Zeroth law of thermodynamics

Zeroth law of thermodynamics

1. ZEROTH LAW OF THERMODYNAMICS

2. Temperature Thermodynamics – science of temperature Temperature – is associated with ability to differentiate the hot from cold. Thermal equilibrium – When two bodies at deferent temperature are brought into contact, some time later they attain a common temperature – now they are in thermal equilibrium.

3. Zeroth law of Thermodynamics Base for all temperature measurement. Thermal equilibrium is the key word for zeroth law. Definition: When a body A is in thermal equilibrium with a body B, and also separately with a body C, then B and C will be in thermal equilibrium with each other

4. Zeroth law of Thermodynamics Temperature measurement procedure – A reference body is used, and a certain physical characteristic of this body which changes with temperature is selected. – The changes in the selected characteristic may be taken as an indication of change in temperature. The selected characteristic is called the thermometric property, expansion of the mercury in the tube is used as the thermometric property – The reference body which is used in the determination of temperature is called the thermometer

5. Thermodynamic Equillibirium

6. Temperature Scales All temperature scales are based on some easily reproducible states such as the freezing and boiling points of water, which are also called the ice point and the steam point Celsius Temperature Scale Fahrenheit scale Kelvin scale

7. Celsius Temperature Scale The Celsius temperature scale employs a degree of the same magnitude as that of the ideal gas scale, but its zero point is shifted, so that the Celsius temperature of the triple point of water is 0.01 degree Celsius or 0.01°C. If t denotes the Celsius temperature, then t = T- 273.15 Thus the Celsius temperature ts at which steam condenses at I atm. pressure ts = Ts - 273.15 = 373.15 - 273.15= 100.00°C Similar measurements for ice points show this temperature on the Celsius scale to be 0.00°C. The only Celsius temperature which is fixed by definition is that of the triple point

8. Fahrenheit scale the freezing point of water  is 32 degrees Fahrenheit (°F) and the  boiling point is 212 ° F. On the Celsius scale, the freezing and boiling points of water are 100 degrees apart. A temperature interval of 1 °F is equal to an interval of ​ 5 ⁄ 9  degrees Celsius. The Fahrenheit and Celsius scales intersect at −40° (i.e., −40 °F = −40 °C ). f  °Fahrenheit to  c  °Celsius : ( f  − 32) °F × 5°C/9°F =  ( f   − 32)/1.8 °C =  c   °C 

9. Kelvin Scale T he lower standard point on this scale is 273K and upper standard point is 373K. There 273 divisions below lower standard point. Its 0K is equal to -273 o C. To convert celsius temperature to kelvin temperature following formulas is used. Temperature in Kelvin = Temperature in  o C + 273 For example, 30 o C = (30+273) = 303K Conversely, (353-273) = 80 o C To convert temperature from celsius scale to kelvin scale the value is increased numerically by 273. Therefore, the temperature on the kelvin scale and celsius scale are related to each other as follows. T(K) = T( o C ) + 273 The celsius and the kelvin scales are used all over the world for scientific measurements. These scales are also found to be more convenient than the scales introduced by Fahrenheit and Reaumer which were commonly used till recently. The following formula is used for the conversion between different temperature scales. K-273/100 = C/100 = F-32/180 = R/80

10. Expansion thermometers The expansion thermometers make use of the differential expansion of two different substances. Thus in liquid-in-glass thermometers, it is the difference in expansion of liquid and the containing glass. And in bimetallic thermometers, the indication is due to the difference in expansion of the two solids Liquid-in-glass thermometers (ii) Bimetallic thermometers.

11. Liquid-in-glass thermometer This is a very familiar type of thermometer. The mercury or other liquid fills the glass bulb and extends into the bore of the glass stem. Mercury is the most suitable liquid and is used from – 38.9°C ( melting point ) to about 600°C. The thermometers employed in the laboratory have the scale engraved directly on the glass stem. A usual type of mercury-in-glass thermometer is shown in Fig . 2.8. An expansion bulb is usually provided at the top of the stem to allow room for expansion of mercury, in case the thermometer is subjected to temperature above its range. The upper limit for mercury-in-glass thermometers is about 600°C. As the upper limit is far above the boiling point of mercury, some inert gas i.e ., nitrogen is introduced above the mercury to prevent boiling

12. Bimetallic thermometers In a bimetallic thermometer differential expansion of bimetallic strips is used to indicate the temperature. It has the advantage over the liquid-in-glass thermometer, that it is less fragile and is easier to read . In this type of thermometer two flat strips of different metals are placed side by side and are welded together . Many different metals can be used for this purpose. Generally one is a low expanding metal and the other is high expanding metal . The bimetal strip is coiled in the form of a spiral or helix. Due to rise in temperature, the curvature of the strip changes . The differential expansion of a strip causes the pointer to move on the dial of the thermometer.

