= Charge carrier
{wiki}
A <charge carrier> transports <electric charge> by moving: <electrons> are one example, and <ions> are another. For species $s$ with <number density> $n_s$, <electric charge> $q_s$ and mean <velocity> $\mathbf v_s$, the transport contributions add as $\mathbf J=\sum_s n_sq_s\mathbf v_s$. A stationary charged background contributes to <charge density> but not to this <current density>. https://openstax.org/books/university-physics-volume-2/pages/9-1-electrical-current[Ling, Sanny and Moebs, §9.1: positive and negative carriers]{ref}.
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