## Electric Current

Electric current is due to the flow of charge. Either positive or negative charge can constitute a current, but a current can only consist of positive charge if a material is molten. The atoms of metals are positively charged, while the electrons are free to move. In wires current is always due to the movement of electrons, which are negatively charged. We can derive a simple equation giving the electric current in terms of measurable quantities as shown below.

The average drift velocity of electrons carrying an electric current is surprisingly small – maybe only a mm per second or so: If we assume one free electron per atom.atoms or charge carriers perin a wire of cross sectional areathen

This does not mean that the electric current moves at a few mm per second, because as soon as charge starts to flow, it can flow throughout the wire almost simultaneously.