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Energy in a wave

  1. Jan 10, 2005 #1


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    i just learned this so dont be shy about telling me i have no idea what im talking about;

    anyway if you have a wave, say sound, you can find the intensity of the wave, say I

    now I is in watts/meters^2 so in the example of sound you can find the amount of energy that the wave makes(?), has(?) by multiplying by the area that the wave goes through (for example a window)

    my question is what exactly is the "energy in a wave", and can someone exaplain why the wave can have different "energies" by going through larger openenings(?)

    also im not even sure if there is such a thing as the "energy of wave" so :yuck:
  2. jcsd
  3. Jan 10, 2005 #2


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    Staff Emeritus
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    The measure of intensity -- watts per square meter -- has nothing to do with openings.

    The watt is a unit of power. Power is effectively the movement of energy; one watt is defined as one joule moved per second. The joule is a unit of energy.

    A simple spherical source produces spherically symmetric waves which spread out from it. If you're close to the source, the sound is very loud; if you're far away, the sound is quieter. If the source radiates a constant amount of energy in the form of sound every second -- say, it generates one joule of sound energy per second (that's one watt) -- that energy will be spread over the entire sphere surrounding it.

    A sphere, of course, has a surface area. If you place a listening device one meter away from your one-watt sound source, you can imagine it on the surface of a 1-meter radius sphere surrounding the source. The sound energy is spread over the entire surface area of that sphere, which has a surface area of almost 13 square meters. Thus, the sound intensity at a distance of one meter is one watt spread over 13 square meters, or about 1/13 watt per square meter.

    If you place the listening device 10 meters away instead, the sphere surrounding the source is now 10 meters in radius, and has a surface area of almost 1300 square meters. The sound intensity there is only one watt spread over 1300 square meters, or about 1/1300 watt per square meter.

    If you make your listening device exactly one square meter in size, it will intercept about 1/1300 watts of power when placed 10 meters away from your source. If you make a larger detector, say one that is 10 square meters in size, it will intercept ten times as much of the sound energy.

    - Warren
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