13. Pressure thermometers In pressure thermometers liquids, gases and vapours can all be used. The principle on which they work is quite simple. The fluid is confined in a closed system. In this case the pressure is a function of the temperature, so that when the fluid is heated, the pressure will rise. And the temperature can be indicated by Bourdon type pressure gauge. In general, the thermometer consists of a bulb which contains bulk of the fluid. The bulb is placed in the region whose temperature is required. A capillary tube connects the bulb to a Bourdon tube, which is graduated with a temperature scale. Vapour pressure thermometers Liquid-filled thermometers Gas-filled thermometers

14. Vapour pressure thermometer A schematic diagram of a vapour pressure thermometer is shown in Fig. 2.9. When the bulb containing the fluid is installed in the region whose temperature is required, some of the fluid vapourizes , and increases the vapour pressure. This change of pressure is indicated on the Bourdon tube. The relation between temperature and vapour pressure of a volatile liquid is of the exponential form . Therefore, the scale of a vapour pressure thermometer will not be linear.

15. Liquid-filled thermometer A liquid-filled thermometer is shown in Fig. 2.10. In thiscase , the expansion of the liquid causes the pointer to move in the dial. Therefore liquids havinghigh co-efficient of expansion should be used. In practice many liquids e.g., mercury, alcohol,toluene and glycerine have been successfully used. The operating pressure varies from about 3 to100 bar. These type of thermometers could be used for a temperature upto 650°C in which mercurycould be used as the liquid In actual design, the internal diameter of the capillary tube and Bourdon tube is, mademuch smaller than that of the bulb. This is because the capillary tube is subjected to a temperaturewhich is quite different from that of the bulb. Therefore, to minimise the effect of variation intemperature to which the capillary tube is subjected, the volume of the bulb is made as large aspossible as compared with the volume of the capillary. However, large volume of bulb tends toincrease time lag, therefore, a compensating device is usually built into the recording or indicatingmechanism , which compensates the variations in temperature of the capillary and Bourdontubes

16. Gas-filled thermometers The temperature range for gas thermometer is practically the same as that of liquid filled thermometer. The gases used in the gas thermometers are nitrogen and helium. Both these gases are chemically inert, have good values for their co-efficient of expansion and have low specific heats. The construction of this type of thermometer is more or less the same as mercury-thermometer in which Bourdon spring is used. The errors are also compensated likewise. The only difference in this case is that bulb is made much larger than used in liquid-filled

17. Electrical Resistance Thermometer In the resistance thermometer (Fig. 2.3) the change in resistance of a metal wire due to its change in temperature is the thermometric property. The wire, frequently platinum, may be incorporated in a Wheatstone bridge circuit. The platinum resistance thermometer measures temperature to a high degree of accuracy and sensitivity, which makes it suitable as a standard for the calibration of other thermometer. In a restricted range, the following quadratic equation is often used, R = Ro(l + At + Br) where R0 is the resistance of the platinum wire when it is swrounded by melting ice and A and B are constants.

18. Thermocouple A thermocouple circuit made up from joining two wires A and B made of dissimilar metals is shown in Fig. 2.4. Due to the Seeback effect, a net e.m.f . is generated in the circuit which depends on the difference in temperature between the hot and cold junctions and is, therefore, a thermometric property of the circuit. This e.m.f . can be measured by a microvoltmeter to a high degree of accuracy. The choice of metals depeods largely on the temperature range to be investigated, and. copper-constantan, chromel-alumel and platinum- platinumrhodium are typical combinations in use. A thermocouple is calibrated by measuring the thennal e.m.f . at various known tempera'tures . the reference junction being kept at 0°C. The results of such measureme.1tts on most thennocouples can usually be represented by a cubic.equation of the form .,• e= a+bt+cr+df

19. where £ is the thermal e.m.f . and the constants a, b, c and dare different for each thermocouple. The advantage of a thermocouple is that it comes to thermal equilibrium with the system, whose temperature is to be measured, quite rapidly, because its mass is small.

20. Optical pyrometer An optical pyrometer works on the principle that matters glow above 480 ° C and the colour of visible radiation is proportional to the temperature of the glowing matter . The amount of light radiated from the glowing matter (solid or liquid) is measured and employed to determine the temperature.

21. Radiation pyrometers A device which measures the total intensity of radiation emitted from a body is called radiation pyrometer. The elements of a total radiation pyrometer are illustrated in It collects the radiation from an object (hot body) whose temperature is required. A mirror is used to focus this radiation on a thermocouple. This energy which is concentrated on the thermocouple raises its temperature , and in turn generates an e.m.f . This e.m.f . is then measured either by the galvanometer or potentiometer method. Thus rise of temperature is a function of the amount of radiation emitted from the object.

22. Advantages of the pyrometers The temperatures of moving objects can be measured. A higher temperature measurement is possible than that possible by thermocouples etc. The average temperatures of the extended surface can be measured. The temperature of the objects which are not easily accessible can be measured.