Question

The drift velocity of free electron is very small why does room light turn on at once as the switch is closed

Answer 1

The drift velocity of electrons is extremely low.

However, it is not the movement of electronsfrom the battery to the bulb that ignites the light, but it is moving charge, aka current.

“But Daren”, I hear you ask, “It’s moving electrons that produces current, isn’t it?”

And again, you are right. But let me draw an analogy here.



A bucket brigade. Everyone starts with a bucket full of water, and each person passes his bucket to the next.

Consider 5 people, standing at A,B,C,D and E. Let’s assume every “pass” of a bucket takes 1 second. More than one “pass” can occur at a time. Everyone has a full bucket at all points in time.

A B C D E F

If the water is needed to put out a fire at F, you needn’t wait for the bucket from A to make it’s way to F (5 seconds), you just need the bucket from E to reach F(one second). You need “a” bucket, and not the “first bucket”(at A) to put out the fire.

Similarly, conductors have electrons throughout their length (and cross section!). The electron starting from the battery, just like the bucket from point A, doesn’t need to reach the bulb, for it to glow.

Instead, every electron through the wire moves by a distance dSdS, so that the electron dSdS units away from the bulb hits the bulb at t≈0t≈0.

Electrons travel at vdd, but Information can travel at the speed of light!