C.29.BB:4.1 - Choose an additive quantity and a common interval

Name the quantity Q, its unit or counting rule, the included parts S, and the start and end conditions. Define who or what belongs to each part. Use disjoint parts, or otherwise account for their overlap before summing.

For disjoint parts with amounts q_i, additivity means that Q(S) = sum q_i. The rule needs a basis in the selected model or counting definition. For a population, assigning each included job to one stage can provide it. For physical storage, use the appropriate physical quantity and law.

An average temperature is not an additive heat store. If the question concerns stored thermal energy, construct that quantity from the material and state relations; if it concerns the maximum temperature, preserve the local temperatures needed to obtain the maximum. Choosing a convenient sum changes the question unless the required result can be recovered from it.

Keep all amounts on the same interval. A transfer that leaves one store before the endpoint but reaches another afterward requires an in-transit store or separate boundary crossings. For snapshots at different times, first recover their relation to the requested interval.