Every joule that has ever existed still exists. What changes is where it is and how thinly it is spread: concentrated in a raised weight or a tank of petrol it can do work, smeared through the air as a fraction of a degree it can do nothing at all. Below is a rollercoaster running on a fixed budget. Watch the three bars trade with each other, and watch the total refuse to move.
Every frame of the run, stacked. Green is kinetic energy, blue is gravitational potential energy, red is heat that friction has already dumped into the rails and the air. The white line along the top is the sum of the three, and it is drawn from live simulation numbers rather than pinned there — flat is what conservation of energy looks like. Kinetic and potential energy on their own get a fuller treatment in Lab 09, Forces & Motion.
A Sankey diagram draws every stream at a width proportional to the energy in it, so the fat one is the one you are paying for and not getting. Efficiency is just the fraction of the input that leaves by the door marked useful. These are real published figures, not illustrations.
A joule is about what it takes to lift an apple one metre. Everything below is measured in the same unit, which is the problem: they span twenty-three powers of ten. Click any row to measure the others against it.