Thermodynamics

1. Thermodynamics Temperature, Heat, Heat Transfer, and the Laws of Thermodynamics

2. Kinetic Energy and Temperature • All matter is composed of continually jiggling atoms or molecules (particles). • Because of this random motion, the particles in matter have kinetic energy. • The average kinetic energy of these individual particles causes an effect we can sense – warmth. • Temperature tells how hot or cold an object is compared to some standard. • Nearly all matter expands when its temperature increases and contracts when its temperature decreases. • A common thermometer measures temperature by showing the expansion and contraction of a liquid. • Temperature and kinetic energy are related. • Temperature is a measure of the average kinetic energy of the molecules of a substance.

3. Temperature and Kinetic Energy Temperature is not a measure of the total kinetic energy of all the molecules in a substance. There is twice as much KE in 2 liters of boiling water as in 1 liter, but the temperatures are the same because the average KE of molecules in each is the same. There is more molecular kinetic energy in the bucketful of warm water than in the small cupful of higher-temperature water.

4. Temperature Scales

5. Heat Flow – Thermal Energy • Matter contains internal energy, not heat. • Heat is energy in transit from a body of higher temperature to one of lower temperature. • The energy resulting from heat flow is called thermal energy. • When there is heat flow between objects, the objects are said to be in thermal contact. • Heat flows according to temperature differences (average molecular kinetic energy differences). • The direction of spontaneous energy transfer is always from a warmer substance to a cooler substance. • When two or more objects or substances in thermal contact reach a common temperature, the objects are said to be in thermal equilibrium. • No heat flows between objects at thermal equilibrium.

6. Temperature is NOT Heat • Heat is the net energy transferred from one object to another because of a temperature difference. • An object may have a relatively high temperature and a relatively low internal energy. • An object may have a relatively low temperature and a relatively high internal energy. • It is possible for heat to flow from an object with very little energy (but high temperature) to an object with lots of energy (but low temperature). • When you add heat energy to an object, its temperature may (or may not!) increase.

7. Measurement of Heat • Some two hundred years ago heat was thought to be an invisible fluid called caloric, which flowed like water from hot objects to cold objects. • Caloric appeared to be conserved – that is, it seemed to flow from one place to another without being created or destroyed. • This idea was the forerunner of the law of conservation of energy. • Heat is energy with units of Joules. • The calorie is also a unit of heat. One calorie (symbol cal) is the heat required to raise the temperature of one gram of water one Celsius degree (Chemistry class!).

8. Measurement of Heat The calorie is not to be confused with the Calorie (capital C). A Calorie is equal to one thousand calories and is the unit used in describing the energy available from food. (1 cal = 4.184 J, or 1 J = 0.24 cal) The kilocalorie is also a unit of heat. One kilocalorie equals 1000 calories, or the amount of heat required to raise the temperature on one kilogram of water by one Celsius degree. To the weight watcher, the peanut contains 10 Calories; to the physicist, it releases 10,000 calories (or 41,840 joules) of energy when burned or digested.

9. Heat Transfer • The spontaneous transfer of heat is always from warmer objects to cooler objects. • Heat will continue to be transferred until the temperature between the objects is equalized. • This equalization of temperature is brought about in three ways: • conduction • convection • radiation

10. Conduction • Conduction of heat can take place within materials and between materials in direct contact. • Heat transfer by conduction involves the transfer of energy from molecule to molecule. • Conduction is explained by the jostling between atoms, molecules and electrons. • Metals are the best conductors. • Materials, such as metals, that are composed of atoms with “loose” outer electrons are good conductors of heat and electricity.

11. Conductors and Insulators • Heat from the flame causes atoms and free electrons in the end of the metal rod to move faster and jostle against others, which in turn do the same and increase the energy of vibrating atoms down the length of the rod. • Conductors easily allow the flow of heat. • Insulators resist the flow of heat. • A poor conductor is a good insulator. • Liquids and gases, in general, are good insulators. • Solids that are good insulators include wood, wool, straw, paper, cork and polystyrene (Styrofoam). • No insulator can totally prevent heat from getting through it, it just slows down the flow of heat.A warm blanket does not provide you with heat, it simply slows the transfer of your body heat to the surroundings. • The tile floor feels cold to the bare feet, while the carpet at the same temperature feels warm. This is because tile is a better conductor than carpet. Tile Carpet

12. Convection Convection currents in air keep the air stirred and moving. Convection currents in liquid keep the liquid stirred and moving. • Convection is where heating occurs by currents in a fluid. • Convection occurs in all fluids, whether liquid or gas. • When a fluid is heated, it expands, becomes less dense, and rises. • Convection is an application of Archimedes’ principle, for the warmer fluid is buoyed upward by denser surrounding fluid. • Convection currents keep a fluid stirred up as it heats. Convection currents stirring the atmosphere produce winds.

