= Normal component of a superfluid
{title2=$\mathbf v_n$}
= Normal fluid
{synonym}
The normal component of a <superfluid> describes the momentum and entropy carried by excitations rather than the phase-gradient condensate current. Its <velocity> need not equal the <superfluid velocity>. In phenomenological condensate equations, prescribing its motion supplies a reference flow for dissipation; a full two-component fluid theory would also evolve its momentum and energy.
These are two responses of one <liquid>, not separable kinds of atoms. Their fractions need not equal the <Bose-Einstein condensation> fraction.<The Theory of Superfluidity of Helium II by Lev Davidovich Landau>[Landau (1941), reprint §5, p.506]{ref}
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