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I Does Destructive Interference Cancel Energy?

  1. Aug 27, 2016 #1
    Does it cancel it, or just create an equilibrium of forces upon a point(s)?

    Example: If you had a string and you sent a wave down one end, and the opposite wave from the other, once they meet there will be an instant when the string is flat as if there had never been waves introduced. But, if you were to analyze the fibers and molecules of the string at the point of the interference, would you find that they were under more "stress" (or just more energized) than a string without any waves in it, or does the energy actually cancel at that moment leaving the string stress/energy free and just as it was before the waves were presented?
  2. jcsd
  3. Aug 27, 2016 #2
    The question of stresses on the fibres is better framed in terms of forces which do in fact cancel.
  4. Aug 27, 2016 #3
    Ok, so you're saying that the string wouldn't have and forces/stress/whatever at that moment, and the fibers/molecules would basically be like they were before any waves were present?
  5. Aug 27, 2016 #4
    For a perfectly uniform string yes, if no then as long as the fibres have not exceeded their elastic limit then yes.
    ETA forces are present, the net force is zero.
  6. Aug 27, 2016 #5

    Ok. What are ETA forces?
  7. Aug 27, 2016 #6
    Note your OP is poorly constructed and leaves a lot of wiggle room. You did not define the phase, frequency and amp.

    Google standing wave to see a better question.

    ETA, did I just make a pun, wiggle/wave....get it.
  8. Aug 27, 2016 #7
    I'm not talking about standing waves. I just want to know if when two identical waves destructively interfere with each other perfectly, does the energy/force at that moment vanish, or is it just bound up...?
  9. Aug 27, 2016 #8
    Two identical waves can never destructively interfere. Tighten up your question.
  10. Aug 27, 2016 #9
    Two identical waves 180 degrees out of phase. That better?
  11. Aug 27, 2016 #10
    Then there is no energy propagating from the source.
  12. Aug 27, 2016 #11
    Would it help to think of a speaker as source of sound and the waves driving the speaker.
  13. Aug 27, 2016 #12
    I don't get the complication you're adding to this. Simple: There is destructive interference. There is total destructive interference. All I want to know is does the energy at that moment vanish or is it bound up in the medium...?
  14. Aug 27, 2016 #13
    Bound up??

  15. Aug 27, 2016 #14
    Yeah, like if two men are pushing against each other (let's imagine sumo wrestlers) with the same force they don't go anywhere. However, there is a lot of pressure and energy being exerted. They are bound up against each other. The force is still there even though they are not moving. In destructive interference, is it like the sumo wrestlers in a sense, or does force actually cancel out at the moment of destruction?
  16. Aug 27, 2016 #15
    As previously stated the net force is zero hence the particle feels no interaction.
  17. Aug 27, 2016 #16
    Thank you
  18. Aug 27, 2016 #17


    Staff: Mentor

    The string is moving. It has kinetic energy.

    Energy is conserved. In destructive interference energy is always either moved elsewhere or converted into some other form.
  19. Aug 27, 2016 #18

    Charles Link

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    Your question makes more sense when electromagnetic waves are considered. In quite a number of interference cases, energy is 100% conserved, and whenever there is a cancellation from destructive interference, there are necessarily brighter spots in the energy pattern where constructive interference is occurring. This is the case for both multi-slit interference as well as Fabry-Perot type interference and Michelson interferometer type interference.
  20. Aug 28, 2016 #19


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    At last! :smile:
    The OP is actually describing of a standing wave, involving two waves, travelling in opposite directions. He is denying it because he isn't coming from that direction in his description but, nonetheless, that's what he is describing. Very often, a standing wave occurs due to reflection at each end. What's being described here is a standing wave that's sustained by two energy sources, one each end. The displacement at each end is kept at zero and energy is fed into the system, building up with no limit (in an ideal case). In practice, the natural losses will impose a maximum amount of energy stored in the standing wave when the power supplied is the power lost to the surroundings. The standing wave is the same as an interference pattern from two sources (only it's in just one dimension). In nodes, the energy density is low (zero), where there is cancellation and maximum in the antinodes where there is enhancement.
  21. Aug 28, 2016 #20


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    Something that tends to be ignored about standing waves. You HAVE to acknowledge that Energy must be leaking out of the resonator or it will build up without limit. It's not a problem with 2D Interference patterns because the Energy always goes somewhere (the screen or free space. All resonators lose a fraction of their energy every cycle. Either 1/10, 1/100, 1/1000000 but that is what limits the peak energy.
  22. Aug 28, 2016 #21
    Ok, but I'm really just thinking of a single pulse from each end-- one pulse up, and the other down. When they meet there will be destructive interference. I'm still a bit wondering what's going on in that very instant that they perfectly cancel each other out, leaving just a straight string. It makes more sense now though.
  23. Aug 28, 2016 #22


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    You had the answer to that question up near the top, I think. There may be no displacement where they cross but the string will still be moving and carrying Kinetic Energy in that particular portion, briefly.
    Last edited: Aug 29, 2016
  24. Aug 28, 2016 #23
    Chris, you stated yourself that the string is only flat for an instant. That implies that it never stopped moving. Therefore, the energy does not cancel, and it is expressed through the motion of the string. Analysis of the string's molecules at the moment of destructive interference would reveal that they are indeed under stress due to the fact that they are in motion. The forces are what cancel at that moment.
  25. Aug 28, 2016 #24
    Yeah, I get this better now :) Thanks
  26. Aug 28, 2016 #25
    Yes, I get this better now. Thank you
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