(Thermodynamics) Why Do I Use U For Constant Volume and H For Constant Pressure

[cnoa]

Why Do I Use U For Constant Volume and H For Constant Pressure?

Also, which can be calculated H, Q, U, W?

 

[Andrew Mason]

Why Do I Use U For Constant Volume and H For Constant Pressure?

Also, which can be calculated H, Q, U, W?

For an ideal gas:

$\Delta U = nC_v\Delta T$

$\Delta H = \Delta U + \Delta (PV) = nC_v\Delta T + nR\Delta T = nC_p\Delta T$

I am not sure what you mean by your last question. They can all be calculated if you have sufficient information.

AM

 

[DrStupid]

Why Do I Use U For Constant Volume and H For Constant Pressure?

I can't tell you why you do that but if there is no non-volumetric work the change of U or H is equal to the exchanged heat if you do so.

 

[I like Serena]

In general we have the equations for small changes dP, dV, dU, dH:
dU=dQ - PdV
dH=dQ + VdPFor constant volume dV=0, so we can write the first as:
ΔU=ΔQ

For constant pressure dP=0, so we can write the second as:
ΔH=ΔQ

These are the quantities that we can actually measure.Depending on circumstances we can calculate each of H, Q, U, and W.

 

[sardar]

The symbols used for thermodynamic variables, such as U for internal energy and H for enthalpy, are based on historical conventions and mathematical notations. These symbols were chosen to represent specific properties in thermodynamics equations and help us understand the behavior of substances under different conditions.

The use of U for constant volume and H for constant pressure is due to the fact that these variables are related to specific types of processes. In a constant volume process, the volume of a system remains constant, but the pressure and temperature may change. In this case, internal energy, represented by U, is a more relevant variable to consider, as it reflects the amount of energy contained within the system.

On the other hand, in a constant pressure process, the pressure of a system remains constant, but the volume and temperature may change. In this case, enthalpy, represented by H, is a more useful variable as it takes into account the energy transferred to or from the system through heat at constant pressure.

To answer the second part of your question, all of the variables H, Q, U, and W can be calculated as they are all related to each other through the first law of thermodynamics, which states that the change in internal energy of a system is equal to the sum of heat transferred to or from the system and work done on or by the system. So, depending on the given conditions and known variables, any of these can be calculated using appropriate equations.

In summary, the use of U for constant volume and H for constant pressure is a convention based on the type of process being considered, and all of these variables can be calculated using the first law of thermodynamics.