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How can energy change forms while total energy remains constant?

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A diagram of a cylindrical vessel containing gas, with a movable piston held down by a weight. Countless small dots beneath the piston represent gas particles in rapid, random motion, illustrating Bernoulli's model of gas pressure. — physics As conceived by Daniel Bernoulli in *Hydrodynamica* (1738), gases consist of numerous particles in rapid random motion. He assumed that the pressure of a gas is produced by the direct impact of the particles on the walls of the container.

The principle of conservation of energy states that energy is neither created nor destroyed, but rather transformed from one state to another within a closed system. This concept is formally defined by the first law of thermodynamics, which establishes that the total energy of the universe remains constant.123

Energy changes form through various physical processes. For instance, as an object moves, its potential energy (stored energy) can be converted into kinetic energy (energy of motion), and vice versa. When friction acts on a moving object, kinetic energy is dissipated as thermal energy. Similarly, chemical energy in fuels can be converted into kinetic energy to power a vehicle.12

Mathematically, this is represented by accounting for energy flows across system boundaries. The change in a system's internal energy ( $Δ U$ ) is equal to the heat ( $Q$ ) added to the system minus the work ( $W$ ) performed by it, expressed as $Δ U = Q - W$ . Because energy is a state function, the total energy remains balanced regardless of the specific path taken during these transformations.3