Adiabatic processes

we're now going to consider adiabatic processes so adiabatic processes are processes in which no heat is transferred so as an equation we can say well q is equal to zero now physically there's two ways that we can obtain an adiabatic process one way is to use an insulated container such as a thermos if heat can't transfer through the walls of the container then there is no heat transferred to that system another way to do it is to perform the process quickly so for example if i compress this gas very quickly then that's an adiabatic process because it takes time for heat to transfer heat transfers from the hotter body to the colder body but it does so over a period of time so doing it quickly means that no heat is transferred so our first law of thermodynamics the change in internal energy is equal to q plus w for an adiabatic process becomes that the change in internal energy is equal to w the work done on the system because q is equal to zero now a really boring example of an adiabatic process is adiabatic free expansion so imagine that we had a cylinder and we compressed all the gas molecules into the lower half of the cylinder and topped it off with a really thin membrane now if you imagine very gently breaking that membrane so we're not doing any work in this process we're just very very gently removing the membrane in that case the gas molecules which were confined to the lower half of the cylinder will be free to expand into the upper half of the cylinder as well and so in this case there is no heat transferred and so this is an adiabatic process not all adiabatic processes are this boring have a look at this more exciting example [Applause] what's up so in this example michael was opening up a cylinder of carbon dioxide and letting that gas escape so the gas was quickly expanding so because this process was so quick there was no time for heat to transfer which is why it is an adiabatic process however hopefully you could gather from watching the footage there is a change in temperature in this case we were actually creating dry ice solid carbon dioxide so how this works is that when michael opened the flask the gas rushed out so in this case the gas was expanding and so negative work was done on the gas now if negative work is done on the gas it means that we need to have a decrease in the internal energy of the gas given that there was no heat transfer so if the change in internal energy was negative it means that the final temperature must be lower than the initial temperature so we have a negative change in temperature so this is why as the gas rushes out we get a change in temperature down to a very cold temperature at which carbon dioxide in its solid form dry ice can form

Channel: UNSW Physics

Views: 48,907

Keywords:

Id: wIoUXcRVVYI

Length: 3min 58sec (238 seconds)

Published: Fri Oct 09 2020