The law of conservation of energy, which states that the amount of energy in a system is constant. For questions about Earth's environment, see the climate-science tag instead.
The conservation of energy is a fundamental concept of physics along with the conservation of mass and the conservation of momentum. Within some problem domain, the amount of energy remains constant and energy is neither created nor destroyed. Energy can be converted from one form to another (potential energy can be converted to kinetic energy) but the total energy within the domain remains fixed.
If you take all forms of energy into account, the total energy of an isolated system always remains constant. All the forms of energy follow the law of conservation of energy.
The amount of energy in any system, then, is determined by the following equation:$$U_T=U_i+W+Q$$
$U_T~:$ the total internal energy of a system.
$U_i~~:$ the initial internal energy of a system.
$W~~:$ the work done by or on the system.
$Q~~~:$ the heat added to, or removed from, the system.
It is also possible to determine the change in internal energy of the system using the equation: $~ΔU=W+Q~.$
The law of conservation of energy states that the amount of energy in a system is constant. For questions about Earth's environment, see the climate-science tag instead.
Conservation of energy is linked to time-independence by noethers-theorem, as how momentum conservation is linked to spatial-independence by the same theorem.