14 votes

The big misconception about electricity

16 comments

  1. [10]
    Weldawadyathink
    Link
    Somehow the YouTube comments section for this video is surprisingly good. One question I saw there really has me questioning this explanation.

    Somehow the YouTube comments section for this video is surprisingly good. One question I saw there really has me questioning this explanation.

    Ok I have a very interesting paradox for you. Let's say
    you have this setup where the wires stretch out very
    far. We'll say a light minute or a light hour to further
    nail this point. Someone at the furthest point away
    can secretly decide to either cut the wire, or not cut
    the wire, before you conduct this experiment. When
    you flip the switch next to the battery and lightbulb,
    the lightbulb is supposed to turn on in in 1/c seconds.
    Does this mean:
    a) we broke causality and we were able to determine
    whether the wire was cut faster than the speed of light
    b) the lightbulb can turn on even if the circuit is not
    connected
    c) we were wrong about the lightbulb turning on in
    only 1/c seconds.

    7 votes
    1. [8]
      teaearlgraycold
      (edited )
      Link Parent
      The way I think of it is this: the two long parallel wires are basically linearized coils. You don’t need connections from one end of a transformer to another. You don’t in the circuit from the...

      The way I think of it is this: the two long parallel wires are basically linearized coils. You don’t need connections from one end of a transformer to another. You don’t in the circuit from the video either.

      The comment you quoted is only right about breaking causality if it’s the electrons traveling from A to B that move the energy. The entire video is about how it’s the field not the electrons that move energy.

      To be honest it’s a bit of a trick question. The light wouldn’t actually turn on instantaneously because the amount of induced current on the opposite side would be minuscule to start. And I think more people would be able to intuit the solution if the example circuit had both ends cut. Then the transformer nature of the circuit would be more obvious.

      4 votes
      1. [2]
        mtset
        Link Parent
        This is the critical point here. Derek pulls a neat little sleight of hand at the beginning of the video; at around 0:45, he says "the light bulb has to turn on immediately when current passes...
        • Exemplary

        The way I think of it is this: the two long parallel wires are basically linearized coils. You don’t need connections from one end of a transformer to another. You don’t in the circuit from the video either.

        This is the critical point here. Derek pulls a neat little sleight of hand at the beginning of the video; at around 0:45, he says "the light bulb has to turn on immediately when current passes through it."

        What's he's describing here is a very sensitive oscilloscope or radio, not a light bulb; yes, some current would pass through the measuring device as you turned the switch on, because changing currents create propagating EM disturbances which would be picked up on the other side of the loop, even if cut, inducing a voltage and thus some current flow. But there is no lightbulb in the world that would light up noticeably from that, and it would not stay "lit" for a noticeable time.

        3 votes
        1. vektor
          Link Parent
          At this point I'm actually kind of convinced he's done this and a few other controversial (thought-)experiments (e.g. the windmill-car-thing) intentionally. I mean, he put the interview bits in as...

          At this point I'm actually kind of convinced he's done this and a few other controversial (thought-)experiments (e.g. the windmill-car-thing) intentionally. I mean, he put the interview bits in as well, so he must know that he'll get called out on this one too and that things are much more complex than he lets on in the video.

          The optimist thinks he does it to get people thinking about these things in depth. The cynicist thinks he's doing it for the clicks.

          6 votes
      2. [5]
        vektor
        Link Parent
        Oh, that outcome would still break causality. Basically, if I can detect that the circuit was cut faster than light from there could reach me, I'm in trouble with Mr. Einstein. Consequently, I...

        The comment you quoted is only right about breaking causality if it’s the electrons traveling from A to B that move the energy. The entire video is about how it’s the field not the electrons that move energy.

        Oh, that outcome would still break causality. Basically, if I can detect that the circuit was cut faster than light from there could reach me, I'm in trouble with Mr. Einstein. Consequently, I can't know that the wire was cut until 1h later, consequently the light bulb would "turn on" anyway. Where "turn on" would mean detecting the faintest bit of voltage as a result of these radio antennas interacting. The soonest that cutting the wire can make a difference, no matter my theory of how the energy is transported, is one hour after the wire was cut.

        Put it this way: If the other person cuts the wire synchronously with me flipping the switch, then either this experiment works the same regardless of whether the wire was cut, or I can detect faster than the speed of light whether the wire was cut. The latter would get me in trouble with Einstein, therefore the answer must be that the wires being connected doesn't matter at all. Which is exactly what you concluded as well.

