Radiation - Thermal Properties of Matter, Class 11, Physics PDF Download

RADIATION

6. RADIATION

The process of the transfer of heat from one place to another place without heating the intervening medium is called radiation. The term radiation used here is another word for electromagnetic waves. These waves are formed due to the superposition of electric and magnetic fields perpendicular to each other and carry energy.

Propoerties of Radiation :

(a) All objects emit radiations simply because their temperature is above alsolute zero, and all objects absorb some of the radiation that falls on them from other objects.

(b) Maxwell on the basis of his electromagnetic theory proved that all radiations are electromagnetic waves and their sources are vibrations of charged particles in atoms and molecules.

(c) More radiations are emitted at higher temperature of a body and lesser at lower temperature.

(d) The wavelength corresponding to maximum emission of radiations shifts from longer wavelength to shorter wavelength as the temperature increases. Due to this the colour of a body appears to be changing. Radiations from a body at NTP has predominantly infrared waves.

(e) Thermal radiations travels with the speed of light and move in a straight line.

(f) Radiations are electromagnetic waves and can also travel through vacuum.

(g) Similar to light, thermal radiations can be reflected, refracted, diffracted and polarized.

(h) Radiation from a point source obeys inverse square law (intensity a )

6.1. PREVOST THEORY OF EXCHANGE

According to this theory, all bodies radiate thermal radiation at all temperatures. The amount of thermal radiation radiated per unit time depends on the nature of the emitting surface, its area and its temperature. The rate is faster at higher temperatures. Besides, a body also absorbs part of the thermal radiation emitted by the surrounding bodies when this radiation falls on it. If a body radiates more then what it absorbs, its temperature falls. If a body radiates less than what it absorbs, its temperature rises. And if the temperature of a body is equal to temperature of its surroundings it radiates at the same rate as it absorbs.

6.2 PERFECTLY BLACK BODY AND BLACK BODY RADIATION (FERY'S BLACK BODY)

A perfectly black body is one which absorbs all the heat radiations of whatever wavelength, incident on it. It neither reflects nor transmits any of the incident radiation and therefore appears black whatever be the colour of the incident radiation.

In actual practice, no natural object possesses strictly the properties of a perfectly black body. But the lamp-black and platinum black are good approximation of black body. They absorb about 99% of the incident radiation. The most simple and commonly used black body was designed by Fery. It consists of an enclosure with a small opening which is painted black from inside. The opeining acts as a perfect black body. Any radiation that falls on the opening goes inside and has very little chance of escaping the enclosure before getting absorbed through multiple reflections. The cone opposite to the opening ensures that no radiation is reflected back directly.

6.3 ABSORPTION, REFLECTION AND EMISSION OF RADIATIONS

(a) Absorptive power :

In particular absorptive power of a body can be defined as the fraction of incident radiation that is absorbed by the body.

a =

As all the raditions incident on a black body are absorbed, a = 1 for a black body.

(b) Emissive power :

Consider a small area DA of a body emitting thermal radiation. Consider a small solid angle Dw about the normal to the radiating surface. Let the energy radiated by the area DA of the surface in the solid angle Dw in time Dt be DU. We define emissive power of the body as

Thus, emissive power denotes the energy radiated per unit area per unit time per unit solid angle along the normal to the area.

(c) Spectral Emissive power (E_l) :

Emissive power per unit wavelength range at wavelength l is known as spectral emissive power, E_l. If E is the total emissive power and E_l is spectral emissive power, they are related as follows,

E = and

(d) Emissivity :

e = =

7. STEFAN-BOLITZMANN'S LAW :

Consider a hot body at temperature T placed in an environment at a lower temperature T₀. The body emits more radiation than it absorbs and cools down while the surroundings absorb radiation from the body and warm up. The body is losing energy by emitting radiations and this rate.

and is receiving energy by absorbing radiations and this absorption rate

=

Here, 'a' is a pure number between 0 and 1 indicating the relative ability of the surface to absorbs radiation from its surroundings. Note that this 'a' is different from the absorptive power 'a'. In thermal equilibrium, both the body and the surrounding have the same temperture (say T_c) and,

P₁ = P₂

or

or e = a

Thus, when T > T₀, the net rate of heat transfer from the body to the surroundings is,

Net heat loss =

or ⇒ Rate of cooling

or