An open system in physics is any system that exchanges both energy and matter with its surroundings. A pot of boiling water on a stove is an open system: heat flows in from the burner, and steam (matter) escapes into the air. Most real-world systems, from car engines to living cells, are open systems. The concept is fundamental to thermodynamics but extends into quantum mechanics, earth science, and engineering.
How Open, Closed, and Isolated Systems Differ
Physics classifies systems into three types based on what crosses their boundaries. An open system allows both energy and matter to flow in and out. A closed system allows energy transfer but blocks matter from entering or leaving. An isolated system permits neither energy nor matter to cross its boundary.
A sealed thermos of hot coffee comes close to an isolated system: ideally, no heat escapes and no liquid gets in or out. A sealed pressure cooker acts more like a closed system: heat transfers through the walls, but the lid traps the steam inside. An uncovered pot on the stove is an open system: heat enters from below, and water vapor leaves from the top. The key distinction is always whether stuff (atoms, molecules, fluid) can move across the boundary. If it can, the system is open.
Energy Balance in an Open System
In a closed system, energy accounting is relatively simple: track the heat going in and the work coming out. Open systems add a complication because matter carries energy with it. When fluid flows into a system, it brings internal energy, and it also does work by pushing its way in against the pressure already inside. These two contributions are bundled together into a property called enthalpy, which represents the total energy a parcel of matter carries across a boundary.
The energy balance for an open system says: the change in the system’s energy equals the heat added, minus the work done by the system, plus the energy carried in by incoming matter, minus the energy carried out by departing matter. Engineers refer to an open system as a “control volume,” meaning a fixed region in space with defined boundaries (the “control surface”) through which material streams in and out. A turbine, a nozzle, or a mixing chamber are all analyzed this way.
Mass Conservation Matters Too
Because matter crosses the boundary of an open system, you need to track mass as well as energy. The principle is straightforward: the rate of change of mass inside the system equals the total mass flowing in per second minus the total mass flowing out per second. In a system at steady state, like a garden hose with a constant flow, the mass inside stays constant because inflow and outflow are balanced. In a system that isn’t at steady state, like a bathtub filling up, mass accumulates inside.
Entropy and the Second Law
One of the most interesting features of open systems is that they can become more ordered locally, even though the second law of thermodynamics says disorder (entropy) always increases overall. A growing plant assembles complex molecules from simple nutrients, reducing its own entropy. This is only possible because the plant is an open system: it takes in energy from sunlight and releases heat and waste products, generating more entropy in its surroundings than it reduces internally. The total entropy of the plant plus its environment still goes up.
This point clears up a common confusion. The second law does not say that every part of the universe must become more disordered. It says that when you add up all the entropy changes across every system involved, the net result is always an increase. Open systems can export their entropy to the environment, which is exactly how life, weather patterns, and other complex structures sustain themselves.
Living Organisms as Open Systems
Every living cell is an open system. Cells pull in nutrients and oxygen, use chemical reactions to extract energy, and expel waste products like carbon dioxide. This constant, controlled exchange of substances with the environment is so fundamental that biologists use it as a basic marker of life. A cell that stops exchanging matter with its surroundings is, by definition, dead.
Life depends on reactions that push the system away from chemical equilibrium. If a cell were a closed system, its reactions would eventually reach equilibrium and stop. Because it is open, it continuously imports fresh reactants and removes products, keeping the chemistry far from equilibrium and able to do useful work. This is why organisms need a constant supply of food or sunlight: not just to get energy, but to maintain the flow of matter that keeps their internal chemistry running.
Earth: Open for Energy, Nearly Closed for Matter
Earth is a helpful example of how the open/closed distinction can be nuanced. With respect to energy, Earth is clearly an open system. It absorbs energy from the sun and radiates heat and light back into space, maintaining a rough energy balance. With respect to matter, however, Earth is nearly closed. Some meteors deliver material from space, and a small number of atoms (mostly hydrogen) drift away from the upper atmosphere, but these amounts are negligible compared to the planet’s total mass. NASA describes Earth as approximately closed for matter and open for energy.
Any subsystem within Earth, though, is fully open. The ocean exchanges water vapor with the atmosphere, rivers carry sediment to the sea, and volcanoes release gases from the mantle. Climate science, ecology, and geology all depend on tracking these open-system flows of energy and matter.
Everyday Engineering Examples
Open system analysis is the backbone of engineering thermodynamics. An internal combustion engine draws in air and fuel, burns the mixture, and expels exhaust gases. Mass enters and leaves, and energy converts from chemical to mechanical form. Analyzing the engine as an open system lets engineers calculate efficiency, predict exhaust temperatures, and design better combustion chambers.
Other common examples include jet engines (air and fuel in, thrust and exhaust out), air conditioning systems (refrigerant circulating across system boundaries), and even a kitchen faucet filling a glass. Any device where fluid flows through a defined region is an open system in thermodynamic terms.
Open Systems in Quantum Mechanics
The concept extends beyond classical thermodynamics. In quantum mechanics, an open system is any quantum system that interacts with a larger environment. This interaction has a dramatic consequence called decoherence: the fragile quantum properties of a particle, like its ability to exist in two states simultaneously, get “leaked” into the environment through entanglement.
When, say, an electron interacts with stray particles around it, the quantum interference pattern that defines its superposition becomes spread across the combined system of electron plus environment. The interference hasn’t been destroyed, but it can no longer be observed by looking at the electron alone. This is why quantum effects are so hard to maintain in everyday conditions: almost every real quantum system is an open system, constantly interacting with its surroundings. Quantum computing research is largely an effort to keep systems as close to “closed” as possible, shielding delicate quantum states from environmental interference.