# Why do circuits work so fast?

Drift velocity (explained to me as how fast the electrons are moving) is really slow. My book says the electrons move at around 10 mm/ s.

If electrons move so slowly how do circuits work so fast? If you make a basic circuit with just a light bulb, the bulb lights up almost immediately after you connect the wire to the positive terminal.

The answer is that the whole circuit is full of electrons. I think you may be thinking along the lines of "if I switch a tap on, the water takes time $v/L$ to reach the end of a hose of length $L$. So, if I switch a light on, the electrons must take analogous time the reach the light". Because the circuit is full of electrons, the energy source shoves the electrons near it, which shove electrons further down the line and so on. The energy propagates as an electromagnetic wave. The electrons which first light the light are not the ones that first go through the closed switch.
If you switch on a tap with "slow water" going through it and the hose is full of water when you begin, the water comes out the end at speed $v$ as soon as you switch the tap on (or it would, if the water were incompressible: in practice an acoustic wave runs up the hose and accelerates all the water, so that there is a tiny delay - but it's much, much less than what you'd reckon from $v/L$).