Chapter 17. Work, Heat, and the First Law of Thermodynamics Topics: Chapter Goal: Conservation of Energy Work in Ideal-Gas Processes

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photo) shows where heat energy is escaping from a house.

Chapter Goal: To expand our understanding of energy and to develop the first law of

Topics: It s All About Energy Work in Ideal-Gas rocesses Heat The First Law of Thermodynamics

Conservation of Energy Work in Ideal-Gas rocesses ΔE sys = ΔK + ΔU + ΔE int = W ext + Q + T

1 Chapter 17. Work, Heat, and the First Law of Thermodynamics This false-color thermal image (an infrared photo) shows where heat energy is escaping from a house. In this chapter we investigate the connection between work and heat. Chapter Goal: To expand our understanding of energy and to develop the first law of thermodynamics as a general statement of energy conservation. Topics: It s All About Energy Work in Ideal-Gas rocesses Heat The First Law of Thermodynamics Thermal roperties of Matter Calorimetry The Specific Heats of Gases Heat-Transfer Mechanisms Conservation of Energy Work in Ideal-Gas rocesses ΔE sys = ΔK + ΔU + ΔE int = W ext + Q + T MT + T MW + T ER + T ET Consider a gas cylinder sealed at one end by a moveable piston. Work in Ideal-Gas rocesses If we let the piston move in a slow quasi-static process from initial volume i to final volume f, the total work done by the environment on the gas is or, graphically 1

2 Work in Ideal-Gas rocesses In an isochoric process, when the volume does not change, no work is done. EXAMLE 17.2 The work of an al compression QUESTION: In an isobaric process, when pressure is a constant and the volume changes by Δ = f i, the work done during the process is In an al process, when temperature is a constant, the work done during the process is Heat, Temperature, and Thermal Energy Thermal energy E th is an energy of the system due to the motion of its atoms and molecules. Any system has a thermal energy even if it is isolated and not interacting with its environment. The units of E th are Joules. Heat Q is energy transferred between the system and the environment as they interact. The units of Q are Joules. Temperature T is a state variable that quantifies the hotness or coldness of a system. A temperature difference is required in order for heat to be transferred between the system and the environment. The units of T are degrees Celsius or Kelvin. Which of the following is NOT a state variable? A) Internal energy (thermal energy) B) Heat C) ressure D) Temperature E) Mass density Work and Heat Exercise and calories If you go up the Empire State Building (102 nd floor 1250 feet), how many calories will you burn? (1 Apple 70 Cal) A) About one apple s worth B) About 5 apples worth C) About 10 apples worth D) About 270 apples worth E) About 900 apples worth 2

3 The First Law of Thermodynamics Work and heat are two ways of transfering energy between a system and the environment, causing the system s energy to change. If the system as a whole is at rest, so that the bulk mechanical energy due to translational or rotational motion is zero, then the conservation of energy equation is What can you say about the net work done on the gas? What can you say about the net work done in this process? A) No work is done B) ositive work is done C) Negative work is done A) No work is done B) Work done on the gas (W on >0) C) Work done by the gas (W on <0) When the cycle is completed, what is the change in the internal energy of the gas? When the cycle is completed, what is the change in the thermal energy of the gas? A) No change B) ositive change C) Negative change D) Depends on the heat transfer, which is not known A) No change B) ositive change C) Negative change D) Depends on the heat transfer, which is not known 3

4 When the cycle is completed, how much energy entered into the system by heat? When the cycle is completed, how much energy entered into the system by heat? A) No energy exchange by heat B) ositive amount C) Negative amount (energy left by heat) D) Depends on the numerical values, so cannot be predicted qualitatively A) No energy exchange by heat B) ositive amount C) Negative amount (energy left by heat) D) Depends on the numerical values, so cannot be predicted qualitatively How can we calculate the amount of work done in an ic process? We need to study specific heats before we can do it For the two processes shown, which of the following is true: A. Q A < Q B. B. Q A > Q B. C. Q A = Q B. For the two processes shown, which of the following is true: A. Q A < Q B. B. Q A > Q B. C. Q A = Q B. Temperature Change and Specific Heat The amount of energy that raises the temperature of 1 kg of a substance by 1 K is called the specific heat of that substance. The symbol for specific heat is c. If W = 0, so no work is done by or on the system, then the heat needed to bring about a temperature change ΔT is 4

Temperature Change and Specific Heat hase Change and Heat of Transformation A phase change

The amount of heat energy that causes 1 kg of substance to undergo a phase change is

hase Change and Heat of Transformation Two specific heats of transformation are the heat

vaporization L v, the heat of transformation between a liquid and a gas.

