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.

History

In the past, energy was discussed in terms of easily observable effects it has on the properties of objects or changes in state of various systems. Basically, if something changed, some sort of energy was involved in that change. As it was realized that energy could be stored in objects, the concept of energy came to embrace the idea of the potential for change as well as change itself. Such effects (both potential and realized) come in many different forms; 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 kinetic energy of a moving train. To simply say energy is "change or the potential for change", however, misses many important examples of energy as it exists in the physical world.

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Energy can be used not only to produce observable change, it also is used to prevent change in which case unaided observation of this kind of energy can be difficult. For example, looking at a statue holding a 10 kg weight, the presence of energy needed to do so may not be observable. However, if you are holding up the 10 kg weight instead of the statue the need for energy to accomplish this becomes apparent. You can feel the gravitational force on you both when you are moving the weight up and when you are not moving it.

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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. In the previous example of holding the 10 kg weight, the work you perform to raise the weight is observed as kinetic energy of motion which is converted to potential energy. Letting go of the weight once again transforms this stored potential energy back into kinetic energy as the weight falls under the force of gravity

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