridge classical and quantum views of particle physics.