Thermal Energy & Thermodynamics

1. Thermal Energy & Thermodynamics

2. Heat • Heat – The transfer of thermal energy from one object to another because of a temperature difference. • Flows spontaneously from hot objects to cold objects

3. Heat • Heat – The transfer of thermal energy from one object to another because of a temperature difference. • Flows spontaneously from hot objects to cold objects

4. Temperature • Measure of how hot or cold an object is compared to a reference point. • Celsius Scale: Boiling and freezing points of water • Kelvin Scale: Absolute zero

5. Zeroth Law of Thermodynamics • If objects A and B are separately in thermal equilibrium with a third object, C, then A and B are in thermal equilibrium with each other. • Allows a definition of temperature

6. Temperature from the Zeroth Law • Two objects in thermal equilibrium with each other are at the same temperature • Temperature is the property that determines whether or not an object is in thermal equilibrium with other objects

7. Thermometers • Used to measure the temperature of an object or a system • Make use of physical properties that change with temperature • Many physical properties can be used • volume of a liquid • length of a solid • pressure of a gas held at constant volume • volume of a gas held at constant pressure • electric resistance of a conductor • color of a very hot object

8. Celsius Scale • Temperature of an ice-water mixture is defined as 0º C • This is the freezing point of water • Temperature of a water-steam mixture is defined as 100º C • This is the boiling point of water • Distance between these points is divided into 100 segments or degrees

9. Kelvin Scale • When the pressure of a gas goes to zero, its temperature is –273º C • This temperature is called absolute zero • This is the zero point of the Kelvin scale • –273º C = 0 K • To convert: TC = TK – 273 • The size of the degree in the Kelvin scale is the same as the size of a Celsius degree

10. Pressure-Temperature Graph • All gases extrapolate to the same temperature at zero pressure • This temperature is absolute zero

11. Some KelvinTemperatures • Some representative Kelvin temperatures • Absolute zero has never been reached

12. Comparing Temperature Scales

13. Temperature • Determined by the kinetic energy of the particles in an object. • More thermal energy = faster motion of particles • Faster moving particles lose energy to slow moving particles in collisions

14. Thermal Energy • Depends on: • Mass • Temperature • Phase

15. Thermal Energy vs. Temperature • Which has more thermal energy, a beaker of hot water or the water in my water bottle? • Thermal energy depends on mass

16. Kinetic Theory of Gases • The number of molecules in the gas is large and the average separation between them is large compared to their dimensions • The molecules obey Newton’s laws of motion, but as a whole they move randomly

17. Kinetic Theory of Gases – cont. • The molecules interact only by short-range forces during elastic collisions • The molecules make elastic collisions with the walls • The gas under consideration is a pure substance, all the molecules are identical

18. Specific Heat • The amount of energy needed to raise the temperature of one gram of a material by one degree Celsius. • Water – 4.18 J/g·oC • Plastic (polypropylene) – 1.84-2.09 J/g·oC • Iron – 0.449 J/g·oC

19. Specific Heat

21. In setting up an aquarium, the heater transfers 1,200kJ of heat to 75,000g of water. What is the increase in the water’s temperature?

22. A Consequence of Different Specific Heats • Water has a high specific heat compared to land • On a hot day, the air above the land warms faster • The warmer air flows upward and cooler air moves toward the beach

23. Heat Compared to Internal Energy • Important to distinguish between them • They are not interchangeable • They mean very different things when used in physics

24. Internal Energy • Internal Energy, U, is the energy associated with the microscopic components of the system • Includes kinetic and potential energy associated with the motion and position of the atoms or molecules

25. Heat • Heat is the transfer of energy between a system and its environment because of a temperature difference between them • The symbol Q is used to represent the amount of energy transferred by heat between a system and its environment

26. Units of Heat • Calorie • An historical unit, before the connection between thermodynamics and mechanics was recognized • A calorie is the amount of energy necessary to raise the temperature of 1 g of water from 14.5° C to 15.5° C . • A Calorie (food calorie) is 1000 cal

