Systems: open, closed and isolated
In science we often study one small part of the world. We call it the system. It has a boundary, and everything outside the boundary is its surroundings. What crosses the boundary decides the kind of system. In an open system both energy and matter can cross: a pot of hot soup without a lid loses warmth and vapour. In a closed system only energy crosses: a sealed can in ice water gives heat to the water but no matter leaves. In an isolated system neither energy nor matter crosses. A truly isolated system is an ideal; a good thermos flask comes close, but a little heat still leaks through. The choice of system is ours: for the same cup of tea, we may study only the tea, or the tea and the cup together. Heat is energy that moves because of a temperature difference. It is not “stored” in an object: what an object holds is internal energy, which the next lesson explains.
| System | Energy crosses? | Matter crosses? | Example |
|---|---|---|---|
| open | yes | yes | a lidless pot of soup |
| closed | yes | no | a sealed can in ice water |
| isolated | no | no | an ideal; a good thermos is close |
System sort, with a grown-up. Collect five household objects: a cup with no lid, a closed bottle, a jar with a lid, a thermos flask and a bowl. For each, name the boundary and decide whether it is open, closed or nearly isolated. Write the reason in one sentence. Try the idea on a cup of cold tap water placed in a sunny spot: what crosses its boundary?