Transmission of Heat, Light Electromagnetic Radiation | Science & Technology for UPSC CSE PDF Download

Transmission of Heat
There are three modes of transmission of heat— conduction, convection and radiation.

Conduction: Conduction is the transference of heat from particle to particle without the particles themselves moving. It takes place mostly in solids. Metals are good conductors of heat and nonmetals are poor conductors. Liquids and gases are poor conductors. Poor conductors are also called insulators. Air is a poor conductor. Copper is used in the construction of boilers because, among other things it is a good conductor and therefore heats up the contents inside quickly. Wool and cotton owe their insulating properties chiefly to the air spaces in them. Double walled glass windows with air between them serve as better non-conductors than windows with single thick glasses and are used in air-conditioned rooms, overs, geysers etc. Eskimos are able to live in snow huts because snow is a poor conductor. Thus they are protected from the extreme cold outside.

Convection of heat: Transference of heat takes place in liquids and gases mostly by convection. This is the process by which transference of heat from one part to another takes place by the moving heated particles carrying the heat to the rest of the particles. In the process convection currents are set up and soon the whole of the substance—liquid or gas—is heated to the same temperature. The warmer particles move up and the cooler, heavier particles move into their place and this in the process the entire thing gets heated up. In heating water in kettles and boilers there is circulation of water due to convection currents. Convection currents are responsible for winds, land and sea breeze, ocean currents etc. 

Radiation: We receive heat from the sun by radiation. Most of the space between the sun and the earth is vacuum. Heat from the sun cannot reach us by conduction or convection, since both these processes require presence of material medium.

Radiation is a process of transfer of heat in which the material medium (a medium consisting of matter) is not necessary. The heat transferred from one body to another by this process is called radiant heat or thermal radiation.White or light coloured clothes are more suitable in summer because they absorb very little of the sun’s heat and keep our bodies cool. Black or dark coloured clothes are suitable in winter because they absorb most of the radiant heat of the sun and keep us warm. Electric radiators (room heaters) are provided with polished reflectors. They absorb very little heat and reflect most of it.

Black Body: Anideal black body is a theoretical object that absorbs all the radiant energy falling upon it and emits it in the form of thermal radiation. The power radiated by a unit area of a black body is given by Planck’s Radiation Law, and the total power radiated is expressed by Stefan-Boltzmann Law.

Thermos flask: The thermos flask is a special kind of bottle or flask which is made in such a way that it keeps cold things cold and hot things hot for a long time.
The loss of heat due to conduction, convection and radiation are reduced.

Greenhouse: Very hot bodies such as the sun emit most of their heat radiation in the form of visible light and short wavelength infra-red rays which easily pass through glass without being absorbed. These rays are absorbed by the earth and objects inside the greenhouse which, in turn, raise the temperature of air by conduction and convection. The objects inside also radiate heat, but, owing to their comparatively low temperature the infrared rays they emit are of longer wavelength and cannot penetrate the glass.

It deals with the measurement of the amount of vapour pressure present in the atmosphere.
Hygrometry 

Relative Humidity. It is the ratio of mass of water vapour in a given volume of air to the mass required to saturate it at the same temperature.

Relative humidity is expressed in percentage,Relative Humidity

= {Pressure of water vapour present}/{Saturated vapour pressure at the same temp.} * 100

Dew point. It is the temperature at which dew begins to form. It is defined as the temperature at which maximum pressure of water vapour is equal to the pressure of water vapour actually present in the atmosphere at the ordinary temperature.

Hygrometers. These are the instruments which are used for measurement of relative humidity of air. They are:

1. Chemical Hygrometer

2. Wet and Dry Bulb Hygrometer Dew-point Hygrometers are

(i) Daniell’s Hygrometer

(ii) Regnault’s Hygrometer

Heat Engine

Specific Heat of Gases. Specific heat at constant volume is defined as the amount of heat required to raise the temperature of unit mass of gas through 1°C, the volume remaining constant.

Laws of Thermodynamics

1st Law. Whenever work is converted into heat or vice versa, the ratio of two is constant. By this law whenever heat is converted into mechanical work or any other form of energy and vice versa the quantity of energy which disappears in one form is always equal to energy which appears in other form.

2nd Law. It states that heat can be transferred from one body to another at higher temperature unless some work is done.

Heat Engines. It is a device by means of which heat energy is converted into mechanical energy.

