Chapter 16. Copyright 2010 Pearson Education, Inc.

Transcription

1 Chapter 16 Temperature and Heat

2

3 Units of Chapter 16 Temperature and the Zeroth Law of Thermodynamics Temperature Scales Thermal Expansion Heat and Mechanical Work Specific Heats Conduction, Convection, and Radiation

4 16-1 Temperature and the Zeroth Law of Thermodynamics Definition of heat: Heat is the energy transferred between objects because of a temperature difference. Objects are in thermal contact if heat can flow between them. When the transfer of heat between objects in thermal contact ceases, they are in thermal equilibrium.

5 16-1 Temperature and the Zeroth Law of Thermodynamics The zeroth law of thermodynamics: If object A is in thermal equilibrium with object B, and object C is also in thermal equilibrium with object B, then objects A and C will be in thermal equilibrium if brought into thermal contact. That is, temperature is the only factor that determines whether two objects in thermal contact are in thermal equilibrium or not.

6 16-1 Temperature and the Zeroth Law of Thermodynamics

7 16-2 Temperature Scales The Celsius scale: Water freezes at 0 Celsius. Water boils at 100 Celsius. The Fahrenheit scale: Water freezes at 32 Fahrenheit. Water boils at 212 Fahrenheit.

8 16-2 Temperature Scales Converting between Celsius and Fahrenheit: Converting between Fahrenheit and Celsius :

9 16-2 Temperature Scales The pressure in a gas is proportional to its temperature. The proportionality constant is different for different gases, but they all reach zero pressure at the same temperature, which we ecall absolute absoutezero: eo

10 16-2 Temperature Scales The Kelvin scale is similar il to the Celsius scale, except that the Kelvin scale has its zero at absolute zero. Conversion between a Celsius temperature and a Kelvin temperature: t

11 16-2 Temperature Scales The three temperature scales compared:

12 16-3 Thermal Expansion Most substances expand when heated; the change in length or volume is typically proportional to the change in temperature. The proportionality p constant is called the coefficient of linear expansion.

13 16-3 Thermal Expansion Some typical coefficients i of thermal expansion:

14 16-3 Thermal Expansion A bimetallic strip consists of two metals of different coefficients of thermal expansion, A and B in the figure. It will bend when heated or cooled.

15 16-3 Thermal Expansion The expansion of an area of a flat substance is derived from the linear expansion in both directions: Holes expand as well:

16 16-3 Thermal Expansion The change in volume of a solid is also derived from the linear expansion: For liquids and gases, only the coefficient of volume expansion is defined:

17 16-3 Thermal Expansion Some typical coefficients i of volume expansion:

18 16-3 Thermal Expansion Water also expands when it is heated, except when it is close to freezing; it actually expands when cooling from 4 C to 0 C. This is why ice floats and frozen bottles burst.

19 16-4 Heat and Mechanical Work Experimental work has shown that heat is another form of energy. James Joule used a device similar to this one to measure the mechanical equivalent of heat:

20 16-4 Heat and Mechanical Work One kilocalorie (kcal) is defined as the amount of heat needed to raise the temperature of 1 kg of water from 14.5 Cto15.5 C. Through experiments such as Joule s, it was possible to find the mechanical equivalent:

21 16-4 Heat and Mechanical Work Another unit of heat is the British thermal unit (Btu). This is the energy required to heat 1 lb of water from 63 Ft to 64 F.

22 16-5 Specific Heats The heat capacity of an object is the amount of heat added to it divided by its rise in temperature: Q is positive if ΔT is positive; that is, if heat is added to a system. Q is negative if ΔT is negative; that is, if heat is removed from a system.

23 16-5 Specific Heats The heat capacity of an object depends on its mass. A quantity which is a property only of the material is the specific heat:

24 Here are some specific heats of various materials: 16-5 Specific Heats

25 16-5 Specific Heats A calorimeter is a lightweight, insulated flask containing water. When an object is put in, it and the water come to thermal equilibrium. If the mass of the flask can be ignored, and the insulation keeps any heat from escaping: 1. The final temperatures of the object and the water will be equal. 2. The total energy of the system is conserved. This allows us to calculate the specific heat of the object.

26 17-5 Latent Heats When two phases coexist, the temperature remains the same even if a small amount of heat is added. Instead of raising the temperature, the heat goes into changing the phase of the material melting ice, for example.

27 17-5 Latent Heats The heat required to convert from one phase to another is called the latent heat. The latent heat, L, is the heat that must be added to or removed from one kilogram of a substance to convert it from one phase to another. During the conversion process, the temperature of the system remains constant.

28 17-5 Latent Heats The latent heat of fusion is the heat needed to go from solid to liquid; the latent heat of vaporization from liquid to gas.

29 17-6 Phase Changes and Energy Conservation Solving problems involving phase changes is similar il to solving problems involving i heat transfer, except that the latent heat must be included as well.