# Four-force/4-current

(Redirected from 4-current)
Given that an observer both moving with and local to a bit of charge finds it to have a charge density of ${\displaystyle \rho _{0q}}$, the product of this scalar quantity with its 4-velocity according to a given frame will constitute a four-vector known as the 4-current given by
 ${\displaystyle j^{\mu }=\rho _{0q}U^{\mu }}$