Chapter 16 Temperature and Heat

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

3 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.

4 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.

5 16-2 Temperature Scales The Celsius scale and The Fahrenheit scale: Water freezes at 0 Celsius = 32 Fahrenheit. Water boils at 100 Celsius = 212 Fahrenheit. The plot of T F vs T C is a straight line with a slope of T F / T C =9/5 From 0 Celsius to 100 Celsius, temperature increases 100 degree C, From 32 F to 212 F, temperature increases 180 F Every 5 Celsius of temperature change is equal to 9 Fahrenheit of temperature change.

6 16-2 Temperature Scales The pressure in a ideal gas is proportional to its temperature. The proportionality constant (slope) is different for different gases, but they all reach zero pressure at the same temperature, which we call absolute zero: The Kelvin scale is similar to the Celsius scale, except that the Kelvin scale has its zero at absolute zero. SI unit: K, Kelvin

7 16-2 Temperature Scales The three temperature scales compared: Every 5 Celsius of temperature change is equal to Fahrenheit of temperature change, and is equal to Kelvin of temperature change.

8 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 constant is called the coefficient of linear expansion.

9 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:

10 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: In general what expends more due to same temperature change? Liquid or Solid?

11 16-3 Thermal Expansion Some typical coefficients of volume expansion: Water also expands when it is heated, except when it is close to freezing; it actually expands a little bit (0.01% )when cooling from 4 C to 0 C.

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13 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:

14 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 C to 15.5 C.

15 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 T is positive; that is, if heat is added to a system. Q is negative if T is negative; that is, if heat is removed from a system.

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17 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:

18 16-5 Specific Heats When a hot object is put in water, 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: The final temperatures of the object and the water will be equal, T f. The total energy of the system is conserved. Water raises T, so that Q w > 0. Block lowers T, so that Q b > 0. Q w +Q b =0 m w c w (T f -T w ) + m b c b (T f -T b ) = 0 (Use T final minus T initial)

19 Q w +Q b =0 m w c w (T f -T w ) + m b c b (T f -T b ) = 0 (Pay attention to the sign of Q, Use T final minus T initial) Which one of the following is incorrect? m w c w (T f -T w ) = m b c b (T f -T b ) m w c w (T f -T w ) = m b c b (T b -T f )

20 16-6 Conduction, Convection, and Radiation Conduction, convection, and radiation are three ways that heat can be exchanged. Conduction is the flow of heat directly through a physical material.

21 16-6 Conduction, Convection, and Radiation Experimentally, it is found that the amount of heat Q that flows through a rod: increases proportionally to the crosssectional area A increases proportionally to the temperature difference from one end to the other increases steadily with time decreases with the length of the rod The constant k is called the thermal conductivity of the rod.

22 16-6 Conduction, Convection, and Radiation Some typical thermal conductivities: Substances with high Substances with high thermal conductivities are good conductors of heat; those with low thermal conductivities are good insulators.

23 16-6 Conduction, Convection, and Radiation Convection is the flow of fluid due to a difference in temperatures, such as warm air rising. The fluid carries the heat with it as it moves.

24 16-6 Conduction, Convection, and Radiation All objects give off energy in the form of radiation, as electromagnetic waves infrared, visible light, ultraviolet which, unlike conduction and convection, can transport heat through a vacuum. Objects that are hot enough will glow first red, then yellow, white, and blue. Objects at body temperature radiate in the infrared, and can be seen with night vision binoculars.

25 16-6 Conduction, Convection, and Radiation The amount of energy radiated by an object due to its temperature is proportional to its surface area and also to the fourth (!) power of its temperature. It also depends on the emissivity, which is a number between 0 and 1 that indicates how effective a radiator the object is; a perfect radiator would have an emissivity of 1. Here, e is the emissivity, and σ is the Stefan- Boltzmann constant:

26 Summary of Chapter 16 Heat is the energy transferred between objects due to a temperature difference. Objects are in thermal contact if heat can flow between them. Objects that are in thermal contact without any flow of heat are in thermal equilibrium. Thermodynamics is the study of physical processes that involve heat. If objects A and B are both in thermal equilibrium with C, they are in thermal equilibrium with each other.

27 Summary of Chapter 16 Temperature determines whether two objects will be in thermal equilibrium. Celsius scale: water freezes at 0 C, boils at 100 C Fahrenheit: water freezes at 32 F, boils at 212 F The lowest attainable temperature is absolute zero. Kelvin: absolute zero is 0 K; water freezes at K and boils at K

28 Summary of Chapter 16 Temperature scale conversions: Most substances expand when heated. Linear expansion: Volume expansion: Water contracts when heated from 0 C to 4 C.

29 Summary of Chapter 16 Heat is a form of energy: Heat capacity of an object: Specific heat is heat capacity per unit mass: Energy is conserved in heat flow.

30 Summary of Chapter 16 Conduction: heat exchange from one part of a material to a cooler part, with no bulk motion of the material. Heat exchanged in time t: Convection is heat exchange due to the bulk motion of an unevenly heated fluid. Radiation is heat exchange due to electromagnetic radiation.

31 Summary of Chapter 16 Optional Radiated power as a function of temperature: Stefan-Boltzmann constant: