Energy And Energy-Conservation:

The term an physical quantity energy goes back to the Greek word „energeia“ and means something like "urge" (but the ancient Greeks also included things like happiness in their understanding of energeia). But that we still use this term today shows that energy urges things, that it urges processes. Energy is a necessary requirement that processes work and run. Energy is used to describe the condition at the beginning and at the end of a process. During the process energy is transformed into different forms of energy. The energy difference between the original and final condition tells us how much energy has been transformed.
You might wonder what exactly energy is. Well that is very difficult to say. Nobody knows exactly, we can only describe its properties with mathematics and physics. Energy can be saved, transformed and also devalued. However, energy can NEVER disappear, get lost or destroyed. Neither can we produce or create energy „out of nothing“. Energy exists in different forms, for example
- kinetic energy,
- potential energy (if a body is lifted up or a spring is strained),
- electric energy,
- internal energy (thermal energy, chemical energy and nuclear energy combined)
- light energy.
In this list you have all necessary and important energy forms that we need to describe our everday life. But the complete list is much longer.
Scientists have found out through many experiments, calculations and observations that the total amount of energy within a system stays always the same, unless energy is added from outside or unless it is released out of the system (law of the conservation of energy). In systems of movements (mechanics) energy is often transformed into warmth due to friction. This is called devaluation of energy because this amount of energy cannot be transformed back into mechanical or electrical energy.
Only if a force influences a body during some kind of process, only then can the body's energy be transformed. This can be described with the energy difference between the original state and the final state. This energy difference is called „physical work“.

The physical quantity energy is measured in the unit Joule (short: J).
1 Joule resembles the energy, which is added to a body, if a constant force of one Newton (1 N) acts on the body over a distance of one meter (1 m) (along or parallely to this distance). In the formula it looks much more clear: 1 J = 1 Nm.
Most energy forms, that describe the state of a body, can be calculated with the help of formulae. But we really don't have discuss that now.