Here is a compilation of essays on ‘Thermal Radiation’ for class 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Thermal Radiation’ especially written for school and college students.
Essay on Thermal Radiation
Essay Contents:
- Essay on the Meaning of Thermal Radiation
- Essay on the Salient Features and Characteristics of Radiation
- Essay on Absorptivity, Reflectivity and Transmissivity of a Material
- Essay on the Spectral and Spatial Energy Distribution of Radiation
- Essay on Gray Body and Selective Radiation Emitters
- Essay on Solar Radiations and its Effect
Essay # 1. Meaning of Radiation:
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Thermal radiation is the transmission of thermal energy without any physical contact between the bodies involved. Unlike heat transfer by conduction and convection, transport of thermal radiation does not necessarily affect the material medium between the heat source and the receiver.
An intervening medium is not even necessary and the radiation can be affected through vacuum or a space devoid of any matter. Radiation exchange, in fact, occurs most effectively in vacuum. A material present between the heat source and the receiver would either reduce or eliminate entirely the propagation of radiant energy.
Energy released by a radiating surface is not continuous but is in the form of successive and separate (discrete) packets or quanta of energy called photons. The photons are propagated through space as rays; the movement of swarm of photons is described as the electromagnetic waves.
The photons (as carriers of energy) travel with unchanged frequency in straight paths and with speed equal to that of light. For propagation in vacuum c = 3 × 108 m/s. When the photons approach the receiving surface, there occurs reconversion of wave motion into thermal energy which is partly absorbed, reflected or transmitted through the receiving surface. The magnitude of each fraction depends upon the nature of the surface that receives the thermal radiation.
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Essay # 2. Salient Features and Characteristics of Radiation:
Some salient features and characteristics of thermal radiation are enumerated below:
(i) The electromagnetic waves are emitted as a result of vibrational and rotational movements of the molecular, atomic or sub atomic particles comprising the matter. The emission occurs when the body is excited by an oscillating electrical signal, electronic or neutronic bombardment, chemical reactions etc. The emission of thermal radiations is associated with thermally excited conditions which depend upon the nature of surface and its absolute temperature.
(ii) The distinction between one form of radiation and another lies only in its frequency and wavelength which are related by-
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c (speed of light) = λ (wavelength) × f (frequency)
Consequently longer wavelengths correspond to lower frequencies and a shorter wavelengths to higher frequencies. Again, a high temperature body will have a high frequency quantum and so shorter wavelengths.
(iii) The general phenomenon of radiation covers the propagation of electromagnetic waves of all the wavelengths, from short wavelength gamma rays, X-rays and ultraviolet radiation to the long wavelength microwaves and radio waves. Thermal radiation is limited to range of wavelength between 0.1 and 100 µm; it thus includes the entire visible and infrared, and a part of the ultraviolet spectrum.
The sun with an effective surface temperature of 5600°C emits most of its radiations at the extreme lower end of the spectrum 0.1 to 4 µm. The radiations from a lamp filament are in the frequency range 1 to 10 µm. Most solids and liquids have a continuous spectrum; they emit radiations of all the wavelengths.
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Gases and vapours radiate energy only at certain bands of wavelength and hence are called the selective emitters. The emission of thermal radiation depends upon the nature, temperature and state of the emitting surface. However with gases the dependence is also upon the thickness of emitting layer and the gas pressure.
(iv) Thermal radiations exhibit characteristics similar to those of visible light, and follow optical laws. They can be reflected, refracted, and are subject to scattering and absorption when they pass through a media. They get polarised and weakened in strength with the inverse square of radial distance from the radiating surface.
(v) All matter emits radiant energy and bodies at high temperature emit at a greater rate than bodies at low temperature. Normally a body radiating heat is simultaneously receiving heat from other bodies as incident radiation.
The net exchange of heat between two radiating surfaces is due to the fact that one at higher temperature radiates more and receives less energy for its absorption. An isolated body which remains at constant temperature emits just as much energy by radiation as it receives. The entire system is then in a state of mobile thermal equilibrium.