5 Temperature Change and Specific Heat hase Change and Heat of Transformation A phase change is characterized by a change in thermal energy without a change in temperature. The amount of heat energy that causes 1 kg of substance to undergo a phase change is called the heat of transformation of that substance. hase Change and Heat of Transformation Two specific heats of transformation are the heat of fusion L f, the heat of transformation between a solid and a liquid, and the heat of vaporization L v, the heat of transformation between a liquid and a gas. The heat needed for these phase changes is The symbol for heat of transformation is L. The heat required for the entire system of mass M to undergo a phase change is 5

6 1 kg of ice at -10 C is dropped in a container that has 2 kg of water at 20 C. What is the final temperature of the mixture? A) Less than zero B) Zero C) More than zero Suppose to systems start at different temperatures T 1 and T 2. Heat energy will naturally be transferred from the hotter to the colder system until they reach a common final temperature T f. Calorimetry The Specific Heats of Gases It is useful to define two different versions of the specific heat of gases, one for constant-volume (isochoric) processes and one for constant-pressure (isobaric) processes. We will define these as molar specific heats because we usually do gas calculations using moles instead of mass. The quantity of heat needed to change the temperature of n moles of gas by ΔT is The Specific Heats of Gases where C is the molar specific heat at constant volume and C is the molar specific heat at constant pressure. The Specific Heats of Gases The Specific Heats of Gases = R 6

7 Cp-Cv=R Thermal Energy Change between two temperatures ( ΔE th ) = Q = nc ΔT ( ΔE th ) = Q + W = nc ΔT pδ p = nrt pδ = nrδt (constant pressure) ( ΔE th ) = ΔE th ( ) nc ΔT = nc ΔT nrδt C C = R. ΔE th = nc ΔT True for all processes! Which of the processes shown in the diagram requires the highest amount of heat transfer to reach the final state at T f? Which of the processes shown in the diagram requires the highest amount of heat transfer to reach the final state at T f? C C D. All require the same amount of heat D. All require the same amount of heat A gas cylinder and piston are covered with heavy insulation. The piston is pushed into the cylinder, compressing the gas. In this process, the gas temperature A gas cylinder and piston are covered with heavy insulation. The piston is pushed into the cylinder, compressing the gas. In this process, the gas temperature A. decreases. B. increases. C. doesn t change. D. There s not sufficient information to tell. A. decreases. B. increases. C. doesn t change. D. There s not sufficient information to tell. 7

8 1 st Law of Thermodynamics and three special processes Work in Adiabatic rocess ΔE th = W = nc ΔT p i γ γ i = p f f GAS γ = C Monatomic Gases 1.67 Diatomic Gases 1.40 C Adiabatic Compression and Expansion Adiabatic Compression or Expansion problem solve!!! Heat Transfer Mechanisms Conduction 8

Conduction Conduction For a material of cross-section area A and length L, spanning a temperature difference ΔT = T H T C, the rate of heat transfer is where k is the

10 Keeping a freezer cold QUESTION: Convection Air is a poor conductor of heat, but thermal energy is easily transferred through air, water, and other fluids because the

This heated water expands, becomes less dense than the water above it, and thus rises to the surface, while cooler, denser water sinks to take its place.

Radiation All objects emit energy in the form of radiation, electromagnetic waves generated by oscillating electric charges in the atoms that form the object.

9 Conduction Conduction For a material of cross-section area A and length L, spanning a temperature difference ΔT = T H T C, the rate of heat transfer is where k is the thermal conductivity, which characterizes whether the material is a good conductor of heat or a poor conductor. EXAMLE Keeping a freezer cold QUESTION: Convection Air is a poor conductor of heat, but thermal energy is easily transferred through air, water, and other fluids because the air and water can flow. A pan of water on the stove is heated at the bottom. This heated water expands, becomes less dense than the water above it, and thus rises to the surface, while cooler, denser water sinks to take its place. The same thing happens to air. This transfer of thermal energy by the motion of a fluid the well-known idea that heat rises is called convection. Radiation All objects emit energy in the form of radiation, electromagnetic waves generated by oscillating electric charges in the atoms that form the object. If heat energy Q is radiated in a time interval Δt by an object with surface area A and absolute temperature T, the rate of heat transfer is found to be The parameter e is the emissivity of the surface, a measure of how effectively it radiates. The value of e ranges from 0 to 1. σ is a constant, known as the Stefan-Boltzmann constant, with the value σ = W/m 2 K 4. 9

Temperature of a hot iron bar is doubled from 500 C to 1000 C. The amount of power radiated changes by The greenhouse effect is a result of light received from the Sun absorbed by the atmosphere. A.

10 Temperature of a hot iron bar is doubled from 500 C to 1000 C. The amount of power radiated changes by The greenhouse effect is a result of light received from the Sun absorbed by the atmosphere. A. Twice B. Four.mes C. About seven.mes D. About sixteen.mes A. TRUE B. FALSE A cubical box 20 cm on a side is constructed from 1.2 cm-concrete panels. A 100- W lightbulb is sealed inside the box. What is the air temperature inside the box when the light is on if the syrrounding air is at 20 C? 10

CHAPTER 17 WORK, HEAT, & FIRST LAW OF THERMODYNAMICS

CHAPTER 17 WORK, HEAT, & FIRST LAW OF THERMODYNAMICS CHAPTER 17 WORK, HEAT, and the FIRST LAW OF THERMODYNAMICS In this chapter, we will examine various thermal properties of matter, as well as several mechanisms by which energy can be transferred to and