PH0101 UNIT 4 LECTURE 9 GAS THERMOMETER CONSTANT VOLUME GAS THERMOMETER DEMERITS AND MERITS PH 0101 UNIT 4 LECTURE 9

MEASUREMENT OF LOW TEMPERATURES When we sit in the sun or near fire, we feel warm or hot. When things are kept on fire, they also become hot. It is heat which makes us and the objects feel warm or hot. Heat is a form of energy which gives the sensation of warm or hot. A body becomes hot when it gains heat energy and becomes cold when it leaves out heat energy. The measure of the degree of hotness or coldness of a body is known as its temperature. PH 0101 UNIT 4 LECTURE 9

MEASUREMENT OF LOW TEMPERATURES When a substance receives heat the following effects are observed. Change in volume Change in pressure Change in electric resistance Change in radiation Change in thermo electric e.m.f., and Change in colour. PH 0101 UNIT 4 LECTURE 9

MEASUREMENT OF LOW TEMPERATURES Any of these changes can be used for the measurement of temperature. The most accurate method of measuring temperature is based on the change in volume of the substance. The instrument used to measure the temperature by making used of expansion of a substance is known as thermometer. The substance use is known as thermometric substance. Since solids, liquids and gases expand on heating any of these three can be used to construct a thermometer. Solids should be heated to a high temperature. PH 0101 UNIT 4 LECTURE 9

MEASUREMENT OF LOW TEMPERATURES So, the expansion of solids is used to measure high temperatures. Liquids expand at moderate temperatures and so the thermometers making use of expansion of liquids are used to measure moderate temperatures. In the case of gases, the expansion is very large even at low temperatures or for a small change in temperature. So, thermometers making use of gas as thermometric substance are used to measure low temperatures. PH 0101 UNIT 4 LECTURE 9

Constant Pressure Gas Thermometer Constant Volume Gas Thermometer. GAS THERMOMETERS The gas thermometers can be classified into two based on the method by which the gas can be heated. They are, Constant Pressure Gas Thermometer Constant Volume Gas Thermometer. The constant pressure gas thermometer is not used in practice because of the errors which arise due to the non – uniformity of tube bore. The constant volume gas thermometer is the most accurate laboratory thermometer and is used for the calibration of other thermometers. PH 0101 UNIT 4 LECTURE 9

CONSTANT VOLUME GAS THERMOMETER When we heat a gas keeping the volume constant, its pressure increases and when we cool the gas its pressure decreases. The relationship between pressure and temperature at constant volume is given by the law of pressure. According to this law, the pressure of a gas changes by of its original pressure at 0oC for each degree centigrade (or Celsius) rise in temperature at constant volume. PH 0101 UNIT 4 LECTURE 9

CONSTANT VOLUME GAS THERMOMETER If Po is the pressure of a given volume of a gas at 0oC and Pt is the pressure of the same volume of the gas (i.e., at constant volume) at toC, then i.e., (or) (1) Where is constant and is known as coefficient of increase of pressure PH 0101 UNIT 4 LECTURE 9

CONSTANT VOLUME GAS THERMOMETER It consists of a glass bulb B connected to a tube A, through a capillary glass tube ‘C’. The tube A is connected to a mercury reservoir R which is clamped on the board and can be lowered or raised whenever required to keep the volume of the air constant. The capillary tube C is provided with a three way stopper S and can be used to connect capillary and bulb as well as to disconnect tube from bulb B. A pointer is provided such that the end P is projecting inside from the upper part of A. A scale calibrated in 0oC is provided between A and R. PH 0101 UNIT 4 LECTURE 9

CONSTANT VOLUME GAS THERMOMETER PH 0101 UNIT 4 LECTURE 9

CONSTANT VOLUME GAS THERMOMETER The whole apparatus is levelled by adjusting the levelling screws. By adjusting the stopper, the bulb ‘B’ is filled with air or some gas and the pointer is adjusted so that tip of the pointer just touches the level of mercury in the tube A. After filling the bulb, it is kept in an ice bath for some time till the air inside the bulb attains the temperature of ice at which the mercury level becomes stationary. Now the reservoir R is adjusted so that the level of mercury in the tube A just touches the tip of the pointer P. PH 0101 UNIT 4 LECTURE 9

CONSTANT VOLUME GAS THERMOMETER The difference between the mercury levels in the two tubes is noted and let it be ho. If Po is the pressure exerted by the air in the bulb, then (2) Now ice bath is removed and the bulb B is surrounded with steam. Again when the mercury level in both the tubes becomes stationary, tube R is adjusted so that the mercury column in this position in tube A touches the tip of the pointer. PH 0101 UNIT 4 LECTURE 9

CONSTANT VOLUME GAS THERMOMETER Let the difference between the mercury column levels in A and R be h100. The pressure of the enclosed gas in B will be given by, (3) If is the pressure coefficient of the air, then from equation (1), we have (4) PH 0101 UNIT 4 LECTURE 9

CONSTANT VOLUME GAS THERMOMETER Finally the bulb is brought in contact of the body whose temperature is to be found out. After the mercury level becomes stationary, the tube R is adjusted so that the mercury level touches the tip of the pointer again. Let ht be the difference between the heights of mercury levels and if Pt is the pressure of the air inside the bulb B, then we have, (5) PH 0101 UNIT 4 LECTURE 9

CONSTANT VOLUME GAS THERMOMETER and (or) (6) Dividing equation (6) by equation (4), we get (7) PH 0101 UNIT 4 LECTURE 9

CONSTANT VOLUME GAS THERMOMETER Substituting and in equation (7), we get, (8) PH 0101 UNIT 4 LECTURE 9

They work uniformly and regularly over a wide range of temperature. MERITS Gas thermometers are more sensitive when compared to other thermometers. They work uniformly and regularly over a wide range of temperature. They can be used to measure low and high temperatures and have a wide range of operation. (–200oC to 1600oC) Since the expansion of glass is negligible when compared to the expansion of the gas the error due to expansion of glass is negligible. PH 0101 UNIT 4 LECTURE 9

They are not easily portable. DEMERITS It takes time to measure the temperature and is not a quick thermometer. These thermometers can be used only in some cases where the bodies can be brought in contact with the bulb. They are not easily portable. PH 0101 UNIT 4 LECTURE 9

Physics is hopefully simple but Physicists are not PH 0101 UNIT 4 LECTURE 9