Current

Here is an electrical circuit.

There are two conditions for current to flow: There must be a complete circuit There must be a supply of energy Both of these conditions are met so current flows, from the positive of the power supply, round the circuit and back to the negative. We know a current flows through the bulb because it lights up.

So what exactly is flowing in this circuit?

In a river current is the flow of water. It is measured in kg / s or m³ / s, i.e. the amount of water which flows past a point every second.

In a circuit current is the flow of charge. It can be measured in coulombs per second where 1 C / s = 1 amp. The amount of charge which flows past any point in the circuit every second is the current at that point.

The charge in the wire is carried by tiny negatively charged particles called electrons. The electrons actually flow from negative to positive but we imagine that something positive flows round the circuit the other way. There are situations where the charge is carried by positively charged particles, when this happens current flow is in the same direction.

Here's a very good friend of mine. Colin the Coulomb. He has helped me to explain electricity for many years now.

Nevertheless, when a current of 1 A flows through something that is 1 C of charge per second. An awful lot of electrons.

Current (A) = Charge (C)  / time (s)        I = Q / t

Does this mean that they flow very quickly?

Actually no. They are jiggling about randomly inside the metal very quickly but they drift though the wire very slowly. About 1 millimetre per second typically. The current is large because there are a LOT of electrons flowing, not because they travel quickly.

Study these circuits carefully and try to understand why the current is what it is at different points.