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: Specific heat (I'm not sure if it's capacity or latent)

  1. Jan 15, 2012 #1
    1. The problem statement, all variables and given/known data
    Part a. A 500 W kettle contains 300g of water at 20°C. Calculate the time it would take to raise the temperature of the water to boiling point.

    Part b. The kettle is allowed to boil for 2 minutes. Calculate the mass of water that would remain in the kettle.

    State any assumptions you make.

    (Specific heat capacity of water = 4.18x103 J kg-1 °C-1, specific latent heat of vaporisation of water = 2.26x106 J kg-1.)

    2. The attempt at a solution
    I have the answer for part a. It's (rounded) about 201 seconds.

    Problem: I don't know where to start from for part b. Someone help.
  2. jcsd
  3. Jan 15, 2012 #2
    Oh and I'm making two assumptions.
    1. There's no energy lost to the surroundings.
    2. All the vapour formed escapes the kettle.
  4. Jan 15, 2012 #3
    I agree with your time for part (a)
    Once the water is boiling there is no increase in temperature ( boils at 100C) so all of the energy supplied ( in 2 minutes) is used to convert liquid water into water vapor ( steam)
    The usual assumptions about no heat loss etc.
  5. Jan 15, 2012 #4
    Yep yep I can think about that. Fine. But, the only formula I've got in my head is E=mL, and the only way I'm thinking of finding the energy change [E in the equation] is power equals work done by time taken. Using that, power being 500 W and time taken being 120 seconds, the work equals 60kJ [I'm not even sure if I should be doing that or not].

    So I've got energy and I've got specific latent heat. What next? Put 'em in the formula?

    mass equals energy by specific latent heat, and I get 37.7 [is this kg or grams?].

    Seriously, I'm COMPLETELY at sea with this.
  6. Jan 15, 2012 #5
    Oh btw the answer is supposed to be 273g, rounded off to 270g. No idea where it comes from though.
  7. Jan 15, 2012 #6
    You are doing the right thing.
    60kJ in 2 mins is correct so the mass of steam = ( 60 x 10^3)/2.26 x 10^6
    which is 0.027 kg
    (where did you get the answer to be 273g?)
    Last edited: Jan 15, 2012
  8. Jan 15, 2012 #7
    I could've sworn I was doing the same thing over and over again and not getting that value! Or maybe I was looking at the energy in a wrong way [I have the value 59892.5 J written on the notebook, no clue where I got it from though]. But thanks! Question solved :D
  9. Jan 15, 2012 #8
    great....don't you wish all problems were so easy:wink:
  10. Jan 15, 2012 #9
    Of course :p
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook