Heat and Thermodynamics PHYSICS 1-2 MR. CHUMBLEY CHAPTERS 9 AND 10 Temperature and Thermal Equilibrium CHAPTER 9, SECTION 1 P. 298-304 Defining Temperature While many of us are familiar with temperature as a measure of how hot or cold something is, defining how to measure that temperature is measured is more complex When heat is added or removed, there is often times a change in temperature This is in effect adding or removing of energy from the system Defining Temperature The temperature of a substance is a measure of the average kinetic energy of the particles in a substance While we think of kinetic energy as particles moving faster, there are multiple ways in which this can happen Form of energy Description Energy Type Translational Linear motion Kinetic Rotational Rotation about some axis Kinetic Vibrational Bending and stretching of bonds Kinetic and potential Internal Energy When looking at an whole system, all of the different types of energies need to be considered Internal energy is the energy of a substance due to both the random motions of its particles and the potential energy that results from the distances and alignments between the particles Thermal Equilibrium However, measuring the temperature as a specific value only has meaning when it is stable Thermal equilibrium is the state in which two bodies in physical contact with each other have identical temperature When not in thermal equilibrium, measuring the change in temperature, and the rate of change of temperature are more useful quantities Measuring Temperature There are many different ways in which temperature can be measured degrees Fahrenheit (˚F) degrees Celsius (˚C) Kelvin (K) These scales are calibrated and established on the basis of fixed temperatures Converting between the different measurement systems can be done using specific relationships Fahrenheit and Celsius To convert from Fahrenheit to Celsius 𝑇𝐶 = 𝑇𝐹 − 32 × 5 9 To convert from Celsius to Fahrenheit 𝑇𝐹 = 𝑇𝐶 × 9 5 + 32 Kelvin and Celsius Kelvin is an absolute scale of temperature where a change of 1 K is equal to 1 ˚C To convert to Kelvin from Celsius 𝑇 = 𝑇𝐶 + 273.15 To convert to Celsius from Kelvin 𝑇𝐶 = 𝑇 − 273.15 Sample Problem 9A (p. 303) What are the equivalent Celsius and Kelvin temperatures at 50.0˚F? Homework! Practice A (p. 303) #1-5 p. 304 #3 The highest recorded temperature on Earth was 136˚F, at Azizia, Libya in 1922. Express this temperature in degrees Celsius and kelvin. Defining Heat CHAPTER 9, SECTION 2 P. 305 – 311 Heat and Energy The phenomena that are described by thermal physics are a result of microscopic interaction affecting macroscopic systems When two objects are in contact and have a difference in temperature, there will be an exchange of energy between them Heat is the energy transferred between objects due to a difference in temperature Heat and Thermal Equilibrium On the microscopic level, as particles of one object collide with particles of another, the kinetic energy is transformed into heat When the kinetic energy transferred between particles happens equally in both direction, the object has reached thermal equilibrium As heat is transferred from one object to another, the temperature changes as well Units of Heat Since head is the transfer of energy, it has units of energy Unit Equivalent Value Use joule (J) m2 1 kg ∙ 2 s SI unit of energy calorie (cal) 4.186 J non-SI unit historical unit kilocalorie (kcal) 4.186 × 103 J non-SI unit Calorie 4.186 × 103 J food and nutrition Brittish Thermal unit (Btu) 1.055 × 104 J Used in engineering, air conditioning, refrigeration therm 1.055 × 108 J 100 000 Btu Thermal Energy Transfer Thermal energy can be transferred in a variety of ways Conduction Thermal Radiation Convection Conduction is the transfer of thermal energy by direct contact Thermal radiation is the transfer of thermal without direct contact Convection is the transfer of thermal energy by the motion of matter as a result of temperature differences Thermal Energy Transfer Heat and Work When work is done on objects, the internal energy of objects can increase Because of this, internal energy is included in the Law of Conservation of Energy ∆𝑃𝐸 + ∆𝐾𝐸 + ∆𝑈 = 0 Sample 9B (p. 310) A Joule’s apparatus is used to measure the amount of mechanical work that can be turned into internal energy. If a total mass of 11.5 kg falls 1.3 m and all of the mechanical energy is converted into internal energy, what is the increase in the internal energy of the water? Homework! Practice B (p. 311) #1-4 Changes in Temperature and Phase CHAPTER 9, SECTION 3 P. 313 – 319 Specific Heat Capacity As heat is transferred to/from a material, the resulting change in temperature is dependent on the material itself Specific heat capacity (c) is the quantity of heat required to raise a unit mass of homogeneous material 1 K or 1˚C in a specified way given constant pressure and volume 𝑄 𝑐= 𝑚∆𝑇 or 𝑄 = 𝑐𝑚∆𝑇 Figure 3.2 on p. 314 has specific heat values for many common materials Sample Problem How much energy is required to raise 2 L of water from room temperature to the normal boiling point? Calorimetry Calorimetry is an experimental procedure used to measure the Energy transferred from one substance to another as heat The premise of calorimetry is the energy released as heat by one object is absorbed by another 𝑒𝑛𝑒𝑟𝑔𝑦 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 = 𝑒𝑛𝑒𝑟𝑔𝑦 𝑟𝑒𝑙𝑒𝑎𝑠𝑒𝑑 𝑐1𝑚1∆𝑇1 = 𝑐2𝑚2∆𝑇2 Sample 9C (p. 315) A 0.50 kg aluminum bolt is heated to an unknown initial temperature. It is ten dropped into a calorimeter