# Ηow does a current density becomes a charge density?

The transformations in special relativity of current and charge density fields are given as follows: $\bar J' = \begin{matrix} \gamma(J_x - Vρ) \\ J_y \\ J_z \end{matrix}$

$ρ' =\gamma (ρ - {V J_x \over c^2})$ with the relative motion happening in the x axis and J is the current density, V is the volume and ρ is the charge density.

The one term I cannot understand is that of the tittle of this question: It's the second term in the charge density in the moving reference frame, that is as you can see: $V J_x \over c^2$

I believe I understand how a charge density gives a current in relative moving frame- it's doe to the relative velocity that has as a result the appearance of a new current.

Question: I cannot understand how a current density gives at the moving reference frame a "new" term of charge density. What is the different thing that the moving observer sees and makes him count this term?

Thank you.

Note. I have read the mathematical proof, but I don't understand what happens and this term makes an appearance.