13. Radiation • Any energy, including heat, that is transmitted by radiation is called radiant energy. • Radiant energy is in the form of electromagnetic waves. • It includes radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays. • Radiant energy can be emitted and absorbed • All objects continually emit radiant energy in a mixture of wavelengths. • The sun’s heat is transmitted by the process of radiation. • Do not confuse heat radiation with radioactive (nuclear) radiation, which is given off by the nuclei of radioactive atoms.

14. An Interesting Experiment Boiling Water When the test tube is heated at the top, convection is prevented and heat can reach the ice by conduction only. Since water is a poor conductor, the top water will boil without melting the ice. Steel Wool Ice Cubes

15. Try this at Home! • Find a pair of metal containers, one with a white or mirror-like surface and one with a black surface. • Fill the containers with hot water and place a thermometer in them. • The black container will cool faster because the black surface is a better emitter of radiant energy.

16. Why a Thermos Works The outer silvered walls of a thermos reflect radiant energy coming from surroundings, helping to keep cold fluids cold. The vacuum prevents heat conduction in either direction, helping to keep hot fluids hot and cold fluids cold. Inner silvered walls reflect radiant energy back to the fluid and your favorite Starbuck’s drink is kept hot!

17. Newton’s Law of Cooling • The rate of cooling of an object depends on how much hotter the object is than the surroundings. • A warm house will lose heat to the cold outside at a greater rate when there is a large difference between the inside and outside temperatures. • The rate of cooling (and warming) is approximately proportional to the temperature difference T between the object and its surroundings. Newton’s law of cooling: rate of cooling ~ T

18. Global Warming and the Greenhouse Effect • The greenhouse effect is a warming effect caused trapped long wavelength energy. Short wavelength radiant energy from the sun can enter the atmosphere and be absorbed by Earth more easily than long wavelength energy from Earth can leave. • Although the greenhouse effect is natural to the planet – and even necessary for life as we know – present environmental concerns are that increased levels of carbon dioxide and other gases may produce an unfavorable thermal balance in the future.

19. Thermodynamics • Thermodynamics is the study of heat. The word thermodynamics stem from Greek words meaning “movement of heat.” The science of thermodynamics was developed in the mid-1800s, before the atomic and molecular nature of matter was understood. • Previously, the study of heat has been concerned with microscopic behavior of jiggling atoms and molecules. However, thermodynamics bypasses the molecular details of systems and focuses on the macroscopic level – mechanical work, pressure, temperature, and their roles in energy transformation. • The foundation of thermodynamics is the conservation of energy and the fact that heat flows from hot to cold, and not the other way around.

20. Zeroth (0th) Law of Thermodynamics • The 0th Law of thermodynamics states that: • If object A is in thermal equilibrium with object B • and object B is in thermal equilibrium with object C • then object A is in thermal equilibrium with object C

21. First Law of Thermodynamics • The law of conservation of energy applied to thermal systems is called the first law of thermodynamics. • It states that the change in internal energy of a system is equal to or balanced by the heat added to the system minus the work done by the system.

22. Second Law of Thermodynamics • Heat will not flow spontaneously from a cold object to a hot object. The natural direction of heat flow is from a body (or reservoir) at a higher temperature to a body (or reservoir) at a lower temperature. • A device that converts thermal energy, by means of heat flow, into mechanical energy (useful work) is called a heat engine. • You cannot create a heat engine which extracts heat and converts 100% of that heat into useful work. • Any system which is free of external influences becomes more disordered with time. This disorder can be expressed in terms of the quantity called entropy.

23. Entropy • Natural Systems tend to proceed toward a state of disorder. • Example: A light bulb gives off energy and that energy is absorbed and transferred by the air molecules until it is dissipated into the surroundings. That energy is no longer usable energy. • Entropy (S) is the measure of the amount of disorder. As disorder increases, entropy increases. • Organized structures in time become disorganized messes, things left to themselves run down. • Whenever a physical system is allowed to distribute its energy freely, it always does so in a manner such that entropy increases while the available energy of the system for doing work decreases.

24. Absolute Zero and the Third Law • As the thermal motion of atoms increases, temperature increases. • There is believed to be no upper limit of temperature. • There is a lower limit: • The lower limit of thermal motion is Absolute Zero (The point where all thermal motion ceases to occur). • Absolute Zero is defined as: 0 K (Kelvin), -273°C, -459.69°F. • Absolute Zero is considered impossible to reach because at absolute zero there is zero kinetic energy – even at the subatomic level. • The Third Law of Thermodynamics states that it is impossible to reach absolute zero.

25. Thermostats A thermostat is a type of valve or switch that responds to changes in temperature and that is used to control the temperature of something. A bimetallic strip is used in thermostats to break and complete circuits. It is composed of two strips of different metals, such as one of brass and one of iron, welded or riveted together into one strip. Because the two substances expand at different rates, when heated or cooled the strip bends.

26. Expansion of Water Almost all liquids will expand when heated. Ice cold water does the opposite! Water at the melting point will actually contract when the temperature increases. Once 4C is reached, water will then began to expand.

27. Water Density Water is most dense at 4C and least dense when frozen.

28. Calculating Heat Energy The amount of heat energy needed to raise the temperature of a a substance depends on: • The amount of the substance • The specific heat of the substance • The change in temperature Q =mcDTwhere: Q = heat energy added/removed m = mass c = specific heat capacity T = change in temperature

29. Change of Phase • Matter exists in three common phases: • Solid – molecules are arranged in a rigid structure • Liquid – takes the shape of container • Gas – molecules far apart with a lot of kinetic energy • Matter can change from one phase (or state) to another. • The phase of matter depends upon its temperature and the pressure that is exerted upon it. • Changes of phase usually involve a transfer of energy.

30. Energy and Changes of Phase • Energy must be put into a substance to change its phase in the direction from solid to liquid to gas. • Energy must be extracted from a substance to change its phase in the direction from gas to liquid to solid.

31. Phase Changes of Water Temperature will remain constant during the change of phase until the change is complete.

32. Review Questions • Distinguish between temperature and heat. Answer: Temperature is a measure of the average kinetic energy of the molecules and atoms of a system. Heat is the amount of energy that is transferred from one place to another because of a temperature difference. • Distinguish between heat and internal energy. Answer: Internal energy is the sum of all energies stored in a substance. Heat is the energy that is transferred because of a temperature difference.

33. Review Questions • Why doesn't the Kelvin scale have any negative numbers? Answer: When the molecules and atoms of a system have an average kinetic energy of zero, they cannot give any net energy to their surroundings. The system cannot lose any energy and is at its coldest possible temperature. By agreement, the system is at zero Kelvin and cannot get any colder.

34. Review Questions • What determines the direction of heat flow? Answer: Heat always flow from high temperature objects to low temperature objects. Answer: It has a low specific heat capacity. • Distinguish between calorie and Calorie. Answer: One Calorie equals 1000 calories. Nutritionists use the term Calorie instead of kilocalorie. • Does a substance that heats up quickly for its mass, have a high or low specific heat capacity?

35. Emission of Radiant Energy • Radiation consists essentially of electromagnetic waves. • Good absorbers are also good emitters, poor absorbers are poor emitter. For example, a radio antenna is a good absorber and emitter of radio waves. • If a good absorber were not also a good emitter, then black objects would remain warmer than lighter colored objects and never come to thermal equilibrium with them. • When objects come to thermal equilibrium, each object is emitting as much energy as it is absorbing. • The rate at which an object radiates energy has been found to be proportional to the fourth power of the Kelvin temperature, T. • The rate of radiation is also proportional to the area A of the emitting object, so the rate at which energy leaves the object, ΔQ/Δt, is ΔQ Δt σ = Stefan-Boltzmann constant = 5.67 x 10-8W/m2K4 e= emissivity = a number between 0 and 1 that is characteristic of the material. Very black surfaces have emissivity close to 1 whereas shiny surfaces have e close to zero. = eσAT4(Stefan-Boltzmann equation)

36. PAP Thermodynamic Processes • Work = area under PV curve • Isothermal process: • is a process carried out at constant temperature • T= constant • ΔU = 0; Q = W • Isobaric process: • P = constant • W = PΔV • Isochoric process: • V = constant • W = 0 • Adiabatic process: • the process of compression or expansion of a gas so that no heat enters or leaves a system (no heat is exchanged). • When work is done on a gas by adiabatically compressing it, the gas gains internal energy and becomes warmer. • When a gas adiabatically expands, it does work on its surroundings and gives up internal energy, and thus becomes cooler. • Q= 0

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38. Net Flow of Heat Radiation • The net rate of radiant heat flow from the object is given by the equation 1 2 σ = Stefan-Boltzmann constant = 5.67 x 10-8W/m2K4 e = emissivity at temperature T1 A=surface area of the object T1=temperature of the object T2=temperature of the surroundings ΔQ Δt = eσA(T4 – T4)

41. Warm-up 1-5-2017Draw the picture or write the paragraph

45. Warm-up 1-6-17