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Homework Help: Difference in Potential Energy between two Reservoirs

  1. Sep 13, 2013 #1
    1. The problem statement, all variables and given/known data

    3. Water is pumped from one reservoir to another 100m away. The water level in the second reservoir is 20m above the water level of the first reservoir. What is the increase in specific potential energy of the water in J/kg?

    2. Relevant equations

    The relevant equation I've been given in my notes is

    P = mgh

    This problem is for a renewable energy course. In class I only received a definition of potential energy and the above equation, the teacher didn't do any problems with potential energy in class either. The given textbook doesn't touch on the quantitative side of renewable energies. I've been trying to look through old physics textbooks but can't find anything too helpful yet.

    3. The attempt at a solution
    I have been away from physics for almost 8 years, so physics problems aren't very intuitive for me anymore.

    So far I've assumed I'm calculating the change in P. Setting up the equation as :
    ΔP = (mgh)2nd reservoir - (mgh)1st reservoir

    Δ (mass*g) = (20m* ? * density of water)*9.8m^2/sec
    Δh = 100m

    I can't figure out how to get the Δ in the volume of water, which I need to get the mass. Or since the teacher wants the answer in J/kg, should I be assuming that I should set up the equation as:
    ΔP/m = Δ(g*h) ?

    Any help would be great.
    Last edited: Sep 13, 2013
  2. jcsd
  3. Sep 13, 2013 #2


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    Hello and welcome to PF!

    Yes. This is very good.

    Note that you can write this as ΔP = mgh2 - mgh1 = mg(h2 - h1) = mgΔh.

    You can imagine that you skim a mass of m = 1 kg of water off the surface of the first reservoir and move it to the surface of the second reservoir. You just need to calculate ΔP for this situation.
  4. Sep 14, 2013 #3
    I attempted the question again keeping in mind I need the specific potential energy.
    This would give me:

    ΔP/Δm = gΔh = 9.80 * 100m = 980 m2/s2 = 980 J/kg.

    However this doesn't use the 20m water level difference. At this point in the course I don't think the teacher would give us useless info. Does this mean I need to use a trig function to get actual difference in height? Or should I be incorporating the 20m into the Δm?
  5. Sep 14, 2013 #4


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    In the equation P = mgh, h is the vertical height. So, Δh is the change in vertical height, or the change in water level of the two reservoirs. There is no change in gravitational potential energy when a mass is moved horizontally. You can think of moving 1 kg of mass 100 m horizontally and then 20 m vertically. Only the 20 m vertical change contributes to the change in potential energy.
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