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Homework Help: Find the electrical energy supplied in kJ and kWhr

  1. Jul 6, 2014 #1
    1. The problem statement, all variables and given/known data
    a mass of 15kg of air in piston-cylinder assembly is heated from 25°C to 77°C by passing current through a resistance heater inside the cylinder. the pressure inside the cylinder held constant at 300kPa during the process and heat loss of 60kJ occurs. (Cv= 0.718 kJ/kgK)

    i) find the electrical energy supplied in kJ and kWhr

    2. Relevant equations

    3. The attempt at a solution
    W-60k = 15 (0.718)(52)
    W= 620.04kJ
    so to find the energy in kWhr, take 620.04 /3600? is it correct?
  2. jcsd
  3. Jul 6, 2014 #2
  4. Jul 6, 2014 #3
    For the internal energy. I should apply cp or cv? The pressure is constant
  5. Jul 6, 2014 #4
    Most conveniently, you could use Cp to get the enthalpy change (which is equal to the heat added at constant pressure). You can also work with Cv to get the change in internal energy, but in this case, you would need to include the PΔV=nRΔT to calculate the heat load. Either way, you get the same answer. Do you know the relationship between Cp and Cv for an ideal gas? If so, use it.

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