# Internal energy Internal energy is the total energy held inside a system — the kinetic energy of its particles plus the potential energy of whatever binds them. It is a state function: it depends only on the system's present condition, never on how it got there, which is what makes the [[First_law_of_thermodynamics]] a usable accounting identity rather than a path-dependent mess. ## Microsim — p5.js Sidecar: `Internal_energy.p5.js` · route leaf: `microsim/p5js/Internal_energy__20260910T0400Z`. Built to the Betterfire Standard v0 (single `ARTICLE` constant, HUD title and Wikitube URL, control hints, parameter readout and equation). | Control | Does | |---------|------| | `T slider` | temperature, which sets particle speed | | `N slider` | number of particles | | `[d]` | toggle monatomic / diatomic | *What to watch:* Raise the temperature and the particles speed up; the internal-energy bar tracks the sum of their kinetic energies exactly. Switch to diatomic and the same temperature holds more energy. ## A property, not a transfer Heat and work are things that happen at a boundary; internal energy is a thing a system *has*. For an ideal monatomic gas it is simply proportional to temperature, U = (3/2)NkT, which is [[Kinetic_theory_of_gases]] saying that temperature is nothing but average molecular kinetic energy. Add molecular rotation and vibration and the count rises, which is why [[Heat_capacity]] differs between a noble gas like [[Helium]] and a diatomic one — the same joules get shared among more ways of moving. ## Where it hides Chemical bonds are potential energy, so a [[Redox]] reaction is internal energy rearranging: [[Combustion]] releases it, [[Electrolysis]] stores it back. Phase change moves it without changing temperature, which is what latent heat measures and why [[Properties_of_water|water]]'s enormous latent heats matter for [[Heat_transfer]]. Only differences in U are ever measured — the absolute zero point is a convention, not an observable. **Reads with:** *Thermodynamics and Chemistry, 2nd ed. (Howard DeVoe, 2020)* — [OTL record](https://open.umn.edu/opentextbooks/textbooks/thermodynamics-and-chemistry) · [download](https://www2.chem.umd.edu/thermobook/downloads.htm) · CC BY 4.0. Section 6 of the [[PORTAL_Thury_Hydrodynamics_Apex_Spine|Apex Spine]] book shelf. **On the spine:** [[First_law_of_thermodynamics]] · [[Enthalpy]] · [[Heat_capacity]] · [[Kinetic_theory_of_gases]] · [[WT!Thury_Hydrodynamics_Compendium]]. <!-- MATTERSIM:BEGIN g33 — Matter & Energy Cluster microsim (framework build, specs/sims/Internal_energy.json); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *Internal energy* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/matter/Internal_energy.html" data-title="Internal energy"></div> *Built from `MICROSIM_GUIDE/specs/sims/Internal_energy.json`; part of the [[PORTAL_Matter|Matter portal]] spine (section sims and See-also variants).* <!-- MATTERSIM:END --> ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Internal_energy) : [Wikitube](https://en.wikitube.io/wiki/Internal_energy) ## Previous hub tags Hubs: `Life_Physics`, `Systems`. Portals: [[PORTAL_Thury_Hydrodynamics_Apex_Spine]], [[PORTAL_Physics]], [[PORTAL_Energy]]. --- *Book-section wave · 2026-09-10 · article + p5 microsim shipped together.*