energy saver

Energy is a fundamental concept in physics that is often defined as the capacity to do mechanical work — a definition that is contested by many physicists (see below). The concept of energy has applications throughout the natural sciences.

Energy is subject to a strict local conservation law. That means energy cannot be created or destroyed. The only way the energy content of a given region can change is by the flow of energy to or from adjacent regions. There is also a global law of energy conservation, which says that the total energy of the universe cannot change. The global law is a corollary of the local law (and not vice versa).^{[1] [2]} Conservation of energy is related to a symmetry of the laws of physics, namely invariance with respect to a shift in time, via Noether's theorem.

In classical physics (pre-20th-century), energy was considered a scalar quantity, having no direction in space. In special relativity energy is also a scalar and is one component of more general quantity - [energy, momentum] 4-vector (so energy is associated with the timelike direction).^[3] To say the same thing another way, energy is invariant with respect to spacelike rotations, but not invariant with respect to boosts.

The total energy of a system can be subdivided and classified in many ways. For example, it is sometimes convenient to distinguish kinetic energy from potential energy. It may also be convenient to distinguish gravitational energy, electrical energy, thermal energy, et cetera. These classifications overlap; for instance thermal energy is usually partly kinetic and partly potential energy.