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fermi Line breaks: fermi
Pronunciation: /ˈfəːmi/

Definition of fermi in English:

noun (plural same)

A unit of length equal to 10−15 metre (one femtometre), used in nuclear physics. It is similar to the diameter of a proton.
Example sentences
  • In order to fuse two hydrogen nuclei together, though, we must force another proton into the hydrogen atom within a distance of 1 fermi from the atom's proton.
  • About half a century later, Robert Hofstadter won the 1961 Nobel Prize in physics for his work verifying that the radius of an atomic nucleus is about one fermi.
  • It acts between two neutrons, between two protons, or between a neutron and a proton, as long as they are no farther apart than a distance of one fermi.


Early 20th century: named after E. Fermi (see Fermi, Enrico).

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