Isolated System
A system that exchanges neither energy nor matter with its surroundings.
Formula / equation
ΔU = 0
Unit
Dimensionless
In depth
An isolated system's internal energy is constant, so any process within it must simply redistribute energy internally. True isolation is unattainable, but a well-insulated vacuum flask approximates it closely enough for practical calorimetry. The entropy of an isolated system can never decrease.
Examples in the real world
A sealed vacuum flask over a short period; the universe as a whole, taken as the ultimate isolated system.