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The Kelvin scale is the unit to be used in basic scientific work. This temperature is a trulyfundamental physical quantity, whose definition is independent of theproperties of the specific materials. Therefore, the temperature obtained by extrapolating the measured temperaturesto the case of no gas in the bulb is a temperature that does not depend on thespecific properties of the materials involved. However, if smaller and smaller amounts of thegas are used, the measured temperature converges for different gasses. The temperature obtained in this manner will depend slightly on the amount andthe type of gas in the bulb. In a similar fashion we can measure the temperature ofthe triple point cell in order to obtain our reference temperature:Ĭombining the two measurements we obtain for T The temperature of the body is defined asįigure 16.1. Where p0 is the atmospheric pressure and ] is the density of mercury inthe manometer. Thepressure of the gas in the bulb can be obtained by measuring the leveldifference h of the manometer A reservoir of mercury is raised orlowered such that the volume of the gas in the bulb remains constant. The bulb is filled with a gas and in thermal contact with the bodywhose temperature is to be measured. It consists of a bulb connected by a capillary tube to amanometer. This device is called the constant volume gas thermometer, seeFigure 16.1). The standard thermometer, against which all other thermometers are to becalibrated, is based on the pressure exerted by a gas confined to a fixedvolume. Water can boil or freeze at differenttemperatures (depending on the pressure) and therefore boiling or freezing ofwater can not be used to define a standard temperature. The triple point of water hasbeen assigned a temperature of 273.16 K. Ice, water and vapor canonly coexist at one temperature and pressure. Thereference point is the triple point of water. The unit of temperature will be the Kelvin.
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The first step in defining a temperature scale is to pick out somereproducible thermal environment and, quit arbitrarily, assign a certaintemperature to it. Thetemperature is a property of a body, and two bodies are found to be in thermalequilibrium if their temperatures are equal. This is called the zeroth law of thermodynamics. If the thermometer is also inthermal equilibrium with a second body than the two bodies are also in thermalequilibrium. After making contact, every measurableproperty of the thermometer and the body assumes a stable value, and the bodiesare said to be in thermal equilibrium. In order for a thermometer to measure the temperature of a body, it must be inintimate contact with the body. When the temperature increases, thevolume of a liquid increases, the length of a metal rod increases, theelectrical resistance increases, the pressure of a confined gas increases, etc.If we know the change in these parameters as a function of the temperature, wecan use them to measure the temperature. Properties of many bodies changeas their thermal environment is altered. The central concept ofthermodynamics is the temperature T. Thermodynamics deals with the internal energies of systems and is governed bya set of laws (similar to Newton's law for mechanics). TEMPERATURE 16.TEMPERATURE 16.1.Thermal Equilibrium