Energy

Energy is a fundamental quantity that every physical system possesses. Energy of physical system in a certain given state is defined as the amount of work W needed to change the state of the system from some initial state (called reference state or reference level) to the given state.

Related Topics:
Physical - System - Work

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

For example, it takes W = (1/2)mv² amount of work to accelerate a bullet from zero speed to speed v — thus the quantity (1/2)mv² is called a kinetic energy of a bullet. Or it takes W = mgh amount of work to elevate mass m at height h in gravitational field g, thus the amount mgh is called gravitational potential energy.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Other examples are the electrical energy stored in a battery, the chemical energy stored in a piece of food, the thermal energy of a water heater, or the potential energy of elevated water behind a dam.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Energy can be readily transformed from one form into another; for instance, using a battery to power an electrical heater converts chemical energy into electrical energy, which is then converted into thermal energy. Or letting elevated water move down transforms stored potential energy into kinetic energy of moving water and turbine, which in turn transforms into electric energy by generator. The law of conservation of energy states that in a closed system the total amount of energy, corresponding to the sum of a system's constituent energy components, remains constant. This law follows from translational symmetry of time (=independence of any physical process on the moment it started). Some works (thus some forms of energy) are not easily measured by the unaided observer.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~