Why the electric bulb turns on almost instantly when the switch is closed?

Question

The electron drift speed is estimated to be very low.How could there is current almost the instant a circuit is closed??
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By the discussions it is known that The information about beginning of the flow of current is transmitted through the propagation of electromagnetic waves(electric impulse)and not with the drift velocity of the electrons.
But I want any one to explain how this process takes place.CURIE:)

i think it has got to do something with electric field, i learned about it when i was learning about how transformers work. — Muhammad Umer, Oct 16 '13 at 05:25
possible duplicate of How can I calculate the wave propagation speed in a copper wire? — John Rennie, Oct 16 '13 at 06:21
@John Rennie.I want you to explain how the propagation of electromagnetic waves can transmit the information of beginning of the flow of current.CURIE:) — Sensebe, Oct 16 '13 at 06:40
OK I've given an answer in general terms. If you want specifics of the electron transport that's going to be an altogether longer answer and should probably be addressed by a new question. — John Rennie, Oct 16 '13 at 07:25

score 6 · Answer 1 · answered Oct 16 '13 at 07:24

6

A commonly used analogy is to represent the electric circuit with pipes filled with water. The electrical current is modelled by flow of water in the pipes, and the voltage is modelled by the pressure. This is known as the hydraulic analogy.

Anyhow, if you have a pipe filled with water and you suddenly increase the pressure at one end, e.g. by opening a valve, the pressure propagates down the pipe at the speed of sound in water. Note that it's not the water itself that's travelling - it's the pressure wave.

This is what happens in your electrical circuit. When you close the switch electrons flow into the wire where they bump into the electrons already in the wire and push them along. The voltage wave (analogous to the pressure wave) travels along the wire at somewhere between a tenth and a half of the speed of light depending on the type of wire, and reaches the light bulb in a few nanoseconds. That's how the electrons in the bulb filament start moving within a few nanoseconds of you closing the switch.

answered Oct 16 '13 at 07:24

John Rennie

355,118

.Up to my knowledge we can not compare wave propagation in liquids with solid conductors.Because,the molecules in the liquids will be quite loosely arranged than solid conductors.Thus,movement of wave would not be as easy as you said within few nanoseconds with respect to the description you gave.It would be better if you can give answer by considering electrons in specific in the present asked question.CURIE:) – Sensebe Oct 16 '13 at 07:56
2

As long as the mean free path of the particles is well below the wavelength of the wave, waves propagate in a liquid in a very similar way to the solid. This is because the timescales for molecular motion are much shorter than the period of the wave so the details of the liquid structure are smeared out. – John Rennie Oct 16 '13 at 08:24
@John Rennie The electrons (far from the high potential terminal of the battery) start moving as soon as the switch is flipped on but the potential drop across the circuit elements wouldn't be uniform since the circuit has not yet reached steady state. When the steady state is achieved will there be any change in the bulb's intensity? – Aditya Ahuja Aug 18 '20 at 14:50

Why the electric bulb turns on almost instantly when the switch is closed?

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