My Chemistry textbook says that in free expansion there is no external pressure and thus the change in internal energy of the system is equal to the heat that we supply to it.

But i think that the only way to apply heat to a system is to make the external molecules loose their kinetic energy to the molecules of the system. ( NOTE - I am not talking about increasing the heat by adding radiant energy to the system )

If the external molecules loose some of their KE then they must exert a force on the system and thus the external pressure must not be equal to zero!

Is this correct? If not, then please tell me where I am wrong.

  • $\begingroup$ So you are talking about conduction of heat to a fluid? $\endgroup$ – Steeven Dec 17 '16 at 11:56
  • $\begingroup$ @Steeven In general, this happens. Well to be precise, I was reading about gases when this doubt popped up in my head. $\endgroup$ – Aaryan Dewan Dec 17 '16 at 11:58
  • $\begingroup$ No external pressure practically means no material surrounding it. So the statement is itself assuming only heating from radiation. So, unless I misunderstood your question, the answer would be "yes". $\endgroup$ – Steeven Dec 17 '16 at 12:00
  • $\begingroup$ @Steeven , So in free expansion is there no way to increase the temperature of a system except by providing it some heat through radiations ? $\endgroup$ – Aaryan Dewan Dec 17 '16 at 12:04
  • $\begingroup$ If free expansion means no external pressure, then there is no "touching" surrounding material. Conduction or convection is not possible. $\endgroup$ – Steeven Dec 17 '16 at 12:12

If no external pressure means no material surrounding it, the statement is itself assuming only heating from radiation. So, unless I misunderstood your question, the answer would be "yes".

With no "touching" surrounding material, conduction or convection is not possible.

In a comment @ChesterMiller points out that a gas can be kept in a container with a free direction to expand along (as with a free piston). Here there is free expansion but possibly conduction as well.

To your question in this case, the "collisions" between more energitic molecules in the container wall with the gas molucules does indeed provide tiny pressure on the gas. It will most likely be negligible, but your thinking is strictly correct. In text-books I've been using, such container (and piston) is usually described as thermally insulating.

| cite | improve this answer | |
  • $\begingroup$ This is incorrect. You can have zero external pressure over part of the boundary where the motion is occurring (e.g., a massless frictionless piston with vacuum outside the cylinder) while you are applying conductive heat through the rigid walls of the cylinder (say by means of an electrical heater in contact with the cylinder). $\endgroup$ – Chet Miller Dec 17 '16 at 13:41
  • $\begingroup$ @ChesterMiller Yes, of course. AaryanDeWan see this comment. I am adding it to the question accordingly. $\endgroup$ – Steeven Dec 17 '16 at 13:57
  • $\begingroup$ @Steeven , What do you mean by conduction here? Heat going out of the system into the vacuum as the piston goes up? $\endgroup$ – Aaryan Dewan Dec 20 '16 at 10:19
  • $\begingroup$ @AaryanDewan What I mean by conduction? I you heat up the surrounding container, the container walls in contact will transfer heat to the gas as the walls interfaces. $\endgroup$ – Steeven Dec 20 '16 at 11:39
  • $\begingroup$ How can you heat the surrounding if there is vacuum outside? ( NO radiation here ) $\endgroup$ – Aaryan Dewan Dec 20 '16 at 14:04

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.