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How quickly does light turn on once you've hit the switch?

  1. Oct 30, 2015 #1
    Once you flipped the switch how quickly does the electricity reach the light bulb? I've read somewhere that it's at the speed of light but I might be wrong, I've also read somewhere that electrons flow very very slow so I can't understand how will lights will turn on instantly.
  2. jcsd
  3. Oct 30, 2015 #2


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    Staff: Mentor

    The change in the electrical field is what matters, and it propagates at a significant fraction of the speed of light - so we're talking maybe tens or hundreds of nanoseconds from switch closing to voltage across the filament of the light bulb.

    The drift velocity of the electrons in the wire is much smaller, but that's something different. If you and I are holding opposite ends of a long metal rod and I pull on my end, you'll feel it almost immediately whether I pull quickly or slowly.
  4. Oct 30, 2015 #3
    Oh, thank you! that clarifies a lot. One more question, regarding to the electrical field propagating "at a significant fraction of the speed of light", exactly how quickly is the propagation in comparison to the speed of light.
  5. Oct 30, 2015 #4


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    2017 Award

    Staff: Mentor

    Roughly 2/3, but it depends on the circuit.

    The time until there is actual light emitted can be much longer, by the way. With LEDs it is fast, but other types of lamps need some time to heat up before they work.
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