Defining thermodynamic SYSTEMS

Systems, thermodynamic states, variables, and more...

"System"

He drew a circle that shut me out
- Heretic,rebel, a thing to flout.
But love and I had the wit to win:
We drew a circle and took him In!
--Edwin Markham
  • Harold and the Purple CrayonImagine (or draw) any boundary in space.
  • We'll call the volume inside the boundary a thermodynamic system.
  • The space outside the boundary is the surroundings.

You may define the boundary of a system in whatever way is easiest to deal with a particular problem!

Closed systems

Think of the system boundary as the latex of one of the balloons.

If the matter, or other stuff inside the system can *not* be exchanged with the surroundings, the system is closed.

"Stuff" might be atoms or radiation (photons).

If the "stuff" is atomic, it might be in any phase (solid / liquid / gas / plasma).

Note: In some textbooks you'll hear talk of a "change of state" when a material goes from from liquid to gas. In this course we'll call that a phase change. The word "state" has a different, and very specific meaning in thermodynamics.

The "stuff" might be atoms that re-arrange their chemical bonds.

... vs Open systems

Think of the system as consisting of the space inside the cooler when the lid is on.

If matter *can* be exchanged with the surroundings, the system is open.

Adiabatic systems

Think of the system as the space inside the thermos (not in the insulation), when the lid is on.

A boundary is adiabatic if heat cannot cross the boundary of the system.

An isolated system is both:

  • adiabatic (no heat exchange), and
  • closed (matter cannot cross the system boundary).

...vs Diathermal

Think of the system as the inside of the ice pack when the lid is on.

Heat (or cold) *can* cross a Diathermal system boundary.

Problem 1.1

To do: Classify the following systems as open, closed, or isolated:

  1. A mass of gas in a container, with rigid, impermeable, diathermal walls.
  2. A mass of gas in a container, with rigid, impermeable, adiabatic walls.
  3. A sugar solution enclosed by a membrane that is permeable to water, that is immersed in a large container of water.

Fixed or changing volume

pistonThe boundary may be moving or changing with time.

Think of our system as consisting of the space above the (moving) piston and below the inlet/outlet valves. The volume of this space is changing with time.

To do: Sketch approximately $V(t)$.

In general physics, work is given in terms of displacement and force: $$\delta W = \myv{F} \cdot \delta \myv x$$

To do: Under what circumstances can a system not possibly do any mechanical work on its surroundings (or vice versa).

Image credits

Frederick Wilson, Flickr users Jaxxon, Swamibu, Matus Kalisky, Windows to the Universe, Wikimedia user Zephyris