Find the electrical energy supplied in kJ and kWhr

Click For Summary

Homework Help Overview

The problem involves a piston-cylinder assembly containing a mass of air that is heated while maintaining constant pressure. The task is to determine the electrical energy supplied in both kilojoules and kilowatt-hours, considering heat loss during the process.

Discussion Character

  • Exploratory, Conceptual clarification, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the calculation of electrical energy supplied, with one attempting to apply the first law of thermodynamics. Questions arise regarding the appropriate specific heat capacity to use, given the constant pressure condition.

Discussion Status

Some participants have offered guidance on the use of specific heat capacities and the relationship between them for ideal gases. There is an exploration of different methods to approach the problem, but no consensus has been reached on the best approach yet.

Contextual Notes

Participants are considering the implications of heat loss and the specific heat capacities (Cv and Cp) in their calculations. The problem context includes specific values and conditions that may influence the approach taken.

udontai
Messages
19
Reaction score
0

Homework Statement


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

Homework Equations





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?
 
Physics news on Phys.org
For the internal energy. I should apply cp or cv? The pressure is constant
 
udontai said:
For the internal energy. I should apply cp or cv? The pressure is constant
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.

Chet
 

Similar threads

Replies
8
Views
2K
  • · Replies 2 ·
Replies
2
Views
7K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
Replies
9
Views
4K
  • · Replies 1 ·
Replies
1
Views
3K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 8 ·
Replies
8
Views
2K
  • · Replies 15 ·
Replies
15
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K