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Thermodynamics Help Understanding Controlled Mass/Volume

  1. Aug 16, 2009 #1
    Thermodynamics Help .... Understanding Controlled Mass/Volume

    I don't understand the these processes. Please let me know if I am correct or not. If not, please correct me. Thanks.

    From what I know, when you have a controlled volume process the pressure and temperature increase when heat is added. This is in a rigid container so no mass leaves or enters. So it is controlled mass as well, right?

    But I do not understand how this is referred to as controlled volume. When you heat air then the volume increases and the density decreases, resulting in the mass remaining the same, correct.

    Also, if you know the energy of individual fluids and 'x' amount of energy is added to the above system, and then you know the resulting total energy after the addition. How do you paredo out how much energy goes to which fluid or the new temperature.
  2. jcsd
  3. Aug 16, 2009 #2
    Re: Thermodynamics Help .... Understanding Controlled Mass/Volume

    Control volume does exactly what it says on the tin. It's simply an arbitrary volume that does not change in which the mass of the fluid remains constant.

    If you had a rigid container and heater the air, it can't increase in volume as its contained. The volume and mass remain the same so the pressure increases.

    I dont get what you dont understand, could you rephrase it please?
  4. Aug 16, 2009 #3
    Re: Thermodynamics Help .... Understanding Controlled Mass/Volume

    I thought that since you are in a rigid container the heating of air results in the molecules separating (becoming less dense) and moving faster. Therefore increasing in volume and causing the pressure to increase.

    To tell you the truth I really don't understand. I just keep reading different things and need help understand what happens when you heat air in a fixed container.

    The only consistent thing I have read is that both pressure and temperature increase but I do not know what happens to the volume and density. I thought there were both changing in a way that allows for the mass to remain constant, mass = volume * density.

    Thank you for the reply. I really appreciate it.
  5. Aug 16, 2009 #4
    Re: Thermodynamics Help .... Understanding Controlled Mass/Volume

    If you have a box 10*10*10 cm. You have 1 L of volume. The box is rigid and filled with air at 1 bar.

    We heat the contents of the box, the air increases in temperature. If there were no box constraining it, it would remain at 1 bar but take more volume according to Pv=nRT.

    Now as the box is rigid no matter how much you increase the temperature the volume of air connot exceed 1L. As such the pressure increases according to PV=nRT.

    In both cases the mass of the gas remains the same. In the control volume case, density and volume also remains the same. only pressure changes with temperature.

    If the gas is free to expand, its pressure will remain constant, but its volume will increase making the density decrease.
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