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(vi) Heat transfer by conduction and convection depends primarily on the temperature difference gradient and is little affected by temperature level. With other factors (k, h and A) remaining constant, the heat transfer due to conduction and convection from a hot source at 1000°C to the surroundings at 200°C would practically remain same if the hot source and the surroundings take up the temperature values as 900°C and 100°C respectively. However, even with the same temperature difference, the heat exchange by radiation gets enhanced at elevated temperature of the source and the surroundings.
The most vivid evidence of radiation transfer is that represented by the solar energy which passes through interstellar space (conditions close to that for perfect vacuum) on its way to the earth surface. Solar radiation plays an important part in the design of heating and ventilating systems.
Heat transfer by radiation is encountered in boiler furnaces, billet reheating furnaces and other types of heat exchangers. The design and construction of engines, gas turbines, nuclear reactors and solar collectors is also significantly influenced by the radiation heat transfer.
Essay # 3. Absorptivity, Reflectivity and Transmissivity of a Material:
The total radiant energy (Q0) impinging upon a body would be partially or totally absorbed by it (Qa), reflected from its surface (Qr), or transmitted through it (Qt) in accordance with the characteristics of the body. By the conservation of energy principle, the total sum must be equal to the incident radiation. That is-
Absorptivity, Reflectivity and Transmissivity
Where,
α = absorptivity or fraction of total energy absorbed by the body.
ρ = reflectivity or fraction of total energy reflected from the body.
τ = transmissivity or fraction of total energy transmitted through the body.
The factors a, p and x are dimensionless and vary from 0 to 1. The value depends upon the nature of the surface of the body, its temperature and wavelength of incident rays. The response of the body to incident radiations is, however, completely independent of and unaffected by the simultaneous emission from the body.
Black surfaces are effective absorbers of radiation in the wavelengths that are encountered in heat transfer. Accordingly the name black body is assigned to a perfect absorber of radiation. The thermal radiations impinging upon a black body are totally absorbed by it; the radiations are neither reflected from the surface nor transmitted through it.
For a black body α = 1 and ρ = τ = 0. Incidentally this implies that a black body is a perfectly non-reflecting and non-transmitting surface. Snow, with its absorptivity 0.985, is nearly black to thermal radiations. The absorptivity of surfaces can be increased to 90-95% by coating their surfaces with lamp black or a dark rough paint. In actual practice there does not exist a perfectly black body which will absorb all the incident radiations.
The absorptivity of a surface depends upon the direction of incident radiation, temperature of the surface, composition and structure of the irradiated surface and the spectral distribution of incident radiation. When a surface absorbs a certain fixed percentage of impinging radiations, the surface is called the gray body.
The absorptivity of a gray body is necessarily below unity, but it remains constant over the entire range of temperature and wavelength of incident radiation. This condition of constant absorptivity too is not satisfied by the real materials and as such even a gray body remains a hypothetical concept like the black body.
A body that reflects all the incident thermal radiations is called a specular body (if the reflection is regular) or an absolutely white body (if the reflection is diffused). For such bodies ρ = 1, and α = τ = 0. The specular and diffused type of reflections have been indicated in Fig. 7.2.
Regular (specular) reflection implies that angle between the reflected beam and the normal to the surface equals the angle made by the incident radiation with the same normal. Reflection from highly polished and smooth surfaces approaches specular characteristics. In a diffused reflection, the incident beam is reflected in all directions, i.e., there is directional independence of the reflected beam.
Most of the engineering materials have rough surfaces, and these rough surfaces give diffused reflections. Diffused reflection is sometimes likened to the situation in which incident energy is absorbed near the surface and then re-emitted.
A body that allows all the incident radiations to pass through it is called transparent or diathermanous. For such bodies τ = 1, and α = ρ = 0. Transmissivity varies with wavelength of incident radiation. A material may be non-transparent for a certain wavelength band and transparent for another. A thin glass plate transmits most of the thermal radiations from the sun, but absorbs in equally great measure the thermal radiations emitted from the low temperature interior of a building.
