## February 07, 2012

### Expert Corner: Equilibria Everywhere

Kurt Rosenkrantz is a science curriculum writer and Gizmo designer for ExploreLearning. Kurt holds a Master of Science in Geology from the University of Cincinnati, and a bachelor's degree in Earth Science from Harvard. He taught high school and middle school science for eight years before joining ExploreLearning in 2005.

In much of life we seek to balance opposing forces: money earned vs. money spent, weight gained vs. weight lost, heat vs. cold. When opposing forces are perfectly balanced, the system is said to be in equilibrium and no overall change is observed. For example, if you spend exactly as much as you earn, your total amount of wealth won't change.

The concept of equilibrium plays a prominent role in every field of science: from the delicate balance of Earth's climate to the forces that keep a satellite in orbit. A simple example of equilibrium occurs when an object is hung from a spring, as shown in the Determining a Spring Constant Gizmo. The force of the spring pulling upward increases as the spring is stretched, while the pull of gravity on the object is constant. Eventually the spring is stretched enough that the spring force is exactly equal to the weight of the object. At this point the spring force and gravitational force are in equilibrium and the object remains in place.

When a system in equilibrium is perturbed, the system will often oscillate, or swing back and forth from one state to another. Oscillations can be seen when you pull down a spring or release a pendulum, as illustrated in the Simple Harmonic Motion Gizmo.

A similar oscillation can occur in the populations of predators and prey in isolated environments, such as the classic case of the wolves and moose of Isle Royale, shown at left. As predator populations rise, prey populations are depleted until the predator population crashes. With fewer predators, prey populations recover and the cycle begins again. These oscillations are illustrated beautifully in the Food Chain Gizmo.

Recently we published two new Gizmos which directly address the topic in the field of chemistry: Equilibrium and Concentration and Equilibrium and Pressure. Both Gizmos explore reversible chemical reactions. In a reversible reaction, the rates of the forward and reverse reactions depend on the concentrations of reactants and products. As the forward reaction proceeds, the concentration of products increases. This causes the rate of the reverse reaction to increase as the forward reaction slows. In the Gizmos, you can measure the rate of each reaction by observing the blue flashes (representing the forward reaction) and red flashes (representing the reverse reaction) over time. As equilibrium is approached, the rates of each reaction become approximately equal.

See how many examples of equilibrium you can find in your class!

Posted by ExploreLearning at 12:59 PM in Science, Using Gizmos | Permalink