Question

Why a bulb glow so rapidly even if electrons move at very low speed?

Answer 1

The speed at which electrons move through a wire is called "electron drift," and it happens at around 0.1-0.4 millimeters per second. So with that speed it would take hours for the electron to reach the other end of the wire. But when we switch on a bulb it glows instantly.

A typical copper wire contains lots of electrons (8.5x10^28 per cubic meter, to be exact). Those electrons are packed in so tightly that even a small movement will travel down the wire from electron to electron at an impressive speed, letting you turn on the lights without having to wait for electrons to travel the whole way there.

Think of the wire in comparison to a pipe full of marbles. If we push another marble into a filled pipe, then one marble would have to exit the other end. Electrons are like that in a wire. If one moves they all have to move. Thus when you turn on a switch an electrical potential difference immediately causes a force that tries to move the electrons. If you make one electron move when you turn on a switch, the electrons throughout the wire move, even if the wire is miles long. Therefore when you turn on a switch, the electrons in the light start moving "instantly" as far as we are concerned, i.e. something starts to happen throughout the electrical system. Although the electrons are actually moving through the wire slowly, we say that the speed of electricity is near the speed of light (extremely fast). What we really mean is that the effects from the electricity occur "instantly." The light comes on the instant you flip a switch. You do not have to wait for electrons to flow from the switch to the light.