In presenting these various energy descriptions of familiar processes, we have been careful to study one process at a time, moving from one snapshot of a system to another. This restricts attention to analysing the energy shift from store A to store B or perhaps from store A to store C. We recommend that you follow this practice and are not drawn into constructing long energy chains. Doing so can over-complicate the simple accountancy of the energy system and may lead to situations where stores are introduced for which there is no change. Here we discuss two examples where you might be tempted to consider intermediate stages.
The first example consists of an electric motor being used to lift a mass.
The energy description involves the quantity of energy removed from the chemical store (associated with the cell) as a result of the reaction of the chemicals in the cell. This energy is shifted to the gravitational store of the mass (in the Earth's gravitational field) as a result of the force acting on the mass through the specified distance.
The speed at which the mass travels upwards is not important to these changes, since the energy gained by the mass is fixed (as it is raised through a fixed distance). We think that you should always try to keep these energy descriptions as simple as possible and focus on the snapshots at the beginning and the end. This may well be in contrast to existing approaches:
Commonly used approach:
There is an energy transfer: Chemical energy (battery) to … Electrical energy (motor) to … Kinetic energy (mass) to … Gravitational energy (mass)
Energy is shifted from the chemical store (of the cell) to the gravitational store (of the mass).
Note that there are two differences in the approaches. Firstly, according to the recommended approach, electrical stores do not exist (although electromagnetic stores do exist – but these are for static situations where charged particles or magnets are held apart) so there is no
electrical energy term. Secondly, kinetic stores have no part to play in the description simply because there is no change in the quantity of energy in that store (the mass rises steadily and so the energy in the kinetic store is constant). In other words we do not have the situation where one store is being emptied and another filled. The kinetic store does not change.
The only development which might be made to this recommended description is to recognise that not all of the energy shifted from the chemical store will end up in the gravitational store, so justifying adding a thermal store (of the surroundings) to which some of the energy is shifted.