Energy Stores & Transfers
Energy Resources & Energy Transfers
Energy Stores and Transfer Pathways
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
| Store | Where it sits |
|---|---|
| Kinetic | Any moving object |
| Gravitational potential | An object raised above the ground in a gravitational field |
| Elastic potential | A material that has been stretched, squashed, bent or twisted |
| Magnetic | Magnetic materials interacting through their fields |
| Electrostatic | Charged particles or objects interacting through their fields |
| Chemical | Chemical bonds inside fuels, foods, batteries and other reactants |
| Nuclear | The bonds holding protons and neutrons together inside the nucleus |
| Thermal (internal) | The random kinetic and potential energy of the particles inside any object above absolute zero |
The four transfer pathways
| Pathway | Mechanism |
|---|---|
| Mechanical | A force acting on an object as the object moves (pushing, pulling, stretching, squashing) |
| Electrical | A current carrying charge through a potential difference |
| Heating | Energy transferred from a hotter object to a colder one by conduction or convection |
| Radiation | Energy carried away from a surface by electromagnetic waves (most commonly infrared) |
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
Describing an energy transfer in stores and pathways
Question: Describe the energy transfers in a situation (a falling car, a marble run) using stores and pathways, or name a store that increases or decreases, or the method of transfer (1–5 marks).
Asked in 14 of the 24 papers. Name every store and every pathway in order: for a falling car, energy goes from its gravitational store to its kinetic store, mechanically (by a force such as weight); a person lifting a marble that then rolls moves energy from their chemical store to its gravitational store, then its kinetic store, mechanically each time. Then the wasted share: the thermal store of the object and surroundings increases, by heating or radiation (sound counts as radiation). Each pathway is a separate mark, so stores alone leave marks behind.
When asked which store changes, give its direction as well as its name: the gravitational store of a falling object decreases while its kinetic store increases. Say thermal, not heat, which one scheme rejects.