User:TrulyBlue/Equilibrium

A system is said to be in equilibrium if it is static or balanced. The term applies to many types of system, including biological, physical, or ecological systems. Examples of equilibrium include:

• in ecology, the balance between the sizes of predator and prey populations
• in chemistry, the balance between the concentrations of different reactants in a solution
• in physics, the balance between electromagnetic and friction forces in an electric motor.

Stable Equilibrium

A stable equilibrium is one where a small change from the equilibrium position causes some negative feedback process to return the system to the point of equilibrium. An example would be a sphere lying in a bowl: a small movement of the sphere will cause it to move up the side of the bowl, and gravity will take it back to its position at the bottom.

Unstable Equilibrium

An unstable equilibrium is one where a small change from the equilibrium position causes some positive feedback process to move the system further from the point of equilibrium. An example would be an inflated balloon with a weight under it, underwater. At some point the buoyancy of the balloon will exactly match the weight of the system. If the balloon is moved deeper it will compress, reducing the buoyancy and causing the system to sink faster and faster. Conversely if the system rises the balloon expands, causing it to rise faster and faster.

Neutral Equilibrium

A neutral equilibrium is one where a small change from the equilibrium position has no particular effect: the system establishes a new equilibrium in the new position. An example is a cylindrical pen on a level table. Rolling it across the table causes the pen to come to rest in a new position with no overall change to the equilibrium.

References

• Van Nostrand's Scientific Encyclopedia, John Wiley & Sons, Inc., 2002