Vocabulary notes

chase, v переслідувати

valour, n доблесть

wander, v ходити скрізь, бродити, блукати

rove, v блукати, тинятися

strath, п гірська долина

valley, п долина

torrent, п потік

flood, n повінь, розлив

Section 3

Text 2. Radiation

Key terms: conduction, convection, radiation, transfer, visible light, ultraviolet rays, infrared light, microwaves, radio waves, electromagnetic spectrum, vacuum, wavelength, radiator, absorber, emitter

Suppose you touch the surface of a hot water tank with your hand. Heat travels from the hot water through the metal to your hand by conduction. Conduction can be defined as the movement of heat as a result of collisions between molecules. Hold your hand above the water tank. Heat is now carried to your hand by convection currents of air. Convection is the transfer of heat by movement of the molecules from one place to another. Hold your hand off to thé side of the tank. Most of the heat reaches your hand by a process called radiation.

Radiation is the transfer of energy by means of electromagnetic waves. Electromagnetic waves include gamma rays, X- rays, ultraviolet rays, visible light, infrared light, microwaves and radiowaves. Figure 3 shows the approximate wavelengths of the electromagnetic spectrum. Heat radiation is the infrared portion of this spectrum. Electromagnetic waves travel through a vacuum at a speed of light and can also travel through gases. This is how the energy from the Sun reaches us.

The surface of any object continually emits radiant energy over a range of wavelengths. The energy emitted by the object depends on the nature of the surface,^nd on its temperature. At room temperature the emission is small, occurring chiefly in the form of long infrared waves. These wavelengths are invisible to the eye. The rate of emission by a hot incandescent filament at a temperature of 3000°C is much larger. Although most of the energy is still in the form of infrared waves, it now also contains much shorter wavelengths that the eye is able to detect.

The best radiators have black surfaces. For this reason, car radiators and heat exchangers, such as cooling coils on the back of refrigerators, are painted black. The best emitter of radiant energy is also the best absorber. That is why the cool absorbing surface of a solar collector, trapping- radiant energy from the sun, is black. Radiant energy absorbed by any surface becomes thermal energy.,

The poorest emitter and absorber о radiant energy is the best reflector. This explains why we we^ 'ite clothing in the summer. It also explains why the inside of i... .t.ws bottles are silvered and why the outside of kettles are light coloured.

Every object continually absorbs and emits radiant energy. The greater the temperature difference between a body and its surroundings the faster the transfer of energy. A body cooler than its surroundings absorbs more radiant energy than it emits and, as a result, heats up. Conversely, a body hotter than its surroundings emits more radiant energy than it absorbs, and so cools down. A cup of hot tea loses radiant energy much faster than a cup of warm tea.