A physical system is any portion of the universe that you separate, conceptually or physically, from everything else in order to study it. It could be a single particle, a cup of coffee, a planet, or an entire galaxy. What makes it a “system” is that you’ve drawn a boundary around it and decided to track what happens inside. Everything outside that boundary is called the surroundings, and the interplay between system and surroundings is what most of physics and engineering aims to describe.
Boundaries and Surroundings
Every physical system starts with a boundary. This boundary is often imaginary, chosen for convenience rather than corresponding to a physical wall. A boundary is simply a closed surface surrounding the system through which energy and matter may enter or leave. You pick the boundary based on what you want to analyze. Studying the thermodynamics of a car engine? The boundary wraps around the engine. Studying a satellite in orbit? The boundary encloses the satellite.
Everything outside that boundary is the surroundings, or the environment. The relationship between the system and its surroundings, specifically what crosses the boundary, determines what type of system you’re dealing with.
Open, Closed, and Isolated Systems
Physical systems fall into three categories based on what they exchange with their surroundings:
- Open systems exchange both energy and matter with their environment. A boiling pot without a lid is open: steam (matter) escapes, and heat (energy) flows in from the stove. Most real-world systems, including living organisms, are open.
- Closed systems exchange energy but not matter. A sealed pressure cooker lets heat pass through its walls but keeps all the water and steam inside.
- Isolated systems exchange neither energy nor matter with anything outside them. No interactions with the environment are possible at all. A perfectly insulated, sealed container would qualify, though truly isolated systems are an idealization. The universe as a whole is the closest real example.
These categories matter because the type of system dictates which physical laws apply and how you can predict its behavior. The rules governing energy flow in a closed system, for instance, are simpler than those for an open system where matter is also streaming in and out.
State Variables: Describing a System’s Condition
To describe what a physical system is doing at any given moment, scientists use state variables. These are measurable properties like temperature, pressure, volume, and mass. If you know just two independent properties of a simple system, you can typically figure out all the others. Knowing the temperature and pressure of a gas, for example, lets you calculate its volume and density.
State variables come in two flavors. Intensive properties, like temperature and pressure, don’t depend on how much material you have. A cup of water at 80°C has the same temperature whether you measure the whole cup or a single drop. Extensive properties, like mass and total volume, scale with the amount of material. Double the gas in a container and you double its mass.
Some important properties can’t be measured directly with an instrument. Internal energy, entropy (a measure of disorder), and related quantities are calculated from the measurable ones. These calculated properties are often the ones that matter most for understanding how a system will change over time.
The Laws That Govern System Behavior
Physical systems don’t behave randomly. Their behavior follows well-established laws, and which laws dominate depends on the system’s scale and speed.
For everyday objects, Newton’s laws of motion are the foundation. The first law says an object stays at rest or keeps moving in a straight line unless a force acts on it. The second law ties force to acceleration: push harder on a lighter object and it accelerates more. The third law requires that every force has an equal and opposite reaction. These three rules, combined with a description of the forces acting on a system, let you predict the motion of everything from baseballs to spacecraft.
For systems involving heat and energy transfer, thermodynamics takes over. The second law of thermodynamics is especially important: in any isolated system, disorder (entropy) always increases or stays the same over time. It never spontaneously decreases. This is why ice melts in a warm room but never spontaneously re-freezes, and why a shuffled deck of cards doesn’t sort itself. The total entropy of the universe always moves in one direction, giving time itself an arrow.
At atomic and subatomic scales, quantum mechanics replaces Newtonian physics entirely. The condition of a quantum system is described not by position and velocity but by a mathematical object called a state vector, which encodes probabilities rather than certainties.
Equilibrium: When a System Stops Changing
A physical system reaches equilibrium when its state variables stop changing over time. Pressure, temperature, volume, and chemical composition all settle into steady values. This requires three conditions to be met simultaneously. Thermal equilibrium means the temperature is uniform throughout the system. Mechanical equilibrium means there’s no net movement of parts. Chemical equilibrium means the concentrations of different substances aren’t shifting.
If you were to isolate a system that has reached equilibrium, nothing would spontaneously change. Mathematically, equilibrium corresponds to either a maximum in entropy or a minimum in free energy, a quantity that captures the system’s capacity to do useful work. Systems naturally evolve toward equilibrium, which is another way of saying they tend toward the state where entropy is maximized.
Scale: From Particles to Planets
One of the powerful things about the concept of a physical system is that it applies at every scale. A single electron is a physical system with measurable properties like charge, spin, and energy. A molecule is a system of atoms. A cell is a system of molecules. Earth’s climate is a system of interacting oceans, atmospheres, ice sheets, and landmasses, characterized by processes that range from molecular interactions to planetary-scale energy flows.
The same system can often be studied at multiple scales simultaneously. NASA, for instance, studies microscopic particles in the atmosphere while also tracking their large-scale effects on weather patterns and ocean ecosystems. The choice of scale depends on what question you’re trying to answer.
How Physical Systems Are Modeled
Scientists and engineers represent physical systems mathematically so they can predict future behavior. The most common approach uses state-space models, which describe the system as a set of linked equations. Each equation tracks how one state variable changes over time based on the current values of all the other variables and any external inputs.
In compact form, this looks like: the rate of change of all state variables equals some function of the current state plus any outside influences. For simpler systems where the relationships are proportional (linear), these equations reduce to straightforward matrix algebra. For more complex systems, the equations become nonlinear, meaning small changes in one variable can produce disproportionately large effects elsewhere.
Nonlinear Systems and Chaos
Many real physical systems are nonlinear, and this introduces behaviors that linear models can’t capture. Nonlinear systems can exhibit multistability, where multiple stable states exist and the system can settle into any one of them depending on conditions. They can also exhibit chaos, where tiny differences in starting conditions lead to wildly different outcomes over time. Weather is the classic example: the atmosphere follows deterministic physical laws, yet long-range forecasting is inherently limited because minuscule measurement errors compound rapidly.
Entropy, in the context of complex systems, serves as a measure of how complicated the dynamics are relative to internal parameters, initial conditions, and external forces. High-entropy dynamics are harder to predict and richer in structure, which is why complex physical systems remain one of the most active areas of study across physics, biology, and engineering.