The absorption of a radiation is a surface phenomenon; it occurs in a very thin layer (approx. 1 µm thick) of material near the surface. Since most of the solids and liquids encountered in engineering are thick enough to cover this layer, they can be considered non-transparent (opaque, diathermanous) to thermal radiations. Exceptions are few solid substances like glass, quartz, rock salt and most liquids in the visible and near infrared range. For opaque bodies τ = 0 and so α + ρ = 1.
This result does suggest that good absorbs are bad reflectors and vice versa. However gases have relatively high transmissivity, they transmit an appreciable portion of radiation even in layers of fairly large thickness. Further, the gases are known to reflect very little of the radiation impinging on their interface. Therefore, for gases reflectivity can be neglected and so α + τ = 1. In a transparent medium, the absorption occurs throughout the material.
Consider a large hollow sphere or cylinder provided with only one small opening and let it be maintained at a uniform temperature. The inner surface of the cylinder is coated with lamp black which absorbs about 95% of the incident radiation. A beam of thermal radiation entering the hole strikes the inner surface.
Since the surface has a high absorptivity, the major portion of the radiation is absorbed and only a small fraction is reflected. The weak reflected ray does not find any way out and again strikes the inner surface. Here it is again partly absorbed and partly reflected. Likewise the reflected radiation is successively absorbed and finally when it escapes out, it has only a negligible amount of energy associated with it.
Let Q represent the radiant energy that enters the hole. This radiation gets reduced to (1 – α) Q = ρQ after first internal reflection, ρ2Q after second internal reflection, ……………….. ρnQ after the nth reflection and this approaches zero at n → ∞. The few rays that emerge from the hole will have suffered many reflections; the emergent flux then becomes essentially zero and that give absorptivity α = 1 for the hole.
A small hole leading into a cavity (Hohlraum) thus acts very nearly as a black body because all the radiant energy entering through it gets absorbed. The smaller the opening, better the approximation to black body behaviour. For most experimentation, a hole of 2.5 cm diameter in the end of a hollow cylinder 25 cm long and 7.5 cm diameter would suffice. Isothermal furnaces, with small apertures, approximate a black body and are frequently used to calibrate heat flux gauges, thermometers and other radiometric devices.
Essay # 4. Spectral and Spatial Energy Distribution of Radiation:
The distribution of radiant energy is non-uniform with respect to both wavelength and direction.
(i) Spectral Distribution:
The radiation emitted by a surface consists of electromagnetic waves of various wavelengths, and the term spectral refers to the variation in thermal radiations with wavelength. Magnitude of the radiation at any wavelength (monochromatic) and the spectral distribution are found to vary with the nature and temperature of the emitting surface.
(ii) Spatial or Directional Distribution:
A surface element emits radiation in all directions; the intensity of radiation is however different in different directions. The surface may emit preferentially in certain directions creating a directional distribution of the emitting radiations.
Example 1:
What happens to the radiant energy when it is incident upon a surface? Give the general equation that relates absorptivity, reflectivity and transmissivity.
Of the radiant energy 350 W/m2 incident upon a surface 250 W/m2 is absorbed, 60 W/m2 is reflected and the remainder is transmitted through the surface. Workout the values for absorptivity, reflectivity and transmissivity for the surface material.
Solution:
Absorptivity α = fractional of total energy absorbed by the surface-
Essay # 5. Gray Body and Selective Radiation Emitters:
Consider two bodies, one absolutely black and the other non-black and let these be at the same temperature. The monochromatic emissive power of a non-black body varies significantly from the black body monochromatic emissive power as illustrated in Fig. 7.8.
These curves indicate that a non-black body radiates less intensively than a black body. Further the radiation spectrum for a non-black body may be similar or radically different from that of a black body.
When the emissivity of non-black surface is constant at all temperatures and through-out the entire range of wavelength, the surface is called a gray body. The radiation spectrum for a gray body, though reduced in vertical scale, is continuous and identical to the corresponding curve for a perfectly black surface; there is no shift in the peak of the curves.
