Exploring Temperature and Thermometers in Thermodynamics

1. Chapter 16 Temperature and the Kinetic Theory of Gases

2. Overview of Thermodynamics • Thermodynamics extend the ideas of temperature and internal energy to systems of many particles. • Concerning with energy transfers between a many-particle system and its environment and the resulting variations in temperature of the system or changes in thermodynamic state. • Kinetic theory explains the bulk properties of matter and the correlation between them and the mechanics of atoms and molecules.

3. 16.1 Temperature • We associate the concept of temperature with how hot or cold an objects feels to the touch. • Our sense of touch provides us with a qualitative indication of temperature. • Our senses are unreliable for this purpose. • We need a reliable and reproducible way for establishing the relative “hotness” or “coldness” of objects that is related solely to the temperature of the object • Thermometers are developed for these measurements.

4. Thermal Contact • Two objects are in thermal contact with each other if energy can be exchanged between them. • The exchanges can be in the form of heat or electromagnetic radiation. • The exchanges in energy are due to a temperature difference.

5. Thermal Equilibrium • Thermal equilibrium between two objects is a situation such that the two objects would not exchange energy by heat or electromagnetic radiation if they were placed in thermal contact.

6. Zeroth Law of Thermodynamics • Two objects A and B are separately in thermal equilibrium with a third object C. A and B are in thermal equilibrium with each other, if they are placed in thermal contact. • Let object C be a thermometer. • Since any two of them are in thermal equilibrium with each other, they must be the same in some physical property.

7. Zeroth Law of Thermodynamics, Application • Object C (thermometer) is placed in contact with A until it they achieve thermal equilibrium • The reading on C is recorded • Object C is then placed in contact with object B until they achieve thermal equilibrium • The reading on C is recorded again • If the two readings are the same, A and B are also in thermal equilibrium

8. 16.2 Temperature • Temperature can be thought of as the property that determines whether an object is in thermal equilibrium with other objects. • Two objects in thermal equilibrium with each other are at the same temperature. • If two objects have different temperatures, they are not in thermal equilibrium with each other

9. Thermometers • A thermometer is a device that is used to measure the temperature of a system with which the thermometer is in thermal equilibrium. • All thermometers make use of some physical property that exhibits a change with temperature and can be calibrated for a measurement.

10. Thermometers, cont • The properties include • The volume of a liquid • The length of a solid • The pressure of a gas at a constant volume • The volume of a gas at a constant pressure • The electric resistance of a conductor • The color of a hot object • The temperature scale of a thermometer can be established on the basis of one of these physical properties.

11. Thermometer, Liquid in Glass • A common type of thermometer is a liquid in a glass capillary tube. • The material in the capillary tube expands as it is heated. • The liquid is usually mercury or alcohol.

12. Calibrating a Thermometer • A thermometer can be calibrated by placing it in contact with some environments that remain at constant temperature. • Common systems involve water • A mixture of ice and water at atmospheric pressure • Called the ice point or freezing point of water • A mixture of water and steam in equilibrium at atmospheric pressure • Called the steam point or boiling point of water

13. Celsius Scale • The ice point of water is defined to be 0oC • The steam point of water is defined to be 100oC • The length of the column between these two points is divided into 100 equal segments, called degrees.

14. Problems with Liquid-in-Glass Thermometers • An alcohol thermometer and a mercury thermometer may agree only at the calibration points • The discrepancies between the two thermometers are especially large when the temperatures being measured are far from the calibration points, because mercury and alcohol have different thermal expansion properties.

15. Gas Thermometer • The gas thermometer offers a way to define temperature by directly relating temperature to internal energy. • Temperature readings are nearly independent of the substance used in the thermometer.

16. Constant-Volume Gas Thermometer • The physical property used in this device is the pressure variation with temperature of a fixed-volume gas. • The volume of the gas in the flask is kept constant by raising or lowering the reservoir B to keep the mercury level at A constant

17. Constant Volume Gas Thermometer, cont • The thermometer is calibrated by using a ice water bath and a steam water bath • The pressures of the mercury under each situation are recorded • The volume is kept constant by adjusting A • The information is plotted

18. Constant Volume Gas Thermometer, final • To find the temperature of a substance, the gas flask is placed in thermal contact with the substance • The pressure is found on the graph • The temperature is read from the graph

19. Absolute Zero • The thermometer readings are virtually independent of the gas in the flask. • If the lines for various gases are extended, the pressure is always zero when the temperature is –273.15o C • This temperature is called absolute zero

20. Kelvin Temperature Scale • Absolute zero is used as the basis of the Kelvin temperature scale. • The size of the degree on the Kelvin scale is the same as the size of the degree on the Celsius scale • To convert: TC = T – 273.15 • TC is the temperature in Celsius • T is the Kelvin (absolute) temperature

21. Kelvin Temperature Scale, 2 • The Kelvin temperature scale is now based on two new fixed points • Adopted in 1954 by the International Committee on Weights and Measures • One point is absolute zero • The other point is the triple point of water • This is the single temperature and pressure at which ice, water, and water vapor can coexist in thermal equilibrium.

22. Absolute Temperature Scale, 3 • The triple point of water occurs at 0.01o C and 4.58 mm of mercury • This temperature was set to be 273.16K on the Kelvin temperature scale. • The unit of the absolute scale is the kelvin

23. Absolute Temperature Scale, 4 • The absolute scale is also called the Kelvin scale • Named for William Thomson, Lord Kelvin • The triple point temperature is 273.16 K • No degree symbol is used with kelvins • The kelvin is defined as 1/273.16 of the temperature of the triple point of water

24. Some Examples of Absolute Temperatures • This figure gives some absolute temperatures at which various physical processes occur • The scale is logarithmic • The temperature of absolute zero has never been achieved. • Experiments only have come close

25. Energy at Absolute Zero • According to classical physics, the kinetic energy of the gas molecules would become zero at absolute zero. • The molecular motion would cease • Therefore, the molecules would settle out on the bottom of the container • Quantum theory modifies this statement and indicates that some residual energy would remain at this low temperature. • This energy is called the zero-point energy

26. Fahrenheit Scale • A common scale in everyday use in the US • Named for Daniel Fahrenheit • Temperature of the ice point is 32oF • Temperature of the steam point is 212oF • There are 180 divisions (degrees) between the two reference points

27. Comparison of Scales • Celsius and Kelvin have the same size degrees, but different starting points TC = T – 273.15 • Celsius and Fahrenheit have difference sized degrees and different starting points

28. Comparison of Scales, cont • To compare changes in temperature • Ice point temperatures • 0oC = 273.15 K = 32oF • steam point temperatures • 100oC = 373.15 K = 212oF