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Homework Help: Sunlight, Intensity, electric field RMS

  1. Dec 4, 2016 #1
    1. The problem statement, all variables and given/known data
    Estimate the rms electric field in the sunlight that hits Uranus, knowing that the Earth receives about 1350 W/m2 and that Uranus is 19.2 times farther away from the Sun (on average) than is the Earth.

    2. Relevant equations

    3. The attempt at a solution
    Since Uranus is 19.2 times farther away then:

    Iearth = 1350 W/m2
    Iearth=cεoEearth rms2
    Euranus rms = Eearth rms / 19.2 = √ ( I / (cεo)) / 19.2
    = 37.14 V/m

    I got this problem wrong several times so I just wanted to verify that I did it right this time. Thanks!
  2. jcsd
  3. Dec 4, 2016 #2
    Be aware that saying the electric field is equal to the electric potential divided by distance implies that you are assuming a constant electric field. In actuality the electric field equals the negative gradient of the potential.
  4. Dec 4, 2016 #3
    But yes, that answer does look right. I just calculated it using the relation between power and intensity, calculating the intensity for a distance 19.2 times greater, and plugging into your equation, and I came out with 37.15 V/m.
  5. Dec 4, 2016 #4
    Thanks! I got it right :)
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