containing 0.15 kg of water with an initial temperature of 21.0 ˚C. The bolt and the water reach a final temperature of 25.0 ˚C. Find the initial temperature of the aluminum bolt. Latent Heat When a substance exchanges enough heat to reach a phase change, additional heat transfer is required A phase change is the physical change of a substance from one state to another at constant temperature and pressure Latent heat (L) is the energy per unit mass that is transferred during a phase change of a substance 𝑄 = 𝑚𝐿 Latent heat of fusion (Lf) is for solid ↔ liquid phase changes Latent heat of vaporization (Lv) is for liquid ↔ gas phase changes Heating Curve Homework! Practice 9C (p. 316) #1-4 Relationships Between Heat and Work CHAPTER 10, SECTION 1 P. 322 – 337 Heat, Work and Internal Energy It is easy to see how work done on objects can increase their internal energy However, internal energy can also be used to do work Systems and Energy Transfer Both heat and work are types of energy being transferred to or from a system A system is a set of particles or interacting components considered to be a distinct physical entity for the purpose of study While we can look at systems in isolation, they rarely exist as such The surrounding environment is the combination of conditions and influences outside a system that affect the behavior of the system Work done by a Gas Gases are a great example of how work is done by changes in internal energy The work done by a gas is equal to the product of the pressure exerted by the gas and the change in volume of that gas 𝑊 = 𝑃∆𝑉 Sample Problem 10A (p. 334) An engine cylinder has a cross-sectional area of 0.010 m2. How much work can be done by a gas in the cylinder if the gas exerts a constant pressure of 7.5 × 105 Pa on the piston and moves it a distance of 0.040 m? Thermodynamic Processes There are different processes that are a result of how internal energy, heat, and work transfer energy An isovolumetric process is a thermodynamic process that takes place at constant volume so that no work is done on or by the system An isothermal process is a thermodynamic process that takes place at constant temperature An adiabatic process is a thermodynamic process during which no energy is transferred to or from the system as heat An isobaric process is a thermodynamic process in which the pressure stays constant Thermodynamic Processes Homework! Practice A (p. 334) #1-4 The First Law of Thermodynamics CHAPTER 10, SECTION 2 P. 338 – 345 Energy Conservation The total energy in an isolated system is conserved ∆𝑃𝐸 + ∆𝐾𝐸 + ∆𝑈 = 0 However, since we have identified that energy can be transferred to or from a system by heat or work, we need to modify this relationship First Law of Thermodynamics The first law of thermodynamics states that the change in internal energy of a system is equal to the difference between the energy transferred to/from a system as heat and the energy transferred to/from the system as work ∆𝑈 = 𝑄 − 𝑊 Work and Heat The signs of Q and W give information about how energy is transferred to or away from a system ∆𝑈 = 𝑄 − 𝑊 Signs of Q and W Q>0 Energy added as heat Q<0 Energy removed as heat Q=0 No transfer as heat W>0 Work done by system W<0 Work done on system W=0 No work done Sample Problem B A total of 135 J of work is done on a gaseous refrigerant as it undergoes compression. If the internal energy of the gas increases by 114 J during the process, what is the total amount of energy transferred as heat? Has energy been added or removed from the refrigerant as heat? Cyclic Processes A cyclic process is a thermodynamic process in which a system returns to the same conditions under which it started ∆𝑈 = 𝑄 − 𝑊 = 0 𝑄𝑛𝑒𝑡 = 𝑊𝑛𝑒𝑡 Heat Engine A heat engine is any device that uses heat to do mechanical work When an engine is connected to a hot reservoir, it uses some of that energy to perform mechanical work, while moving the remaining heat to a cold reservoir Heat Pump A heat pump performs mechanical work to move heat against its temperature gradient This is how refrigerators work The Second Law of Thermodynamics CHAPTER 10, SECTION 3 P. 348 – 353 Limitations of a Heat Engine When looking at heat engines, it seems desirable to use as much heat energy as possible to perform mechanical work However, it is impossible to completely convert heat energy into work Entropy Entropy is the measure of randomness or disorder of a system In nature, energy tends to become more disorderly with time To increase the order of energy in a system, greater work must be performed Second Law of Thermodynamics The second law of thermodynamics states that in any cyclic process, the entropy of a system will increase or stay the same The second law of thermodynamics is sometimes referred to as the law of entropy Efficiency Efficiency is the ratio of usable work to total work done For a heat engine, this means the ratio of the work done to the energy added as heat It can also be described as the ratio of difference in output and total energy to the total energy 𝑊𝑛𝑒𝑡 𝑄ℎ − 𝑄𝑐 𝑒𝑓𝑓 = = 𝑄ℎ 𝑄ℎ Efficiency The efficiency for engines is typically very small Typical efficiencies for engines include: Engine Type Maximum eff Measured eff steam engine 0.29 0.17 steam turbine 0.40 0.30 gasoline engine 0.60 0.25 diesel engine 0.56 0.35 Sample Problem 10C (p. 350) Find the efficiency of a gasoline engine that, during one cycle, receives 204 J of energy from combustion and loses 153 J as heat to the exhaust. Homework! Practice B (p. 341) #1, 3, 5 Practice C (p. 351) #2, 4, 6
