Thermodynamics: Enthelpy and energy change of a column of air

Click For Summary
SUMMARY

The discussion focuses on the thermodynamic analysis of a column of dry air with a cross-sectional area of 1m² and a surface pressure of 1 atm, which gains 2 MJ of energy from radiation and surface heating. The surface pressure remains unchanged due to the absence of mass exchange, while the enthalpy increases as a result of the energy gain. The relationship between enthalpy (H), internal energy (U), and potential energy (PE) is established through the equations H = U + pV and ΔH = ΔU + Δ(pV), highlighting the need for mathematical calculations to determine changes in these energy forms.

PREREQUISITES
  • Understanding of thermodynamic principles, specifically enthalpy and internal energy.
  • Familiarity with the ideal gas law and its implications for internal energy.
  • Knowledge of hydrostatic equilibrium and its effects on pressure.
  • Basic proficiency in calculus for evaluating integrals related to energy changes.
NEXT STEPS
  • Study the derivation and application of the equation H = U + pV in thermodynamics.
  • Learn how to calculate changes in internal energy using the ideal gas law.
  • Explore the concept of potential energy in fluid systems and its relation to height and pressure.
  • Practice solving thermodynamic problems involving energy transfer and equilibrium conditions.
USEFUL FOR

This discussion is beneficial for students in thermodynamics, engineers working with fluid dynamics, and anyone interested in understanding energy changes in atmospheric systems.

Nyfinscyf
Messages
6
Reaction score
0

Homework Statement


A column of dry air, of cross-sectional area 1m2, has a surface pressure of 1atm. Over the course of several hours during the day it gains 2MJ of energy from radiation and surface heating. There are no horizontal exchanges of energy of mass, and it remains in hydrostatic equilibrium.

Does the surface pressure change?
What is the change in enthalpy, internal energy, and potential energy?

Area: 1m2
Energy gain: 2 MJ
Pressure: 1 atm

Homework Equations


H = U + pV
ΔH = ΔU + Δ(pV)
H=U+P = \int^{∞}_{0} ρ(c_v + R)T dz = \int^{∞}_{0} ρh dz
{\frac{∂U}{∂T}}=\frac{3}{2} Nk=C_v

Ideal gas - internal energy does not depend on volume.


The Attempt at a Solution


Surface pressure: Does not change as there is no change in mass of the column and so there is the same volume of air above the location.

I understand enthalpy increases because the internal energy increases from the gain in heating and radiation. But I don't understand how the potential energy increases nor do I understand how the math works. These formulas seem very vague to me.

How does the gain of energy increase the temperature mathematically?
 
Physics news on Phys.org
And how can I calculate the change in enthalpy, internal energy and potential energy mathematically?
 

Similar threads

Replies
30
Views
5K
Replies
4
Views
1K
Replies
5
Views
1K
  • · Replies 1 ·
Replies
1
Views
2K
Replies
12
Views
3K
  • · Replies 5 ·
Replies
5
Views
2K
Replies
1
Views
2K
  • · Replies 4 ·
Replies
4
Views
2K
Replies
5
Views
1K
  • · Replies 5 ·
Replies
5
Views
2K