Letter R

RADIATION - The flow of energy across open space via electromagnetic waves such as light. Passage of heat from one object to another without warming the air space in between.

REACTOR - A device in which a controlled nuclear chain reaction can be maintained, producing heat energy.

RENEWABLE ENERGY - Resources that constantly renew themselves or that are regarded as practically inexhaustible. These include solar, wind, geothermal, hydro and wood. Although particular geothermal formations can be depleted, the natural heat in the earth is a virtually inexhaustible reserve of potential energy. Renewable resources also include some experimental or less-developed sources such as tidal power, sea currents and ocean thermal gradients.

RENEWABLE RESOURCES - Renewable energy resources are naturally replenishable, but flow-limited. They are virtually inexhaustible in duration but limited in the amount of energy that is available per unit of time. Some (such as geothermal and biomass) may be stock-limited in that stocks are depleted by use, but on a time scale of decades, or perhaps centuries, they can probably be replenished. Renewable energy resources include: biomass, hydro, geothermal, solar and wind. In the future they could also include the use of ocean thermal, wave, and tidal action technologies. Utility renewable resource applications include bulk electricity generation, on-site electricity generation, distributed electricity generation, non-grid-connected generation, and demand-reduction (energy efficiency) technologies.

RESISTANCE (ELECTRICAL) - The ability of all conductors of electricity to resist the flow of current, turning some of it into heat. Resistance depends on the cross section of the conductor (the smaller the cross section, the greater the resistance) and its temperature (the hotter the cross section, the greater its resistance).

RESISTOR - Resistors are used to restrict the amount of CURRENT flowing through a circuit. They are often used to protect other delicate devices, such as LEDs, from damaging high voltages. The value of a resistor is measured in OHMS, and we can use this to calculate how much current will pass through it. High value resistors may become very hot, and these may be fitted with fins to help stop them melting. There are two special types, THERMISTORS and LDRs (sometimes called PHOTOCONDUCTORS).