isolated system
Summary
An isolated system exchanges neither matter nor energy (heat or work) with its surroundings, so its total energy stays constant. A closed system differs: it can exchange energy but not matter. In mechanics it also means no net external force, so momentum is conserved.
Detail
Meaning: An isolated system exchanges neither matter nor energy (no heat and no work) with its surroundings. A closed system exchanges energy but not matter, and an open system exchanges both. Because nothing crosses the boundary, the total energy of an isolated system stays constant. In mechanics the same label means no net external force acts, so total linear momentum is conserved too.
Equation: ΔU = Q − W = 0, where ΔU is the change in internal energy (J), Q is heat added to the system (J), and W is work done by the system (J). For an isolated system Q = 0 and W = 0.
On the MCAT: Passages use a sealed, insulated calorimeter or colliding carts and ask what is conserved. Heat lost by the hot object equals heat gained by the cold one; momentum before a collision equals momentum after. Entropy of an isolated system does not decrease in a spontaneous process.
Common confusion: Isolated is not closed; a closed system can still exchange energy. Constant total energy also does not mean constant kinetic energy: in an inelastic collision, kinetic energy becomes thermal energy inside the system.
Sources
- Physics (Cutnell and Johnson)
- University Physics (Young and Freedman)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.