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What to solve for in specific heat energy problem

  1. Mar 11, 2013 #1
    1. The problem statement, all variables and given/known data
    You have a position working with a group investigating biological mechanisms that
    determine a predisposition to obesity. Your assignment is to measure the rate that
    energy is output by certain types of cells when a nutrient is introduced. To begin this
    study, you have decided to use a calorimetric technique. A culture of cells with the
    appropriate nutrient is placed inside a closed container. That container is submerged in
    a water bath and you measure the rate that the temperature of the bath changes.
    To calibrate the apparatus, you decide to use a resistor connected across a known
    voltage as a power source. You know that the power output by the resistor is just the
    current through the resistor times the voltage across the resistor. You then will
    compare that power to the rate that the internal energy of the water bath changes by
    measuring its temperature as a function of time. To accomplish this calibration, you
    calculate the rate of temperature change as a function of the voltage across the resistor,
    the current through the resistor, the specific heat of the water, and the mass of the
    water. You know that even with good insulation, your apparatus will transfer some
    energy to the outside and your measurements will allow you to correct for this.


    So I am honestly not sure what I am trying to solve for. I know that we are measuring energy output and that a resistor is used as a power source and I believe it is water we are measuring in terms of a transfer in energy from the system into the surronding environment. However, I'm confused as to what I'm specifically trying to solve for? What is it that I need to compare and look for in this problem? Any input is appreciated.
     
  2. jcsd
  3. Mar 12, 2013 #2

    ehild

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    The calorimeter loses energy by transferring heat to the surroundings at a rate proportional to the temperature difference between the calorimeter and the surroundings. You need to find that proportionality factor (heat transfer coefficient might be the name). Also, the calorimeter has its own heat capacity. When measuring temperature versus time with a given input power, you can determine both parameters.

    ehild
     
  4. Mar 12, 2013 #3
    So I believe I found a proportionality. Because the problem tells me that energy output can be determined by the power of the resistor and that energy input is determined as a function of temperature change over time of the water bath, energy input and energy output of the system are equivalent. I showed my work for this problem on the attached document. If someone could look at it to see if I'm on the right track with solving it, that would be appreciated.
     

    Attached Files:

  5. Mar 13, 2013 #4
    Just wanted to bump this up to see if anyone could look at my work from my previous post to the problem that I posted above. It'd be really appreciated, thanks!!
     
  6. Mar 13, 2013 #5

    ehild

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    I can not read docx files. Type in your solution.

    ehild
     
  7. Mar 13, 2013 #6
    Solution I came up with:



    Efinal-Einitial = Eoutput-Einput
    Efinal = 0
    Einitial = 0
    0 = Eoutput - Einput
    Einput = Eoutput
    Eoutput = Power of Resistor = Voltage (V) x Current (I)
    dEoutput/dt = (mwater)(cwater)dT/dt
    Power = (mwater)(cwater)dT/dt
    Mwater = mass of water
    Cwater = specific heat of water
    ∆Twater = change in temperature of water
     
  8. Mar 13, 2013 #7

    ehild

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    How do you do this correction for loss of energy?


    ehild
     
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