The law of conservation of energy states that energy cannot be created or destroyed; it can only be transformed from one form to another. In a closed or isolated system, the total amount of energy remains constant over time

. This means that the sum of all forms of energy before an event is equal to the sum of all forms of energy after the event. For example, potential energy can convert into kinetic energy, and kinetic energy can convert into thermal energy, but the total energy stays the same

. Mathematically, this is expressed as:

∑Ebefore=∑Eafter\sum E_{\text{before}}=\sum E_{\text{after}}∑Ebefore​=∑Eafter​

or

Etotal=constantE_{\text{total}}=\text{constant}Etotal​=constant

The principle applies broadly, including mechanical systems, thermodynamics (first law), and even in modern physics such as special relativity and quantum theory

. In practical terms, energy transformations include chemical energy converting to electrical energy in batteries, potential energy converting to kinetic energy in falling objects, and kinetic energy converting to thermal energy through friction. Although energy may be "lost" as heat or sound in some processes, it is not destroyed but merely changes form

. Einstein’s mass-energy equivalence formula E=mc2E=mc^2E=mc2 further confirms that mass itself is a form of energy, reinforcing the conservation principle

. In summary, the law of conservation of energy is a fundamental physics principle that ensures the total energy in an isolated system remains unchanged, only changing forms but never disappearing or appearing from nothing