Refrigerator. Refrigeration principle is the application of the effect of cold caused by evaporation of a liquid. Commonly used refrigerants are ammonia, freon, carbon dioxide and methyl chloride. The criteria of a refrigerant are:

1. It should have a large heat of evaporation.
2. Its vapours should have a small specific volume.
3. It should be non-inflammable and non-toxic. It should not have a chemical reaction with the machinery involved.
4. It should have sufficient vapour pressure at the temperature of the evaporator so that the atmospheric air may not be sucked in through any possible leakage.
5. It should be easily compressed and cooled in the condenser.

Types of Refrigeration. There are two main types of refrigeration which differ in the nature of supply of energy.

1. Compression Type. Working energy in this type is supplied in the form of mechanical energy by a compressor.
2. Absorption Type. In the form working energy is supplied in the form of heat energy by burning of fuel such as coal gas etc.

Compression Type Refrigerator (Ice Plant). It consists of five main parts.

1. Compressor
2. Condenser
3. Reservoir
4. Regulating value
5. Evaporator.

The refrigerant liquified by pressure, is allowed to evaporate through coils surrounded by brins. The heat required for this evaporation of refrigerant which is ammonia in this case, is taken from the brine. As result of this, brine is so much cooled that the water kept in brine solution cans gets freezed.

Solar Cooker. A simple solar cooker is a box made of insulating material like wood, card-board, etc. The box has a glass cover to retain heat in side by the green house effect. The inside of the box is painted dull black to increase heat absorption. The cooking vessel is kept in side the box which, in turn, is kept in the sun. Generally, this type of cooker is used only for warming food but can sometimes be used for cooking rice, pulses, etc.

Thermos Flask. A thermos flask is double walled with a vacuum between the walls. The two inner glass surfaces facing each other are silvered. Silvered surfaces walls prevent heat loss by radiation. The vacuum also prevents heat loss by convection.

Pressure Cooker. The boiling point of a liquid depends on external pressure. When the atmospheric pressure is 76 cm. of mercury, water boils at 100°C. But when the pressure is increased, the boiling point of water is raised up. In a pressure cooker, water boils at temperatures higher than 100° C due to increased pressure. The increased boiling temperature allows water to hold more heat which cooks food faster. At higher altitude, atmospheric pressure is reduced. This lowers the boiling point of water and food takes much longer to cook. Thus, a pressure cooker becomes more essential for cooking on hill stations.

Air-conditioning. Bodily comfort depends on temperature as well as on humidity. The comfortable conditions for an average person are

(a) temperature between 23° and 25°C and

(b) relative humidity between 60 and 65 per cent.

An airconditioner provides these conditions by regulating temperature and humidity.

Light Electromagnetic Radiation

Electromagnetic waves include an enormous range of frequencies—from radio waves with frequencies less than 10⁵ Hz to gamma rays having frequencies greater than 10²⁰ Hz. Visible light is simply electromagnetic radiation in the range of frequencies 4.3 x 10¹⁴ to 7 x 10¹⁴ Hz. The descriptive names of various sections of the spectrum are merely historical classification, otherwise all the waves, from radio waves to gamma rays are same in nature, differing only in frequency and wavelength.

All have the same speed (c = 3 x 10⁸ m/s) in vacuum. The relation, speed = frequency × wavelength holds good for all electromagnetic waves. If the frequency of radio waves sent out by a radio station is known, the wavelength can be calculated by dividing 3 x 10⁸ m/s by the given frequency. For example, the wavelength of radio waves sent out by a radio station at frequency 819 kHz is 336m.

Radio and Television Transmission

Radio waves sent out by radio stations are reflected by the ionosphere (a part of the atmosphere which extends from 60 to 500 km above the earth) and can be received anywhere on the earth. Due to slight absorption in the ionosphere, the radio signals received at far off places are rather weak. At night the radio reception improves because the layers of the ionosphere are not exposed to sunlight and are more settled. High-frequency waves carrying television signals penetrate the ionosphere and are not received like radio signals. Television transmission is therefore accomplished on a “line-of-sight” basis. This explains why television programmes can only be received over a limited range. The curvature of the earth limits the range of television reception. However, geostationary satellites are being used these days for television (and telephone) links between places anywhere on the earth.

Radar

Radar (Radio detection and ranging) employs highfrequency radio waves for detecting objects like ships and aeroplanes. A rotating aerial sends out pulses which are reflected from the objects on which they fall. The time interval between the transmission and receptionof pulses helps determine the distance of the object. A picture of the scanned area is produced on the screen of a special cathode ray tube.