# 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

## 1 Answer

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.

• Positrons do not exist in a copper wire. – my2cts Feb 10 at 21:18
• @my2cts Copper is is number 29 on the periodic table, consists of 29 neutrons and 29 positrons ? – DakkVader Feb 11 at 7:54
• @my2cts Thank you, of course i meant protons. I've changed it now – DakkVader Feb 11 at 7:54
• I cant understand the flow through a section. How we measure the flow in a surface ? Why it isnt define as flow through volume? – ado sar Feb 13 at 1:10
• Current density is by the cross-secitonal area, so if you integrate this current density over the length of the conductor, it becomes over a volume instead. @adosar – DakkVader Feb 14 at 7:58