Energy Stores & Transfers

The store-and-pathway model

• Energy in any physical situation is described as sitting in one of several stores and being moved between those stores by one of a few transfer pathways
• A system is the object (or group of objects) you have chosen to focus on; defining the system fixes the boundaries of which stores and which transfers count for the question
• When a system is in equilibrium, no stores are changing and nothing happens; when something changes, energy moves between stores

The eight energy stores

The four transfer pathways

Reading an energy transfer

• To describe what happens in a system in store-and-pathway language, answer three questions:
  • which store is the energy moving from?
  • which store is it moving to?
  • which pathway is carrying it?

Example A — a wound-up clockwork toy is released and runs across the floor. Identify the stores and pathway.

• Wound up, the toy holds its energy in the elastic potential store of its coiled spring
• Once the spring unwinds, the gears and wheels carry the toy across the floor, so the energy ends in the kinetic store of the moving toy
• A spring force acting through the gear teeth does the work, so the pathway is mechanical
• A small amount also leaks to the thermal store of the gears as friction warms them, and this is the wasted share

Example B — a stone is dropped from a window ledge. Identify the stores and pathway.

• Before release, all the stone's energy sits in its gravitational potential store, because it has been raised above the ground
• As gravity pulls it downwards and it picks up speed, the energy fills its kinetic store instead
• The pull of weight acting along the fall distance is the work being done, so the pathway is mechanical
• Net effect: a mechanical pathway carries the energy out of the stone's gravitational potential store and into its kinetic store