1. Not finding help here? Sign up for a free 30min 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!

Alternating Current Problem

  1. Jun 17, 2009 #1
    1. The problem statement, all variables and given/known data

    On its highest setting a heating element on an electric stove is connected to an ac voltage of 240V. This element has a resistance of 29 ohms.(a) Find the power dissipated in the element.(b) Assuming that three-fourths of the heat produced by the element is used to heat a pot of water(the rest being wasted), find the time required to bring 1.9kg of water(half a gallon) at 15degC to a boil

    2. Relevant equations
    Vmrs = V0 / square root 2

    P=Vrms^2 / R


    3. The attempt at a solution

    Vrms= 240/sqrt2 = 170V

    P= 170^2/29 = 997watts

    I'm not sure how to answer part b.

    I think I would use the equation:
    Q=cm(T1-T2) but I don't know how to solve for time?
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jun 17, 2009 #2

    cepheid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Solving for Q will give you the total amount of energy needed to heat up the pot by that much. You already know how much useful power the heating element produces. What is the relationship between power and energy?
     
  4. Jun 17, 2009 #3
    Power is energy per unit time.
     
  5. Jun 17, 2009 #4

    cepheid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    So if you know how much energy the heating element transfers to the pot per unit time, and you know how much total energy it needs, then...
     
  6. Jun 18, 2009 #5
    so. Q=mc(T-T)
    Q= (1.9kg)(4.186)(100-15)
    Q=676J

    Then Take 3/4 of the P
    (997J/s)(3/4) = 748J/s

    so it takes 676J to heat the water to boil, and the stove produces 748 J/s towards heating the pot.

    676J / 748 J/s = .904 seconds
     
  7. Jun 18, 2009 #6

    Redbelly98

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Actually, 240V is the rms voltage.

    Vrms = 240 V
     
  8. Jun 18, 2009 #7

    rl.bhat

    User Avatar
    Homework Helper

    It should be
    Q= (1.9kg)(4186)(100-15)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Alternating Current Problem
  1. Alternating current (Replies: 36)

  2. Alternating currents (Replies: 2)

Loading...