Energy Transferred by Working
Electricity and Magnetism

Differing accumulation

Physics Narrative for 14-16 Supporting Physics Teaching

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 in the store.

Calculating Accumulations

Changes in certain 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.

Now you can make good on that promise for some stores, paying attention to the physical clue given, and then quantifying:

    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.

These stores are mostly dealt with in post-16 physics (so forcing a delay in making good the promise a little here), 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):

    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.

Energy Transferred by Working
appears in the relation dU=dQ+dW
is used in analyses relating to Working Engines Thermionic Emission
is a special case of Work
has the special case Potential Energy Kinetic Energy
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