# How electric current is defined in a metal wire?

How current is defined if in a metal wire both positive and negative charge exist? Isn't the total charge/time 0?

• current is total charge across (crossing) a surface, and only the electrons move – Wolphram jonny Feb 10 at 15:24

Yes, both positrons and electrons exist in, let's say, a copper wire. The current however is only made up of movable charges, and those charges are made up of electrons only, the protons can't move to 'cancel' out the current from the electrons. The current is defined as $$\frac{dQ}{dt}=I$$, a similar property is a wires current density $$J=\frac{dQ}{dt}\frac{1}{A}[\frac{A}{m^2}]$$ where the A in the equation is the cross-section area and the A in the unit represents ampere.