27. Units of Heat, cont. • US Customary Unit – BTU • BTU stands for British Thermal Unit • A BTU is the amount of energy necessary to raise the temperature of 1 lb of water from 63° F to 64° F • 1 cal = 4.186 J • This is called the Mechanical Equivalent of Heat

28. Calorimeter • One technique for determining the specific heat of a substance • A calorimeter is a vessel that is a good insulator which allows a thermal equilibrium to be achieved between substances without any energy loss to the environment

29. Calorimetry • Analysis performed using a calorimeter • Conservation of energy applies to the isolated system • The energy that leaves the warmer substance equals the energy that enters the water • Qcold = -Qhot • Negative sign keeps consistency in the sign convention of ΔT

30. Phase Changes • A phase change occurs when the physical characteristics of the substance change from one form to another • Common phases changes are • Solid to liquid – melting • Liquid to gas – boiling • Phases changes involve a change in the internal energy, but no change in temperature

31. Latent Heat • During a phase change, the amount of heat is given as • Q = ±m L • L is the latent heat of the substance • Latent means hidden • L depends on the substance and the nature of the phase change • Choose a positive sign if you are adding energy to the system and a negative sign if energy is being removed from the system

32. Latent Heat, cont. • SI units of latent heat are J / kg • Latent heat of fusion, Lf, is used for melting or freezing • Latent heat of vaporization, Lv, is used for boiling or condensing

34. Sublimation • Some substances will go directly from solid to gaseous phase • Without passing through the liquid phase • This process is called sublimation • There will be a latent heat of sublimation associated with this phase change

35. Graph of Ice to Steam

36. Methods of Heat Transfer • Need to know the mechanisms responsible for the transfer • Methods include • Conduction • Convection • Radiation

37. Conduction • The transfer of thermal energy with no transfer of matter. • Between particles in the same material • Between materials that are in contact with each other.

38. Conduction • The transfer can be viewed on an atomic scale • It is an exchange of energy between microscopic particles by collisions • Less energetic particles gain energy during collisions with more energetic particles • Rate of conduction depends upon the characteristics of the substance

39. Conduction example • The molecules vibrate about their equilibrium positions • Particles near the stove coil vibrate with larger amplitudes • These collide with adjacent molecules and transfer some energy • Eventually, the energy travels entirely through the pan and its handle

40. Conduction, cont. • In general, metals are good conductors • They contain large numbers of electrons that are relatively free to move through the metal • They can transport energy from one region to another • Conduction can occur only if there is a difference in temperature between two parts of the conducting medium

41. Thermal Conductors and Insulators • Thermal Conductor – A material that easily conducts thermal energy • Metal • Thermal Insulator – A material that does not conduct thermal energy well. • Wood • Air • Rubber

42. Convection • The transfer of thermal energy when particles of a fluid move from one place to another • Convection Current – Fluid that circulates in a loop as it heats up and cools down.

43. Convection example • Air directly above the flame is warmed and expands • The density of the air decreases, and it rises • The mass of air warms the hand as it moves by

44. Convection Current Example • The radiator warms the air in the lower region of the room • The warm air is less dense, so it rises to the ceiling • The denser, cooler air sinks • A continuous air current pattern is set up as shown

45. Global Ocean Currents

46. Radiation • The transfer of energy by waves moving through space. • All objects radiate energy • The higher an object’s temperature, the more energy it radiates.

47. Radiation • Radiation does not require physical contact • All objects radiate energy continuously in the form of electromagnetic waves due to vibrations of the molecules

48. Radiation example • The electromagnetic waves carry the energy from the fire to the hands • No physical contact is necessary • Cannot be accounted for by conduction or convection

49. Applications of Radiation • Clothing • Black fabric acts as a good absorber • White fabric is a better reflector • Thermography • The amount of energy radiated by an object can be measured with a thermograph • Body temperature • Radiation thermometer measures the intensity of the infrared radiation from the eardrum

50. 1st Law of Thermodynamics • Energy is conserved. • Law of Conservation of Energy