18.20 - Radiation

Radiation: Heat transfer by electromagnetic

waves.

If you place your hand near a red-hot heating element and feel your hand warm up, you

are experiencing thermal radiation: the transfer of energy by electromagnetic waves.

You correctly think of objects like the heating element as radiating heat; in fact, every

object with a temperature above absolute zero radiates energy.

Radiation consists of electromagnetic waves, which are made up of electric and

magnetic fields. Radiation needs no medium in which to travel; it can move through a

vacuum. The wavelength of radiation varies. For instance, red light has a wavelength of

about 700 nm, and blue light a wavelength of about 500 nm. Infrared and ultraviolet

radiation are two forms of radiation whose wavelengths are, respectively, longer and

shorter than those of visible light.

All objects radiate electromagnetic radiation of different wavelengths. For instance, you

see the red-hot stove coil because it emits some visible light. The coil also emits

infrared radiation that you cannot see but do feel as heat flowing to your hand, and it

emits a minimal amount of ultraviolet radiation too.

Although any particular object radiates a range of wavelengths, there is a peak in that

range, a wavelength at which the power output is the greatest. This peak moves to

shorter wavelengths as the temperature of the object increases. Understanding the

exact form of the spectrum of thermal radiation wavelengths requires concepts from

quantum physics, and its derivation was one of the early triumphs and verifications of

quantum theory.

Bodies with temperatures near the temperature of the surface of the Earth emit mostly

infrared radiation. In the photograph in Concept 2, called an infrared thermograph, you

see the radiation emitted by a horse. Since areas of inflammation in the body are

unusually warm, and emit extra thermal radiation, veterinarians can use photographs

like this to diagnose an animal’s ailments. They are created by a digital or film camera

that assigns different (visible) colors to different intensities of (invisible) infrared

radiation in a process called false color reproduction.

Sunlight is a form of radiation and is crucial to life on Earth. The Sun emits massive

amounts of energy in the form of radiation: 3.9×10^26 joules every second. Some of that

strikes the Earth, where it warms the planet and supplies the energy that plants use in

photosynthesis.

The amount of power radiated by a body is proportional to the fourth power of its

absolute temperature, its surface area, and a factor called its emissivity. The Sun emits

tremendous amounts of radiation energy because it is quite hot (about 6000 K) and vast

(with a surface area of about 6×10^18 m^2 ).Only a small portion of the total power emitted

by the Sun reaches the Earth. Even this fraction is an enormous amount: 1.8×10^17

watts, about 100 times what human civilization consumes. The average solar power

striking the Earth’s atmosphere in regions directly facing the Sun is about 1370 watts

per square meter. This value is called the solar constant.

Not all parts of the Earth directly face the Sun, and some radiation is reflected or

scattered by the Earth’s atmosphere before it reaches the surface. Different regions on

Earth receive different average amounts of power per square meter: Measurements

show that on average about 240 W/m^2 (watts per square meter) reaches the Earth's

surface. North America is estimated to receive radiation of 150 W/m^2 , on the average.

Given the dimensions of an average American house, this amount of radiation supplies

about four times as much power as the household consumes. Using this clean

“renewable resource” constructively for human purposes challenges both engineers and

physicists. It also provides a significant opportunity to conserve non-renewable energy

resources that would otherwise be used to heat, cool, and light our houses.

Efforts have been underway to take advantage of this form of energy for years.

Photovoltaic cells convert solar energy directly into electricity.

The energy in sunlight can also be used without transforming it into electricity. The heat

of sunlight can be used to heat both water and the interior of a house. Devices called

solar collectors are becoming increasingly common. They are typically black; you see

one in Concept 4. Solar collectors heat water for uses ranging from hot showers to

warm swimming pools. Typically, the collectors contain tubes or rods that hold water,

and after it is heated the water is circulated to the desired locations in the building.

Current solar collectors come in a variety of shapes and sizes. The engineers designing them have two main challenges. One is to maximize

Radiation

Heat transfer by electromagnetic

radiation

Radiation

All objects emit radiant energy

Sun and Earth

Sun emits enormous amounts of radiant

energy

Some strikes the Earth’s atmosphere

Solar collector

Uses solar energy that reaches the

Earth

(^352) Copyright 2007 Kinetic Books Co. Chapter 18