3. Energy sources.

Energy sources are natural resources that we can use to generate different forms energy. Then we can transform that energy for various purposes. We can classify energy sources into two general categories: renewable and non-renewable.

3.1. Non-renewable energy sources.
Non renewable energy sources come from natural resources that are limited and can be exhausted; it would take millions of years o regenerate these sources of energy. At the moment, the most commonly used energy sources are non-renewable. They include fossil fuels and nuclear energy, wich uses radioactive materials.

3.2. Renewable energy sources.
Renewable energy sources come from natural resources that we cannot use up completely. These include hydroelectric, solar, marine, geothermal and biomass resources, as well as energy that we can produce from solid urban waste.