closed system
Summary
A closed system can exchange energy (heat and work) with its surroundings but not matter. Its internal energy changes according to ΔU = Q − W. Closed is not the same as isolated, which exchanges neither matter nor energy.
Detail
Meaning: In thermodynamics, a closed system has a boundary that matter cannot cross, although heat and work can. A gas sealed in a cylinder with a movable piston is the standard example: it can be heated or compressed, but its moles of gas stay fixed. An open system exchanges matter and energy (a living cell). An isolated system exchanges neither, so its total energy stays constant.
Equation: ΔU = Q − W, 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, so ΔU = 0.
On the MCAT: A passage describes a sealed piston or container and asks for the sign of Q, W, or ΔU, or shows a PV diagram where work is the area under the curve. Know the special cases: isothermal (ΔU = 0 for an ideal gas, so Q = W), adiabatic (Q = 0), isochoric (W = 0).
Common confusion: Closed is not the same as isolated. A closed system blocks matter, but energy can still cross, so a sealed flask on a hot plate is closed yet its energy rises. Only an isolated system has constant energy.
Sources
- Fundamentals of Physics
- University Physics
- Physics for Scientists and Engineers
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.