Science > Physics > Heat Transfer > Convection and Radiation

In this article, we shall study different modes of heat transfer.

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Convection:

Convection is a mode of heat transfer through amaterial medium in which heat energy is carried from one place to another byactual motion “migration” of heated matter.

Consider a beaker containing water small quantity ofsawdust in added to this water. The beaker is then heated it was foundthat the sawdust particles start moving from bottom to top and then from top tobottom in a circular way.


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The equatorial andpolar regions ofthe earth receive unequal solar heat. Air attheearth’s surface near the equator is hot whilethe air in theupper atmosphere of the poles iscool. Aconvection current would beset up, with theair at the equatorial surface rising and movingouttowards the poles, descending andstreaming in towards the equator.

Due to the rotationof the earth, modifies the direction of a convectioncurrent. Because of the rotation of the earth air close to the equatorhas an eastward speed of 1600 km/h, while itis zero close to the poles. As a result, the airdescends not at the poles but at 30° N (North)latitude and returns to the equator. This flow of wind iscalledthe trade wind.

Other Examples:


The exhaust fans, ventilators are always kept at the top portion of the wall.The freezer region is the topmost portion in the freeze

Radiation:

Radiation is a process of transfer of heat in the formof electromagnetic waves for which material medium is not necessary. Thethermal energy which is transferred by radiation is called radiant heat orradiant heat or simply radiations

Characteristics of Radiation:

In the process of radiation thermal energy or heat energy is transferred from one point to other in the form of electromagnetic waves.As radiation is due to electromagnetic waves and electromagnetic waves are capable of passing through a vacuum, there is no necessity of material medium for radiation.Due to the electromagnetic nature of radiation has the same properties as that of light, such as rectilinear propagation, reflection, refraction, interference etc.The velocity of radiant energy in air or vacuum is the same as that of light in vacuum i.e 3 × 108 m/s. Due to this high-speed radiation is the most rapid process of heat transfer.When radiant heat is incident on a matter, it is partly absorbed and converted into heat.Radiationshave a wavelength greater than that of red colour and thus radiation form infrared region of the electromagnetic spectrum.

Diathermanous Substances:

The substances which can transmit the radiant heatincident upon their surfaces are called diathermanous substances. e.g. glass,quartz, gases

Adiathermanous (Athermanous)Substances:


The substances which cannot transmit the radiant heatincident upon their surfaces are called adiathermanous (athermanous)substances. e.g. wood, iron copper etc.

Perfectly Black Body:

A body which absorbs all the radiant heat incidentupon it is called a perfectly black body.

No body exists in nature, which can be called aperfectly black body. For practical purposes, lamp black which absorbs nearly98 % of the heat incident upon it is considered as a perfect black body.

Characteristics of Perfectly Black Body:A perfectly black body which absorbs all the radiant heat incident upon.For a perfectly black body the coefficient of absorption is equal to 1.The blackness of such a body is due to the fact that it does not reflect or transmit any part of heat incident upon it. Thus the coefficient of reflection and coefficient of transmission are zero.

Applications of Radiations:

Black bodies absorb and emitradiant energy better than bodies of lightercolours. We wear white or light colouredclothes in summer so that they absorb the leastheat from the sun. However, during winter, we use dark coloured clothes which absorb heatfrom the sun and keep our body warm.Thebottoms of the utensils for cooking food areblackened so that they absorb maximum heatfrom the fire and give it to the vegetables to becooked.A Dewar flask or thermos bottle isa device to minimise heat transfer between thecontents of the bottle and outside. It consistsof a double-walled glass vessel with the innerand outer walls coated with silver. Radiation from the inner wall is reflected back into thecontents of the bottle. The outer wall similarlyreflects back any incoming radiation. The spacebetween the walls is evacuated to reduceconduction and convection losses and the flaskis supported on an insulator like a cork. Thedevice is, therefore, useful for preventing hotcontents (like milk) from getting cold, oralternatively to store cold contents (like ice).
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Newton’s Law ofCooling:

The rate of loss of heat by a body is directlyproportional to its excess temperature over that of the surroundings providedthat this excess is small.


Explanation:

Let θ and θo, be the temperature of a bodyand its surroundings respectively. Let dQ / dt be the rate of loss of heat. Sofrom Newton’s Law of Cooling,


where k is aconstant.

Thus Newton’s law of cooling statesthat the rateof loss of heat by cooling body is directly proportional to its excess oftemperature over the surrounding, provided this excess is very small.

The alternate statement of the law is that the rate offall of temperature of a cooling body is directly proportional to its excess oftemperature over the surrounding, provided this excess is very small.

dθ/dt∝(θ – θo)


Limitations of Newton’sLaw of Cooling:This law is applicable when the excess temperature of a body over the surroundings is very small (about 40 °C)When the body is cooling the temperature of the surrounding is assumed to be constant. which is not true.The law is applicable for higher temperature using forced convection.Verification of Newton’sLaw of cooling:

Newton’s law of cooling can be verified withthe help of the experimental set-up shown in the figure. The set-up consists of a double-walled vessel (V) containing water in betweenthe two walls. A copper calorimeter (C)containing hot water is placed inside the double-walled vessel. Two thermometers through thecorks are used to note the temperaturesT2ofwater in calorimeter andT1of hot water inbetween the double walls respectively.

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The temperature of hot water in the calorimeter isnoted after equal intervals of time. A graph isplotted between loge (T2–T1) and time (t). Thenature of the graph is observed to be a straightline having a negative slope as shown in the figure.