Seite - 15 - in Water, Energy, and Environment - A Primer

10 billion of what the Sun radiates in all directions into space. While some of this intercepted solar radiation (insolation) bounces off the Earth’s clouds back into space, approximately 70% enters the atmosphere and becomes part of the Earth’s energy balance with the Sun, which determines the Earth’s average temperature (a more detailed discussion of this energy balance and global warming can be found in Chapter 4). In addition, there is considerable geothermal energy under our feet in the form of hot water and hot rock (see Chapter 8). Thus, the reality is that large quantities of energy are available on our planet. What is in short supply is inexpensive energy that people can afford to buy. 2.3 WHAT ISENERGY? Wheredoes theword ‘energy’comefrom,andhowis it defined? It derives from the Greek word ‘energeia’ (as translated) which means activity or operation. When first used, probably by Aristotle in the 4th century BC, it was ‘a qualitative philosophical concept, broad enough to include ideas such as happiness and pleasure (11).’ Leibniz, in the late 1600s, defined something he called ‘vis viva’ (living force) as the product of an object’s mass and its velocity squared, mv2. Today, ½mv2 is referred to as an object’s kinetic energy, as first articulated by Coriolis in 1829. The concept of potential energywas introduced in 1853byRankine. Energy isoftendefinedas ‘the capacityorpower todowork’. In physical/scientific terms it is ‘the quantitative property that must be transferred to an object in order to performwork on, or to heat, the object’ (12). It comes in several broad categories: kinetic energy – energy of motion; potential energy – energy due to an object’s position in a gravitational or electromagnetic field; chemical energy – energy released when a fuel burns; radiation energy – energy carried by electromagnetic waves; elastic energy – energy stored by stretching solid objects; and Energyand itsglobal context 15