1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Optics: How much light required to heat water?

  1. Apr 4, 2014 #1
    1. The problem statement, all variables and given/known data
    How many grams of water could have its temperature raised by 1.13 °C by a mole of photons that have a wavelength of 510 nm?

    2. Relevant equations



    3. The attempt at a solution

    (3x10^8) / (510 x 10^-9) = 5.88 x 10^14 Hz

    multiply by Planck's constant to get E= 3.8977x10^-19 J/photon.

    Now multiply by avogadro's number to get 234719J

    234,719J = m (4.18J/g C)(1.13 C)

    m = 49693g

    supposedly not the correct answer. I don't know the correct answer but the input thing for online homework tells me it wants 3 significant figures in grams and since you can't enter any powers of ten the answer must be less than a kg.
    Any ideas?
  2. jcsd
  3. Apr 4, 2014 #2


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    Your work looks correct to me.

    It could be that they want you to round to three significant figures without using scientific notation (powers of 10). Zeros on the right end of the number would be assumed non-significant.
  4. Apr 4, 2014 #3
    Hah! You were right. That just doesn't seem like the correct way to do it to me though.

    Thank you.
  5. Apr 4, 2014 #4


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    Good. I agree with you. Zeros on the right of an integer are generally ambiguous in regard to being significant figures.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted