# 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
• Imagine (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.

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

The 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