Temperature

1. Temperature Physics 202 Professor Lee Carkner Lecture 12

2. Thermodynamics • We have already learned about kinetic energy due to an object’s motion and potential energy due to forces acting on an object • Thermodynamics deals with the internal energy of a system • Energy due to random motions of a system’s components • We will examine how this internal energy changes due to changes in heat and work • One of the important concepts in thermodynamics is temperature • Temperature is a measure of the internal energy of an object • How do we experience temperature?

3. Thermoscope • Changes in temperature cause changes in other properties • the length of a column of fluid • the electrical resistance of a wire • the length of a metal bar • We can measure these changes to construct a thermoscope, a device to record changes in temperature • If we put it in an oven, the reading goes up, in a refrigerator the numbers go down • The oven is at a higher temperature than the refrigerator • Temperature changes are caused by an exchange of heat

4. Thermal Equilibrium • Consider a thermoscope in a cup of water • When the thermoscope stops changing, it and the water are in thermal equilibrium • They are not transferring heat • They are at the same temperature • If we move the thermoscope to a different cup and it reads the same value we know that it and the second cup are also in thermal equilibrium

5. Zeroth Law of Thermodynamics • What happens if we pour the two cups together? • No heat will be transferred between them • The cups are in thermodynamic equilibrium with each other • Zeroth Law of Thermodynamics If two objects are each in thermal equilibrium with a third object then they are also in thermal equilibrium with each other

6. How Thermal Equilibrium Works

7. Thermometers • The zeroth law is sometimes called the thermometer law • In order for a thermoscope to be a thermometer, it needs to be calibrated • For example, the Celsius scale • Put a tube of mercury in ice water and mark the mercury level “0” • Them put the tube in boiling water and mark that mercury level “100” • Fill in the numbers evenly in between • You now have a Celsius thermometer

8. The Kelvin Scale • In science we normally use the Kelvin scale • The Kelvin scale is designed so that 0 K is at absolute zero and that 273.15 K is at the freezing point of water • The Kelvin and Celsius degrees are the same size TC = TK -273.15

9. The Triple Point of Water • The Kelvin scale is actually defined not from the freezing point, but from the triple point of water • The triple point of water is the only temperature where water can exist in liquid, solid and gas forms simultaneously • One Kelvin is defined as 1/273.16 of the difference between absolute zero and the triple point of water

10. Converting Temperatures TC = TK -273.15 TF = 9/5 TC +32 • A temperature change of 5 Celsius degrees is equal to a temperature change of 9 Fahrenheit degrees

11. Temperature Scales • Gabriel Daniel Fahrenheit invented the mercury thermometer in 1714 • Zero F was the coldest temperature he could measure • Anders Celsius introduced his scale is 1742 • He used 0 for the freezing point and 100 for the boiling point of water to make computations easier • William Thomson, Lord Kelvin, determined from theory that minus 273.15 degrees Celsius is the coldest it can get

12. Three Temperature Scales

13. Which of the following places the temperatures in the correct rank, highest to lowest? • a) 50 X, 50 W, 50 Y • b) 50 X, 50 Y, 50 W • c) 50 W, 50 X, 50 Y • d) 50 Y, 50 W, 50 X • e) 50 Y, 50 X, 50 W

14. What’s Hot, What’s Not

15. Thermal Expansion • Heating an object causes it to expand • Some objects expand more than others when heated • Heating a glass jar makes the lid easier to remove • The degree of expansion depends on the change in temperature and the coefficient of expansion

16. Thermal Expansion of Ruler

17. Linear Expansion • The degree to which the length of an object changes is given by: DL = L aDT • Where a is the coefficient of linear expansion • This applies to all dimensions of a solid length, width and height • Example: bimetal strip • Two strips of metal with different coefficients of linear expansion attached together • As the strip heats up, one side expands more than the other bending the strip • This principle is used in dial thermometers and thermostats

18. Consider 3 aluminum plates, plate A has area of 1X1 m, plate B has area of 2X2 m and plate 3 has area of 3X3 m. If the temperature of the plates is raised by 10 degrees, which plates has the greatest increase in area? a) A b) B c) C d) All tie e) You cannot tell with out knowing the thickness

19. Bimetal Strip

20. Volume Expansion • If the linear dimensions of a solid change then the volume must change: DV = V bDT • Where b=3a • If the volume changes with temperature but the mass stays the same, then the density must decrease • Density in general decreases with increasing temperature • This is what makes a hot air balloon work

21. Today’s PAL • Galileo Thermometer • Filled with water • Four floating balls • From top to bottom: 80, 76, 72, 68 degrees F • How does the Galileo thermometer work? • What are its limitations as a thermometer?

22. Temperature and Heat • Temperature and heat are two different things • Temperature is a measure of the internal (thermal) energy of a system • A higher temperature means faster random motions • Heat (Q) is energy transferred between systems due to a temperature difference • Heat moves from higher temperature to lower temperature objects • Heat can manifest itself in different ways • Heat is not a thing or a condition • Being “hot” does not mean you have a lot of “heat”

23. Heat • Two objects placed together will exchange heat until they are in thermal equilibrium (equal temperatures) • The rate of heat exchange depends on the temperature difference • Run warm water over your hands after being outside on a cold day and the water feels hot • Large temperature difference, large heat flow • You feel hot on a hot day because your body generates heat and can’t get rid of it quickly enough • Small temperature difference, low heat flow

24. What is Heat?

25. Heat Units • Heat is a form of energy and so its SI unit is the joule (kg m2/s2) • 1 joule (J) = 107 ergs • A joule is fairly small so sometimes the kilojoule (kJ) is used • Heat is also given in calories • 1 calorie is the heat needed to raise the temperature of 1 gram of water 1 deg C • 1 cal = 4.186 J • The nutritional Calorie = 1000 standard calories (1 Cal = 1 kilocal)