ENERGY: QUALITATIVE DEFINITION

Energy is a property of matter that

can be converted to work

can be converted to heat

can be converted to radiation

Heat, work and radiation are more palpable forms - or manifestations - of energy:

We want heat, work, and some forms of radiation. Heat and work are used to produce electric energy (electricity), on which our industrialized society depends so much.

Heat is easier to get than work. (If you don't believe this, just rub your hands and you'll convert work into heat. To do the reverse, you have to buy a special device that we call a heat engine.) Successful conversion of heat to work in a practical heat engine has brought about the industrial revolution in the nineteenth century.

We get radiation as a 'byproduct' (of nuclear energy utilization, in which the objective is to produce heat) or as Nature's gift (from the Sun); the latter is useful (for the most part), while the former can be very dangerous (we refer to it as radioactivity or radioactive waste). We also now know how to produce certain forms of radiation ourselves (e.g., TV, microwave and radio signals), and this has brought about the current communications revolution.

These are the energy forms that our industrial society needs, but they are not the ones that are readily available. Heat is available, but not where and when we need it. In summer, when it is plentiful, we want to get rid of it (using air conditioning). In winter, when it is scarce, we can't get enough of it (so we produce it in furnaces).

The following energy forms (referred to as the 'primitive' ones) are readily available to us on Earth:

• gravitational (energy due to the fact that all objects that are close to the Earth's surface tend to fall toward the center of the Earth);
• nuclear (energy stored in the nucleus of an atom);
• chemical (energy stored in molecules of substances called fuels).

So we need to convert these primitive energy forms into more useful energy forms, and this is our principal topic of discussion in this part of the course. We shall thus take a closer look at the more important energy forms and then examine the laws that govern their interconversion.

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