Thermodynamics — ‘thermo’ meaning heat and ‘dynamics’ referring to work —is the branch of physics that focuses on the relationship between heat and other kinds of energy. This video from The Royal Institution introduces the first of four laws of thermodynamics: that energy can’t be created or destroyed, but will instead be converted or transferred:

In this unique animation, chemist Andrea Sella recalls one of Richard Feynman’s descriptions of the law. In a classic analogy, Feynman imagines a child with a set of building blocks, and a mother struggling to ensure he doesn’t lose any. Each day, the total number of blocks remains the same, although some do stray from their original location.

This consistent fact – that the blocks cannot be created or destroyed – allows the mother to calculate how many blocks are in a box without even looking. And this consistent fact, that energy itself is also always conserved, lays the foundation for one of physics’ greatest rules.

This animation is part of the Ri’s Thermodynamics Advent Calendar for 2016.

Next: Watch more illuminating Ri videos on this site.

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