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Can renewable energy sources power the world?
Can renewable energy sources power the world?

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2 Energy definitions and concepts

If someone asked you to define energy, what would you say?

The word energy is derived from the Greek en (in) and ergon (work). Energy is defined as ‘the capacity to do work – that is, the capacity to move an object against a resisting force. The scientific unit of energy is the joule.

The concept of energy reveals the common features in processes as diverse as burning fuels, propelling machines and charging batteries. These and other processes can be described in terms of diverse forms of energy, including:

  • thermal energy (heat)
  • chemical energy (in fuels or batteries)
  • kinetic energy (in moving substances)
  • electrical energy
  • gravitational energy
  • nuclear energy

In everyday language, the word 'power' is often used as a synonym for 'energy', but this is not strictly correct.

Power is the rate at which energy is converted from one form to another or transmitted from one place to another. The scientific unit of power is the watt.

Renewable energy can take a variety of these forms, and can be defined as:

energy obtained from the continuous or repetitive currents of energy recurring in the natural environment

(Twidell and Weir, 1986)

or as

energy flows which are replenished at the same rate as they are ‘used’

(Sorensen, 2000)

Now you’ll look at some frequently used units of energy and power that you’ll come across regularly during this course.