However for many materials the emissivity is different for the various wavelengths of the emitted energy. The radiating bodies exhibiting this behaviour are called selective emitters. The radiation spectrum for a selective emitter does not follow any definite pattern, and it varies entirely from that of a black body.
Stefan-Boltzman law when applied to a gray body takes the form-
E = σT4 … (7.16)
The constant o is different for different bodies and its value depends upon the nature of the body, the state of its surface and temperature. It is always less than the radiation coefficient σb for a black body; its value ranges from 0.0 to 5.67 × 10-8 W/m2 K4.
The emissivity of the gray surface may be expressed as:
Values of emissivities range from 0.0 to 1.0.
The emitted radiant energy flux density for non-black body, as prescribed by equation 7.16 may be rewritten as-
E = ϵ σb T4 W/m2 … (7.18)
Emissivities of Real Bodies:
Emissivity of a surface indicates its ability to emit radiation energy in comparison with a black surface at the same temperature level; it represents the ratio of the emissive power of the surface to the emissive power of black surface at the same temperature.
Based upon the direction and totality of emissive power we have:
(i) Monochromatic Emissivity ϵλ:
Ratio of the monochromatic emissive power of a surface to the monochromatic emissive power of a black surface at the same wavelength and temperature. For a gray body the monochromatic emissivity is independent of the wavelength of the emitted radiation, i.e., the monochromatic emissive power of the surface and the monochromatic emissive power of a black surface have the same ratio for all wavelength of emitted radiation at the same temperature.
(ii) Total Emissivity ϵ:
Ratio of the total emissive power of a surface to the total emissive power of a black surface at the same temperature.
(iii) Normal Total Emissivity ϵn:
Ratio of the normal component of the total emissive power of a surface to the normal component of the total emissive of a black body at the same temperature.
(iv) Mean and Equilibrium Emissivity:
Emissivity of most of the engineering materials is influenced by temperature as well as wavelength. For a particular temperature, the average of monochromatic emissivity at various wavelengths (within a range of wavelength) is called the wavelength-mean emissivity. Like-wise for a prescribed wavelength, average value of all the monochromatic emissivity at various temperatures (within a range of temperature) is called the temperature-mean emissivity.
The emissivity of a material though varying with temperature and the nature of its surface is not affected in any way by the nature of surface surrounding it. The total emissivity remains constant whether the material is in equilibrium with the surroundings or not. The total emissivity is, therefore, sometimes called the equilibrium emissivity.
In general, the emissivity of a material is dependent upon its nature (colour, texture and roughness), temperature, wavelength of radiations, and angle of emission and the nature of the surface which is influenced by the method of fabrication, thermal cycling and chemical reaction with environment. Emissivity of a metallic surface increases with temperature growth, surface roughness and with the formation of a film of impurities and a thin layer of oxide.
Emissivity of a highly polished surface is quite low. For non-metallic surface emissivity values generally decrease with temperature rise. Non-conductors have a comparatively large (generally exceeding 0.6) emissivity. Depending upon specific material, the emissivity of a non-conductor may either increase or decrease with temperature rise?
Example 2:
What physical ratio determines whether a real surface is an almost specular reflector or an almost diffuse reflector?
Define emissivity. How does it vary with temperature for conductors and nonconductors?
The radiant heat transfer from a plate of 2.5 cm2 area at 1250 K to a very cold enclosure is 5.0 W. Determine the emissivity of the plate at this temperature.
Solution:
If the roughness dimension for a real surface is large with respect to wavelength of incident radiation the surface behaves as a diffuse reflector. If the roughness dimension is considerably smaller than the wavelength, the surface reflects secularly.
A real surface has a total emissive power E less than that of a black surface Eb. The ratio of the total emissive power of a surface to that of a black surface at the same temperature is called the total emissivity; e = E/ Eh.
Some common observations about the emissivity of a body are:
(i) The emissivity of the metallic surfaces is very small having the values as low as 0.02 for highly polished gold and silver.
(ii) The presence of oxide layers generally improves the emissivity of metallic surfaces.
