Current: how much electricity flows

You press a switch and the light comes on at once. But the electrons in the wire are not racing like cars. In fact, they hardly seem to move at all. So how does the light come on so fast?

To answer that, we need the idea of current.

Current is how much flows

If voltage is the push, current is the flow. In the pipe picture, current is how much water passes a point each second. In a circuit, it is how much charge passes a point each second.

Current is measured in amperes, called amps (A) for short. One amp means about six billion billion electrons (6,240,000,000,000,000,000) pass a point every second. You never need to remember that number. The point is that it is huge, and that current is a count of how many charges go by.

Small currents are often written in milliamps (mA). One milliamp is one thousandth of an amp.

A traffic jam in a pipe

Think of a long pipe already full of water. Push more water in one end, and water comes out the other end right away, even though each drop moves only a little. The push travels along the pipe much faster than any single drop.

A wire is the same. It is already full of loose electrons. When you close the switch, the battery's push moves all of them at once, all the way round the loop. The bulb lights immediately, because the electrons in the filament start moving at that moment.

How fast do the electrons themselves move? Slowly. In a typical household wire they drift at less than a tenth of a millimetre per second. That's slower than a snail. It is the push that races along the wire, not the electrons.

The same all the way round

Here is a rule that surprises many people. In a simple loop, the current is the same everywhere. It is not used up by the bulb. The electrons that go into the bulb are the same number that come out the other side. What the bulb takes is energy, not electrons.

It's like a loop of pipe with a water wheel in it. The same amount of water flows past every point, but the wheel takes energy from the water as it turns.

Try it

Change the voltage and resistance and read the current in the middle of the picture.

1. What happens to the current when you raise the voltage?
2. What happens when you squeeze the pipe (raise the resistance)?
3. Can you find two different settings that give the same current?

The dots in the pipe move faster when the current is bigger. Current is the amount of flow.