Heat, Temperature Scales & Thermometers, Thermal Expansion | Science & Technology for UPSC CSE PDF Download

Heat is a kind of energy which causes sensation of warmth. Temperature is the degree of hotness or coldness of a body. Heat indicates quantity of heat energy possessed by a body, while temperature denotes intensity regarding thermal conditions of a body.

Latent Heat of Fusion (or Melting): Equal masses of different substances require different latent heats in order to melt. The latent heat of fusion or melting of a substance is the amount of heat in calories per gram (cal/gm). Example: Latent heat of fusion of ice is 80 cal/gm. This means 80 calories of heat are needed to convert 1 gram of ice into water (0°C) without change in temperature. 80 kcal/kg which means that 80 kcal/kg of heat is required to convert 1 kg of ice into water without change in temperature.

Vaporization and Condensation: The change from liquid state into gaseous or vapour state is called vaporization. The latent heat of vaporization of a substance can be defined as the amount of heat in calories required to convert 1 gram of the substance from liquid to vapour state without any change in temperature. The SI unit of temperature is degree Kelvin denoted by K.

Temperature Scales and  Thermometers

Thermometers measure length (as of a mercury column) or pressure or volume (with the gas thermometer at the National Bureau of Standards) or electrical voltage (with a thermocouple).

Celsius and Fahrenheit: The most widely used scales are the Fahrenheit (°F) and the Celsius (°C). The Centigrade scale with °0 assigned to ice water (ice point) and 100° assigned to water boiling under one atmosphere pressure (steam point) was formerly used, but it has been succeeded by the Celsius scale.

In the Celsius scale, the temperature at which pure ice melts at normal atmospheric pressure is taken to be zero degree (0° C) and the temperature at which pure water boils (under normal atmospheric pressure) is taken to be 100°C).

The relation between Celsius (C) and Fahrenheit (F) temperatures are:

Absolute zero: Absolute zero corresponds to -273.16° C or -459.7° F.

In practice, absolute temperatures are measured by using low density helium gas and dilute paramagnetic crystals, the most nearly ideal of real materials.

Choice of liquid for Thermometers

The freezing and Boiling points of mercury are-39°C and 357° respectively while alcohol freezes at -115°C. It is therefore essential to use alcohol thermometer in places where winter temperature persists at or around - 40°C. Alcohol also possesses the advantage of having a coefficient of expansion about six times that of mercury. For extra low-temperature work (down to - 200°C) Pentane is used instead of alcohol. Water is unsuitable for use in thermometers not only because it freezes at 0°C but also because of its irregular expansion. Mercury is preferred to alcohol or any other thermometric liquid for the following reasons:

(i) It does not wet glass. Alcohol tends to cling to the walls of the tube and hence, reading is disturbed.

(ii) It does not, like alcohol, vaporize and distil on to the upper part of the bore.

(iii) Its expansion is nearly uniform and regular at ordinary temperature and therefore makes calibration easier.

(iv) It is opaque and easily seen.

(v) It is a better conductor of heat than alcohol, and therefore responds more rapidly to changes of temperature.

(vi) It has a low specific heat capacity, so does not remove much heat from the body whose temperature is to be measured, therefore is more sensitive.

Thermocouple is a device that uses the voltage developed by the junction of two metals to measure temperature difference. Two wires of dissimilar metals welded together at the ends make up the basic thermocouple.

Pyrometers measures high temperature using emitted radiation. The instrument is not to be in contact with the hot body. They can measure temperature howsoever high and the lower practical limit for radiation. The highest temperature that it can record is 900 K ( = 62 J°C).

 Thermal Expansion

Solids, liquids and gases generally expand when heated and contract when cooled. Some substances such as water from 0°C to 4°C, Silver iodide from 80°C to 141°C and Silica below - 80°C contract on heating. When a substance is heated, the average kinetic energy of its molecules increases and they start moving with increased speed. Consequently, the mean distance between the molecules increases which results in an increase in the volume of the substance.

Expansivity. If we heat a 1-m long iron rod through 1°C (or 1 K), its length increases by 0.000012 m. We say that the linear expansivity of iron is 0.000012/°C.
Linear expansiv-ities of some solids in per degree Celsius are as follows:

 Brass                       0.000019

 Invar                        0.000001

 Glass (ordinary)      0.000009

 Glass (pyrex)          0.000003

Bimetal Strip. A brass bar and an invar bar riveted together form a bimetal strip. When temperature rises, brass expands more than invar and the strip bends with brass on the convex side. When temperature falls, the strip regains its original shape. Thus a bimetal strip can act like a swtich. Bimetal strips are used in thermostats which are used for regu-lating temperatures of electricallyheated rooms, ovens, toasters, etc.

Refrigerators are also equipped with special thermostats.

Anomalous Expansion of Water

Water shows unusual expansion. If we take a cube of ice at - 5°C and heat it, it expands till ice starts melting. During melting its temperature remains 0°C but its volume decreases. If heat is continuously supplied to water at 0°C, it further contracts up to 4°C and then it starts expanding. Thus water has its minimum volume and maximum density at 4°C. The anomalous expansion of water helps preserve aquatic life during very cold weather. When temperature falls, the top layer of water in a pond contracts, becomes denser and sinks to the bottom. A circulation is thus set up until the entire water in the pond reaches maximum density at 4°C. If the temperature falls further, the top layer expands and remains on top till it freezes. Thus even though the upper layers are frozen the water near the bottom is at 4°C and the fishes, etc., can survive in it easily.