(iii) The non-conductors have large value of emissivity, generally exceeding 0.6.
(iv) The emissivity of a conducting material increases with increase in temperature, but emissivity of non-conducting materials decreases with increase in temperature-
Essay # 6. Solar Radiations and its Effect:
The sun is a source of heat radiations and it emits radiations in all directions. The atmosphere absorbs a part of the heat radiations and air, clouds, dust particles etc. in the atmosphere scatter the heat and light radiations falling on them. Obviously, the earth receives only a fraction of the energy emitted by the sun. The solar radiation that is felt at the earth’s surface includes direct radiation that has passed through the atmosphere; diffuse radiation from the sky; reflected radiation from water, snow and other such materials on the surface.
The approximate distribution of the flow of sun’s energy to the earth’s surface is:
i. 9% is scattered
ii. 15% is absorbed in the atmosphere and out of it 4% reaches the earth’s surface by convection
iii. 43% is transmitted to the earth directly and by diffuse radiation
iv. 33% is reflected back to space
When the sum lies at a mean distance from the earth, the heat flux from the sun to the outer edge of the atmosphere has been found to be about 1350 W/m2 and 47% of this (or 635 W/m2) would reach the earth’s surface. A large fraction of the sun’s energy reaches the earth’s surface in ultra-violet and visible wavelength; and re-radiation from the cool surface of the earth would be in wavelength that are generally far larger.
Green House Effect:
Much of the solar radiation is transmitted through the glass or plastic covering and absorbed by the objects within the enclosure. As their temperature rises, they too radiate energy which, however, is mostly in the higher wavelength band to which the glass or plastic is opaque. Most of the thermal radiation emitted at low temperature is reflected back and remains inside.
Because of this one-way action of heat exchange of the glass or plastic, the temperature within the enclosure becomes considerably higher than the ambient temperature outside. The phenomenon is commonly referred to as the “green-house effect”, and obviously it is a manifestation of transmission of low wavelength energy (from the sun) and absorption or reflection of higher wavelength emission at low temperature.
Selective absorbing surfaces have been developed for use in solar collectors; these surfaces absorb much of the incident radiation without reradiating it.
The sky creates a partial green-house effect if it is heavily loaded with CO2, H2O and to a lesser extent, ozone.
Solar Constant:
From the quantity of heat radiations received by the earth, it is possible to estimate the temperature of the sun. For that certain ideal conditions are taken into consideration and a parameter called solar constant is introduced.
Solar constant is the amount of heat energy (radiation) absorbed per unit time by unit area of a perfectly black body surface placed at a mean distance of the earth from the sun, in the absence of the atmosphere; the surface being held perpendicular to the sun’s rays.
The heat energy absorbed by a known area in a fixed time is determined with the help of an instrument called pyrheliometer. The effects of absorption by the atmosphere are eliminated by finding the value of the solar constant at various altitudes of the sun on the same day under similar sky conditions.
The observed solar constant S0, the true solar constant S and the angular elevation (altitude) Z of the sun are related by the expression,
S0 = S asecZ (a is a constant)
Or log S0 = log S + sec Z log a
From the straight line graph between log S0 along the y-axis and sec Z along the x-axis, the intercept log S on the y-axis is found and the true solar constant is evaluated there from. The solar constant varies between 1335 and 1815 W/m2.
Let R = mean distance of earth from the sun
r = radius of the sun
Then the total amount of heat energy received by the sphere of radius R is 4πR2S, and the amount of heat energy radiated by unit surface area of the sun in the same time works out as-
This value represents the effective temperature of the sun acting as a black body.
Temperature of the sun can also be worked out from Wien’s displacement law:
λmax T = 2.89 × 10-3 mK
The wavelength of radiation for which the energy is maximum is 0.49 micron. The temperature of the sun then works out as-
T = 2.898 × 10-3/0.49 × 10-6 = 5914 K
The sun consists of a central hot portion surrounded by the photosphere. The temperature of the photosphere, referred to as effective temperature of the sun, is usually taken as 6000 K.