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Homework Help: On tap boiling water

  1. Mar 9, 2014 #1
    Hello, my problem is this

    It is possible to buy water heaters that provide ‘instant boiling water’ at the turn of a tap. Assume the heater takes in water at 4[itex]^{\circ}[/itex]C and gives out hot water at 100[itex]^{\circ}[/itex] C. Furthermore, assume that the hot water flows out at a rate of 21s[itex]^{-1}[/itex]
    How much power is required to heat the water at this rate?

    So I have attempted a solution as follows,
    [itex]Q=c\Delta T[/itex]​

    Where c is the heat capacity and [itex]\Delta T[/itex] is the temperature difference. My first problem is I wasn't sure what to use for 'c' as I wasn't given a mass for the water. I just used this value from wikipedia for the mass specific heat capacity

    [itex]c=4.1813 \frac{J}{gK}[/itex]

    Obviously [itex]\Delta T=96 [/itex] kelvin​

    So the heat energy required to raise the water by that temperature is

    [itex]Q=4.1813 \textrm{ x }96=401.4 \textrm{ J}[/itex]​

    So to get power I multiplied this result by the rate of water flow quoted in the problem giving

    [itex]P=401.4 \textrm{ x }21=8429.5 \textrm{ J}[/itex]​

    But this all seems wrong. I don't know from the question how much mass of water is flowing per second but I have used a mass specific heat capacity. Plus the power seems like a low value. Could anyone point out where I am going wrong?

    Thanks a lot
  2. jcsd
  3. Mar 9, 2014 #2


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    You'll have to be more specific about the flow of the water out of the heater. 21/s means nothing.
  4. Mar 9, 2014 #3
    I agree, but unfortunately this is, verbatim, the problem I have been set to solve. I am hoping some sense can be made of it.
  5. Mar 10, 2014 #4
    Have I in principle done the correct calculation? So if, say, the rate had been 21g of water per second would I have proceeded in the same way as I have in my example?

  6. Mar 10, 2014 #5


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    Yes, except you want to keep track of the units better. The units of power are J/s.
  7. Mar 10, 2014 #6
    The water is not going to boil when it comes out, because you've only added enough heat to get it up to the boiling point. You need to keep the pressure in the tank higher than 1 atm, and you need to add more heat in the tank. The pressure in the tank has to be high enough to keep it from boiling before it exits.
  8. Mar 10, 2014 #7
    That is true, I will factor that in to my calculations. Thank you all for your responses.
  9. Mar 10, 2014 #8


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    I would guess that 21 s-1 is a typo for 2 ls-1.

    If that is right, you can find the mass flow / second.

    Most people use L as an abbreviation for liter, to avoid this problem.
  10. Mar 10, 2014 #9


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    The question says the water is "at 100° C", not that it is boiling.
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