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Learn the skill
For constant power, E = P x t. Watts multiplied by seconds give joules. Compare alternative runs separately; do not add them as if both happened in sequence. When separate intervals are explicitly successive, add their energies. A longer run at the same power transfers more energy without changing power. Use only paper records, not real equipment or waiting.
Worked example
At constant 3 W, a 2-second run transfers 6 J. A 4-second run instead transfers 12 J. These are alternative runs, so 18 J is not the energy of either one. If a different record explicitly says 2 seconds followed by another 4 seconds at 3 W, that six-second run does transfer 18 J.
