Scientific reading
Begin with an everyday engineering question
A fictional small device receives 500 J of energy over a test interval. Some leaves as its intended useful output, some transfers thermally to surroundings, and some leaves through other pathways such as vibration or sound. An engineer cannot learn much from a claim that a device 'loses energy' without specifying the system boundary, intended output, and receiving pathways.
Draw a complete transfer diagram
For a device with negligible stored-energy change over the chosen interval, write E_input=E_useful+E_thermal+E_other. Every arrow should be named and labeled with joules. Thermal energy is not destroyed energy; it is accounted for outside the selected useful output. Whether a thermal flow is 'useful' depends on a device's purpose—for a heater it may be the intended output.
A numerical Sankey example
Suppose 500 J enters a fictional motion-producing device: 300 J is useful mechanical output, 125 J is transferred thermally, and 75 J is transferred to other forms. These numbers add to 500 J. The diagram's band widths can scale proportionally with the flows, but actual appearance is secondary to the numerical table and proper system boundaries.
What the model cannot measure
The three pathways are assumed, not measured. Actual devices may temporarily store energy, have energy flow through additional forms, or vary across time. In a system with storage change, the simplified output-only ledger must be expanded: E_input=E_outputs+ΔE_stored (with consistent signs). No electrical wiring or running machines is needed for this learning activity.