Energy is defined as the 'capacity to do work'. It can be obtained from a number of different sources, and converted from one form into another, transferred from one object to another, and even moved from one location to another. Throughout the entire Universe, all forms of energy are linked by the 'first law of thermodynamics', one of the cornerstones of science. Despite its grand name, this law just states the obvious - you can't get something for nothing!

Put another way, although energy can be converted from one form into another, it can't be created or destroyed - when you burn coal, for instance, you're only releasing energy that's been stored; you're not actually creating energy. In other words, the total amount of energy in the Universe is constant.

Throwing a book into the air involves a variety of energy transfers and conversions: chemical energy, kinetic energy, gravitational energy, sound , internal energy.

The Earth's main energy sources

Before the late 18th century, British industry had depended on energy (or more precisely, power) harnessed from water, wind, humans or domesticated animals. However, the Industrial Revolution brought new energy sources and through them major changes in the way people lived.

The discovery and exploitation of massive coal reserves beneath the Earth's surface provided a new, flexible source of energy. It could be burnt to heat water to produce steam, which was then used in a variety of ways to drive machinery. Before coal, factories and mills could only be built where there were running water or windmills. Coal, on the other hand, could be transported with relative ease, but, although a relatively convenient way of transporting stored energy, it was still bulky, and canals and railways were built to move it from place to place.

It would have surprised miners working deep underground, to learn that the energy from coal, rivers and even the wind, all originates from the sun.

The sun is the source from which almost all energy on Earth is derived

In each case, the sun’s energy is stored or converted into different forms before being used to do work.

After coal, the next advance in flexible energy sources was the discovery of electricity. Electrical energy can be transported over vast distances at little cost, and can easily be transformed into other forms. It can be used to produce kinetic energy (the energy due to motion) in motors or pumps, sound energy in hi-fi’s, light in light bulbs and heat.

Despite the advent of electricity, coal remained important because the energy released by burning coal is used to generate the electricity. Other fuels, like oil or gas, could also be used to produce electricity, as well as waterwheels that produce hydroelectricity.

One serious drawback is that many methods of generating electricity involve disruption to the environment. Valuable habitats can be destroyed when valleys are flooded to make dams for hydroelectricity, and the products of burning fossil fuels are implicated in global warming and acid rain. Recently, novel methods have been used to generate electricity without such drawbacks.

Einstein’s theory of relativity states that energy and mass are interrelated. That’s what the equation E=mc² is all about. Energy (E) can be transformed into mass (m), and vice versa. Mass is therefore a form of energy, and, when one chemical element is converted into another, the small changes in mass can be converted into huge amounts of energy. This is because the value of c, the speed of light, in the equation E=mc² is so large (300 000 000 metres per second).

As a result, a small value of m in the equation becomes a very large number indeed when multiplied by 300 000 000 squared, to give the corresponding value of E. The sun is a kind of nuclear power station that converts hydrogen atoms into helium atoms by what’s called a fusion reaction. This transformation of hydrogen into helium involves a small loss of mass, which produces colossal amounts of energy. On Earth, nuclear power stations break an atom into two smaller atoms – a fission, rather than a fusion, process. Again, there’s a small loss of mass, leading to the release of vast amounts of energy, which is then used to produce steam to drive the power station’s turbines.

However, there are significant hazards associated with producing nuclear power in this way. Accidents can lead to the release of radioactivity into the atmosphere, and to loss of life. Consequently, even cleaner, safer solutions are being sought.

On Capraia, we castaways were able to make electricity using a simple wind-powered generator, and because we were provided with wire, we could transport the electricity to where it was needed. Similar small, mobile wind-driven generators are commonly used on boats and in caravans. Larger-scale wind-driven generators, waterwheels and tidal power can address some of the environmental problems associated with fossil and nuclear fuels.

Ironically, this takes the energy story full circle, back to pre-Industrial Revolution times.

Perhaps science still has some alternatives up its sleeve – why not convert sunlight directly into electricity? This has been a surprisingly difficult task, especially in the UK, where sunlight can’t be guaranteed. However, technical improvements in the manufacture of photovoltaic cells using advanced semi-conductors now allow us to capture energy directly from the sun using solar panels. So, with luck, and lots more research, we should be able to produce large amounts of energy without the problems of pollution and habitat destruction.

WIND POWER
Different parts of the Earth's surface absorb energy from the sun with different efficiencies. For instance, land warms up more quickly than water. This means that the air just above the land also warms up, becomes less dense and so rises. The warm air is then replaced with colder air from above the sea. This leads to air currents that are powerful enough to turn the sails of a windmill.

WATER POWER
Streams and rivers get their energy through a process that starts when the sun heats up the surface water of the sea, which then evaporates. Air currents carry the water vapour as clouds until it rises, cools and condenses back to liquid water, which falls as rain (or if cold enough, snow).

The water cycle

This cooling often happens as air is forced upwards by hills or mountains. The rain that falls on high ground moves downhill under gravity, and, en route back to the sea, is channelled into streams and rivers, which can be used to turn waterwheels. Again, all the energy that is needed to evaporate and move the water originated in the sun.

POWER FROM COAL
Coal is just one way in which the sun's energy is stored temporarily, albeit for millions of years. All green plants harvest energy from the sun by a process called photosynthesis. This takes place in tiny structures in the leaves, called chloroplasts. Chloroplasts absorb sunlight, and through a series of reactions, break down water and carbon dioxide absorbed from the air, to produce oxygen and carbohydrates (for more on carbohydrates, visit 'You are what you eat'). If plant material is burnt, the carbohydrates react with oxygen to re-generate the carbon dioxide and water. At the same time, energy is released in the form of heat - heat that originated from the sun.

The energy transfers and conversions involved in making a cup of tea. The internal energy in the cup of tea is derived from the chemical energy in the coal.

Coal (and other fossil fuels like natural gas and oil) is derived from plants and trees that died millions of years ago. These became buried and subjected to heat and high pressures, so that over geological time they were transformed into coal. When coal is burned, oxygen is consumed, and the carbon dioxide and water that the plants and trees absorbed all those years ago are recycled into the atmosphere. Energy is again released, but in this case, the energy had its origins in the sun millions of years ago.

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Content last updated: 30/08/2006