You have already discovered that energy cannot be created or destroyed. Where does energy come from? The answer is that on Earth the primary source of energy is the Sun. The Sun is an enormous nuclear reactor producing energy by destroying matter in a process called nuclear fusion. In this process, hydrogen atoms are fused or joined to make larger atoms and the energy produced is radiated into the Universe as electromagnetic radiation. The Earth, being close to the Sun, receives a small portion of this radiated energy.

Energy from the Sun - solar energy
The solar energy arrives at the Earth's atmosphere ranging from radio waves through visible to gamma radiation. (See Electromagnetic spectrum for more information) It is this solar energy that is transformed into the range of energies we rely on here on the Earth. Without the Sun, we would soon be a dead planet.

Solar power to electricity

With solar cells it is possible to transform the Sun's energy into electrical energy. This is particularly valuable where other sources of electrical energy are not available, for example, a telephone box way out in the bush. Solar cell generated electrical energy can also be transformed to kinetic energy, as is done with solar powered cars. A more common use of energy from the Sun is to generate electrical energy by water flowing from lakes or dams. In this case, there is a chain of transformations. First, the Sun's energy evaporates water from the ocean or lakes. This water vapour rises, gaining gravitational potential energy. Clouds form and the falling rain experiences the transformation of potential to kinetic energy. Water collected in dams above sea level still has gravitational potential energy. Water flowing down from the dam through a generator, transforms the stored gravitational potential energy of the water, into kinetic energy of the moving turbine blades in the generator, which is, in turn, transformed into electrical energy. It is a long chain, all begun by the Sun.

Food energy
We all need food to maintain life. The energy in food is chemical energy which is transformed into thermal energy to keep us warm, to drive our internal functions and, with children, to grow. Plants use energy from the Sun's combined with carbon dioxide and water to produce food by a process called photosynthesis. Plants are chemical factories storing energy in the form of sugars so that it can be transformed at a later time.

Stored energy from the past
Energy from food keeps us warm inside and helps maintain our bodies, but we also need an external source of storable energy to run our machines and make our lives more comfortable. This energy can come from plants too, but in a slightly different way. A wood fire burns plant material, largely from trees that have stored solar energy over many years. Coal is even older and is the compressed remains of plants millions of years old. When we burn coal we are using the Sun's energy. Oil is thought to have come from tiny sea creatures, their gathered energy trapped for millions of years under the ground in the form of crude oil and natural gas. This stored energy is transformed by machines into light, thermal energy, kinetic energy and electrical energy to carry the transformation chain even further.