Why is a bulb lit as soon as a torch is switched on?

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Discussion Overview

The discussion centers around the question of why a bulb lights up immediately when a torch is switched on, despite the slow drift of electrons in the wire. Participants express confusion over textbook explanations and explore various analogies to clarify the phenomenon.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant notes that while electrons drift slowly, the vibrations they cause under voltage create heat that quickly heats the filament to white hot.
  • Another participant suggests that although electrons move slowly, the wire is filled with them, and once the first electrons start moving, a pressure wave travels through the wire at the speed of light, causing all electrons to move almost immediately.
  • A participant uses the analogy of water flowing from a faucet to illustrate how the initial movement of electrons leads to a rapid response throughout the wire.
  • One analogy compares the situation to crowded people on a street reacting to a call for "Free Beer," where the initial excitement causes a quick collective movement, similar to how electrons interact in the wire.
  • Another participant likens the movement of electrons to marbles in a tube, where pushing one marble results in another marble emerging from the other end almost instantaneously, despite the slow movement of individual marbles.

Areas of Agreement / Disagreement

Participants do not reach a consensus on a single explanation but present multiple analogies and models to describe the phenomenon, indicating a variety of perspectives on the underlying mechanics.

Contextual Notes

Some assumptions about the behavior of electrons and the nature of electrical signals are not explicitly stated, and the discussion relies on analogies that may not capture all aspects of the physical processes involved.

koat
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The vibrations they cause as they drift under the influence of the voltage creates heat which quickly heats the filament to white hot.
 
The electrons might move relatively slowly, but the wire is already full of them, so once the first ones start to move, a pressure wave moves through the wire (at the speed of light) causing all of them to start moving very soon.

For the same reason, water starts pouring out of your faucet almost instantly after you turn it on.
 
Addendum: Imagine a street full of crowded people milling around. Someone yells Free Beer! from one end of the street creating a desire to move in one direction. They're crowded so they don't move quickly but their bustling and bumping into each other will cause quite a loud ruckus quite quickly. The light from the filament is the electrons yelling for free beer as they bump along. ;)
 
You could also think of it in terms of marbles in a tube. Where the marbles represent electrons and the tube represents the wire. If you push a marble in one end of the tube, another marble will come out the other end almost instantaneously. Even though the marbles are actually moving very slowly, the force moves from one to the other with great speed. It's the same in a wire. Even though the electrons are only moving at 0.03miles/hr, the wave fronts are moving much, much faster.
 
jambaugh said:
Addendum: Imagine a street full of crowded people milling around. Someone yells Free Beer! from one end of the street creating a desire to move in one direction. They're crowded so they don't move quickly but their bustling and bumping into each other will cause quite a loud ruckus quite quickly. The light from the filament is the electrons yelling for free beer as they bump along. ;)

Now that clears it up!
 

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