Energy
Energy is the capacity of a physical system to perform work. It is not produced or consumed, as is often wrongly believed, it is merely transported or converted from one physical state to another, in the form of heat, electrical or mechanical energy, or other forms.
We generally have to distinguish between energy from renewable sources, such as wood, waste, wind, solar radiation, etc. and exhaustible sources, such as fossil fuels. Whereas renewable energy is naturally replenished and can therefore be used by humans more or less indefinitely, provided the rate of consumption is in balance, non-renewable energies tend to be exhaustible and pollutive. The energy in fossil fuels is released upon burning when the carbon and hydrogen within them combines with oxygen in the air to form carbon dioxide (CO2).
Although renewable energies are non-pollutive, the manufacturing processes of technical equipment, e.g. solar cells, the displacing of natural habitats and the large areas of land required in the case of wind or solar farms, also have an impact on our living environment.
The following animated diagram shows several energy-related concepts.
Animation: Energy-related concepts (click to start animation)
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