Electric current and charge

Circuit measuring current, that is, rate of flow of charge.

The first experimental scientists realised that current flowed around a circuit. Unfortunately when they guessed which way, they guessed the wrong way, but the convention still stuck.  

We now know that almost all current flow is caused by negatively charged electrons flowing from negative to positive. However, most circuit analysis is still done in terms of the old “conventional current” flow.


Conventional current flow

Electron flow

Defining a coulomb

Current and charge

The real current is almost always a flow of free electrons in metallic conductors. The outer shell electrons of metals are loosely attached and flow easily from atom to atom. However the charge on an electron is tiny, far too small to be a useful unit in most measurements. We measure charge, that is the total amount of electricity flowing, in much larger units called Coulombs. One coulomb is a charge of about 6.3 x 1018 electrons

Current is the rate of flow of charge so the definition of an ampere (or amp for short) is that the current is 1 ampere (often shortened to amps, unit symbol A) if one coulomb of charge flows past in one second.

Defining one amp


Equation connecting charge and current

The equation

So the simple equation connecting current, charge and time is Current = charge/time in symbols  I = Q/t

Electric charge and current explained

Two other useful pages on electric current and charge are: