Equilibrium — Science definition
Defined in 2 courses:
- The state where two opposite flows match, so the net movement is zero; the particles are still moving and still crossing. (Life Science & Scientific Inquiry)
- The condition in which the net force on an object is zero, so its velocity does not change. (Physics & Environmental Science)
Where it is taught
- Membrane Transport, Diffusion & Osmosis — Cells, Transport & Cellular Energy, Life Science & Scientific Inquiry, Grade 7
- Free-Body Diagrams & Net Force — Forces, Rotation & Equilibrium, Physics & Environmental Science, Grade 12
In use
It stops being one-sided when the crowding evens out, and that state is equilibrium.
Membrane Transport, Diffusion & Osmosis
A special and important case is equilibrium, when all the forces sum to exactly zero.
Free-Body Diagrams & Net Force
Questions using equilibrium
Problem 1
Answer and explanation
No. Motion continues; opposing movements can balance so there is no net concentration change.
Problem 2
Answer and explanation
Initial net water movement tends from A toward B under these simplified conditions. Water crosses; the stipulated solute does not. This can change concentrations and pressures, limiting later flow. At equilibrium water molecules can still move both ways even when there is no net change. The conclusion depends on the stated permeability and pressure assumptions.
Compare nonpenetrating solute concentrations.
Apply the stipulated water permeability.
Separate net movement from individual crossings.
Response criteria
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.