Before you can track energy, you have to say what "it" even is. The line you draw decides what counts as crossing — same physics, different accounting.
MASS crosses
HEAT crosses
WORK crosses
POT
WHISTLE
SEALED
THERMOS
BODY
WINDOW
A pot on a flame
Drag the handle to redraw the boundary. Same pot, same flame — the accounting changes.
SYSTEM
~~~ HEAT IN
🔥
⠿
Pressure cooker, whistle blowing
Mass is visibly leaving the boundary. This is an open system.
SYSTEM
↑ MASS OUT (steam)
The whistle is the boundary, briefly propped open. Mass — real matter — is crossing it. That single fact makes this an open system, no different in kind from a running tap or an open door.
Sealed cooker, before the whistle
Energy crosses. Mass doesn't. This is a closed system.
SYSTEM
🔒
~~~ HEAT IN
🔥
The valve is locked — nothing can leave. But heat is still getting in from the flame. Energy crosses; mass can't. That's a closed system — the definition Chapter 2's First Law is written for.
A thermos
Neither heat nor mass gets through. Near-isolated — the entire reason you own one.
SYSTEM
🍵
No arrows here — deliberately. Nothing is crossing this boundary, in either direction. Real thermoses leak a little heat eventually, which is why "isolated" is always an approximation, never a promise.
Your own body
Draw the boundary around yourself. It's uncomfortably busy.
SYSTEM
🧍
🍽 IN
💨 IN/OUT
~ HEAT OUT
💧 OUT
Food in, air in and out, heat out, sweat out. You are, thermodynamically, a very leaky open system — and you've never once stopped to notice.
A room, window open or shut
Same room. Tap the window — the boundary gets redrawn by a hinge.
SYSTEM (the room)
🪟
→ AIR OUT
TAP TO OPEN THE WINDOW
Shut: nothing crosses. The room is (almost) closed.
Puzzle · What Crossed?
A car engine, running
Draw the boundary around the engine block. Drag each label to where it actually crosses. No rehearsal for this one.
SYSTEM (the engine)
crossings placed: 0 / 4
AIR + FUEL →
EXHAUST →
SHAFT WORK →
RADIATOR HEAT →
Four different crossings, three different kinds — mass in, mass out, work out, heat out — all on one boundary you drew yourself. Chapter 2's control-volume analysis is exactly this picture, made numeric.