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Homework Help: Electric immersion water heater time

  1. Mar 7, 2016 #1
    1. The problem statement, all variables and given/known data
    The mechanical equivalent of heat is 1cal=4.18J. The specific heat of water is 1cal/g*K and its mass is 1g/cm^3. an electric immersion water heater rated at 400 W should heat a liter of water from 10C to 30C in about how much time

    2. Relevant equations

    3. The attempt at a solution
    I know the answer is 3.5 minutes but i do not know how to get there. Im pretty lost.
  2. jcsd
  3. Mar 7, 2016 #2


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    Work out the units. What is a watt? How many grams of water are in 1 liter?

    BTW, the formulas in Section 2 won't help you with this problem, since you don't know voltage, current, or resistance.
  4. Mar 7, 2016 #3

    im still confused i know that a watt is joules per second and that 1000 grams of water in a liter but how do i connect everything?
    Last edited: Mar 7, 2016
  5. Mar 7, 2016 #4
    you should calculate amount of heat required to raise the water from t1 to t2 given -then calculate the time taken by the heater to provide that heat-
    heat req.= related to( massxsp. heatx temp diff ) by calorimetric equation
  6. Mar 7, 2016 #5


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    You need to go back and study the SI system some more. 1 liter contains way more than 100 grams of water.
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