Delia fills a glass bottle with water, screws the cap tight, and stands it on a warm windowsill. Nothing can get into the bottle or out of it, yet the water still warms.
So the boundary stops the water but not the energy. Boundaries differ in exactly what they let cross, and the differences sort systems into three classes.
Check the bottle's boundary, the glass and the cap, for two possible crossings. First, matter: no water or air gets in or out, so matter cannot cross.
Second, energy: the windowsill is hotter than the water, and energy passes in through the glass, so energy can cross.
A system whose boundary lets energy cross but not matter is called a closed system. The capped bottle is closed.
Now an uncovered pot of soup simmering on a stove. Steam carries matter out over the rim, and energy enters at the hot base: matter and energy both cross.
A system whose boundary lets both matter and energy cross is called an open system.
Earlier in the course, you met the condition for a constant total: with every store counted, the total energy of a system that no energy enters or leaves stays constant. Now picture a boundary that nothing crosses at all, no matter and no energy.
A system whose boundary lets nothing cross is called an isolated system. No real container is perfect, but a sealed, heavily insulated cooler comes close for a while.
One case sits near two classes. A boundary that lets nothing cross stops matter, and a closed boundary stops matter too. But the two questions sort each case into exactly one class. For closed, the answer to the energy question is yes. A case that lets nothing cross is sorted as isolated, not closed.
A zipped, uninsulated plastic bag of marinade sits in a fridge. No marinade leaks and no air gets in. Classify the system of marinade in the bag.
Matter: the zip keeps marinade and air from crossing, so matter cannot cross.
Energy: the fridge air is colder than the marinade, and energy leaves through the thin plastic, so energy can cross.
Energy crosses but matter does not: the bag of marinade is a closed system.
An idealized picnic cooler is sealed and so well insulated that for the whole trip nothing crosses its boundary. Classify the system inside it for the trip.
Matter: nothing gets in or out.
Energy: none crosses either.
Neither matter nor energy crosses: for the trip, the cooler's contents are an isolated system.
A fish tank sits uncovered. Water evaporates from the surface, and the warm room air warms the water. Which class is the system of tank water in?
A sealed metal can of beans stands in a pan of hot water, and the beans warm. No beans, water, or air cross the can's wall. Which class is the system inside the can in?
Emrys says: 'For a system to count as isolated, its boundary must stop matter and energy both.' Is Emrys correct?
Which of these is a closed system?
A sealed bottle of water is a closed system. Which new crossing would make it an open system instead?
Ivo wraps a sealed jar of water in a perfect insulating wrap. Now nothing at all can cross the jar's boundary. Which class is the jar's system of water in now?