Constant power: steady accumulation
Power in a pathway accumulates over time to fill or empty a store. A constant power leads to a steady change in the energy of stores. How much energy is shifted can be found from the area under a power/time graph.
Accumulations that are simple to calculate
The energy in these stores can be calculated, and therefore dealt with during studies for those aged 14–16, mostly in physics.
The areas under the graphs again show the energy accumulated in the store as the independent variable is altered:
- For the kinetic store, the speed.
- For the elastic store, the stretch.
- For the thermal store, the temperature.
- For the gravity store, the change in separation.
- For the chemical store, clues about the number of particles that have reacted.
Changes in these independent variables were exactly the changes you were asked to look out for in the SPT: Energy topic when identifying likely variation in the energy in each store. This is what justified identifying these as stores in pre-quantitative work – that you would be able to calculate the energy stored due to particular changes not too far in the (pupil's) future.
Accumulations that are less simple to calculate
These stores are mostly dealt with in post-16 physics, largely pre-19 (magnetic stores are mathematically tricky, and only the electric bit of the electromagnetic store can easily be calculated by A-level students).
Again, the areas under the graphs show the energy accumulated in the store as the independent variable is altered:
- For the vibration store, the amplitude.
- For the nuclear store, the change in mass.
- For the electric and magnetic store, the change in separation.
Again, these changes map onto those used to identify the stores, although looking for the minuscule changes in mass associated with energy shifted to or from the nuclear store is hard to imagine, which is perhaps why that one seemed a bit mysterious in the SPT: Energy topic. Perhaps surprisingly, the magnetic aspect of the electromagnetic store was easiest to relate to the stretched elastic band teaching tip in the SPT: Energy topic, and turns out to be hardest to calculate.