# Electric Current

Electric current is the movement of mobile charge carriers. It is measured in amperescoulombs per second. With positive and negative charge carriers we have to take into consideration the total charge going in each direction. So, if we have a conductor with:

• A - cross-sectional area in m2,
• n - number of mobile charge carriers per m2,
• v - drift velocity of the mobile charge carrier in ms-1,
• t - time in s, and
• q - the charge of a mobile charge carrier in C

then for positive charge we have:

```Q+=qpnpvpΔtA
```

coulombs of charge passing a point in one second. Similarly for negative charge we have:

```Q-=-qnnn-vnΔtA
```

This gives us in total:

```Q++Q- = (qpnpvp + -qnnn-vn)ΔtA
```

Now, current is the rate of charge flow, so:

```I = (Q++Q-)/Δt = (qpnpvp + -qnnn-vn)A
```

Note that in a metal, we only have one type of mobile charge carrier, the electron, so:

```I = Q-/Δt = -qnnn-vnA
```

The current divided by the area (I/A in our notation) gives us what is known as the current density.

## References

Fischer-Cripps. A.C., The Electronics Companion. Institute of Physics, 2005.