# Energy

There are about 20 million households in Britain alone and on average each one of these uses some 21 000 kW h of energy annually! The study of energy is therefore of vital importance in our lives.

### World energy use

An account of the growth in world energy consumption makes sobering reading. If we define a quantity of energy (Q) as 1021J then in the 2000 years up to 1850 the world is thought to have used between 6 and 9 Q. In the following hundred years up to 1950 it had burned a further 6 Q and from then on we have been using at least 1Q every ten years. Fortunately in recent years this rate of increase has slowed down considerably, partly because of the high cost of energy.
Energy conservation is also important. Some 5 per cent of the total energy bill of the western world can be saved by conservation measures such as the proper use of insulation!

To give you some idea of the value of energy, the table below shows the energy used in a number of situations.

 Typical energy values (J) Joules moonlight on face for 1 s 10-3 pressing down a typewriter key 1 house brick raised to shoulder height 30 burning a match 1000 potential energy of a person at the top of the stairs 1500 kinetic energy of a car travelling at 70 mph 500000 electrical energy in a fully charged car battery 2000000 chemical energy in a day's food intake 11000000 chemical energy in one litre of petrol 35000000 first atomic bomb 1013 very severe earthquake 1020 world annual energy consumption (2008) 4.74x1020 Earth's annual share of the Sun's heat 1025 Rotational kinetic energy of the Earth 1029

Relative present energy use per person per day

 USA 10 000 Japan and Europe 4000 Less developed countries 100-1000 World average 2000

Simple estimate of the world's annual energy consumption
Total = 5x109 people x 2000W x 86400x365 = 315x1018 = 3.15x1020 J

### Conservation of energy

You should notice that we talk about the transformation or conversion of energy from one form to another and not its use. This is because although we may use up energy in one form it always reappears as another. This is a most important principle of Physics: that of the conservation of energy. The principle states that:

Energy is never created or destroyed but only changed from one form to another.

The enormous increase in demand for energy this century makes the production and conversion of energy a most important topic. But you may ask: why worry, if energy is always conserved? The trouble is that it is changed from useful or high-grade forms such as chemical or electrical energy to relatively useless forms such as low-grade heat.

### Ultimate recoverable energy deposits (thousands of millions of tonnes or equivalent)

 Cumulative production to 1975 Proven or probable reserves Possible additional reserves Petroleum 48.4 114.8 142.7 Tar sands 0.025 300 Shale oils - 460.3 1415.6 Natural gas 21 54 185 Charcoal n/a 339.9 4904 Lignite n/a 99.3 1656 Total n/a 1368.3 8303.3

(Interdisciplinary Science Reviews 1983)

 Oil 60 year supply remaining Natural gas 100 years supply remaining Coal 200 years supply remaining Uranium 200 years supply remaining Fusion 1 million years supply remaining DD Fusion 10 billion years supply remaining