        3 votes
        1. teaearlgraycold
          Link Parent
          Right. That’s what I’m saying. Although once current runs out of fresh wire the results will differ.

          then either this experiment works the same regardless of whether the wire was cut

          Right. That’s what I’m saying. Although once current runs out of fresh wire the results will differ.

          2 votes
        2. [3]
          deadbeef
          (edited )
          Link Parent
          edit: Meant as a response to the question posted by OP. Misplaced reply. It sounds like you're are assuming that what you observe at the bulb is that it is either on or off, as if the energy...

          edit: Meant as a response to the question posted by OP. Misplaced reply.

          It sounds like you're are assuming that what you observe at the bulb is that it is either on or off, as if the energy transferring field is basically static, apart from expanding in space. But it's not. There's an electric wave travelling along the wire when you close the switch, and it doesn't "see" that you've cut the wire until it actually reaches that point, from which the wave reflects back in the other direction.

          This is basically how radio transmitters work*. It is not weird that you're able to measure any voltage variation on the bulb in such a scenario. The voltage won't stabilize at "on"-level, of course.

          *) For radio transmission, you tune the length of the wire (antenna) according to the frequency to get standing waves.

          1 vote
          1. [2]
            vektor
            Link Parent
            Oh, I completely agree. something like that is also what I arrived at. The "shock front" of voltage starting off from the switch moves along the wire and induces a voltage in the other wire. I...

            Oh, I completely agree. something like that is also what I arrived at. The "shock front" of voltage starting off from the switch moves along the wire and induces a voltage in the other wire.

            I guess I was trying to show how even a relatively simple model of electricity would run into issues with the presented experiment. You don't need to know anything about how electricity is transmitted to figure out that Einstein demands that the far end of the wire plays no role. Hence the simplification.

            2 votes
            1. deadbeef
              Link Parent
              Sorry, I think I somewhat confused your comment and the question posted by OP... 😬 My bad!

              Sorry, I think I somewhat confused your comment and the question posted by OP... 😬 My bad!

              1 vote
    2. vektor
      Link Parent
      Reading through a few comments and thinking about it a fair bit (I came up with a similar counterpoint independently) I think my answer is that the light bulb would indeed turn on for Derek's...

      Reading through a few comments and thinking about it a fair bit (I came up with a similar counterpoint independently) I think my answer is that the light bulb would indeed turn on for Derek's definition of turn on, but the voltage delivered would be very low. Basically, you're relying on radio signals to power a light bulb. It's not actually going to turn on, but with careful instrumentation you'd notice a tiny bit of voltage coming in.

      I'm not really happy with the question. Kicks up a whole lot of questions that the video is not remotely prepared to answer. I think looking at a transformer or capacitor might have been a better example to illustrate his overall point, as that's where the conventional theory breaks down.

      2 votes
  2. [3]
    vaddi
    Link
    Sometimes I wonder if there really is a benefit of lying to students when trying to teach them methods on how to compute results. I don't remember someone telling me "So, in reality this doesn't...

    Sometimes I wonder if there really is a benefit of lying to students when trying to teach them methods on how to compute results. I don't remember someone telling me "So, in reality this doesn't really work like this, but lets assume that it does for now." when I was learning this stuff.

    3 votes
    1. Weldawadyathink
      Link Parent
      I think it works to just tell the students it is a lie, but a useful one. I had a great AP Physics teacher in high school. He would constantly tell us that he was teaching us a lie, give us a...

      I think it works to just tell the students it is a lie, but a useful one. I had a great AP Physics teacher in high school. He would constantly tell us that he was teaching us a lie, give us a sneak peek of the truth, and continue teaching us the lie. His system worked very well for me and the other students.

      4 votes
    2. teaearlgraycold
      Link Parent
      Honestly I think most of how they teach college level physics classes is terrible. The averages on the tests I took were pretty low. If you can’t teach a subject better than that you need to...

      Honestly I think most of how they teach college level physics classes is terrible. The averages on the tests I took were pretty low. If you can’t teach a subject better than that you need to figure out a better way.

      2 votes
  3. [3]
    Apos
    Link
    https://youtu.be/--v5BXmFYv4 This video just popped into my recommendations. I think the explanation is a bit clearer.

    https://youtu.be/--v5BXmFYv4

    This video just popped into my recommendations. I think the explanation is a bit clearer.

    3 votes
    1. [2]
      Apos
      Link Parent
      Looks like there are many more video replies. Here is another good one from ElectroBOOM.

      Looks like there are many more video replies. Here is another good one from ElectroBOOM.

      4 votes