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Thermodynamics

Thermodynamics. Physical Science Chapter 9. 9.1 Thermal Energy – The Total Energy in a Substance. All matter is made of moving molecules If the molecules are moving slowly = solid If the molecules are moving a little more quickly = liquid

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Thermodynamics

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  1. Thermodynamics Physical Science Chapter 9

  2. 9.1 Thermal Energy – The Total Energy in a Substance • All matter is made of moving molecules • If the molecules are moving slowly = solid • If the molecules are moving a little more quickly = liquid • If the molecules are moving very quickly = gas

  3. Name some actions that can make things warmer: • Striking it repeatedly • Putting it in a flame • Rubbing it between our hands • What happens to the molecules as they warm up? • The molecules move faster

  4. Thermal Energy • Thermal energy = the total energy of all the atoms and molecules in an object • Thermal energy consists of the potential (stored) energy and kinetic energy (energy of motion) of the particles in a substance

  5. 9.2 Temperature = Average Kinetic Energy per Molecule in a Substance

  6. Temperature • Temperature is the measure of how warm or cold an object is • Thermometers measure temperature by expansion or contraction of a liquid • Most common type of thermometer = Celsius

  7. How a thermometer works • When a thermometer is put into something hot, thermal energy flows from the hot substance to the thermometer • When thermal equilibrium is achieved, both the thermometer and the substance are the same temperature • That means the thermometer is measuring its own temperature!

  8. Temperature Scales Fill out the chart!

  9. So what happens to molecules when you heat something? • The molecules move more quickly! • What would happen then if you put a closed, sealed can on a hot stove? • The molecules inside would increase in temperature and pressure because the molecules will move more quickly!

  10. Heat, Temperature and Thermal Energy • Thermal energy = the energy due to molecular motion • Heat = the transfer of thermal energy • Temperature = the average kinetic energy per molecule

  11. 9.3 Absolute Zero – Nature’s Lowest Possible Temperature • Absolute zero – nature’s lowest possible temperature • The upper end of the temperature scale is infinite! Solid Liquid Gas Plasma

  12. Absolute Zero • The lower end of the temperature scale has a limit Absolute Zero • Absolute zero = 0 K • (-273° C) • At this temperature, molecules have lost all kinetic energy so the atoms stop vibrating

  13. 9.4 Heat is the movement of thermal energy

  14. 9.4 Heat • If you put your finger in a cup of hot tea, which way will the energy flow? • From tea into your finger • If you touch an ice cube, which way will the energy flow? • From your finger into the ice

  15. 9.4 Heat • True or False: A giant iceberg has more thermal energy than a cup of hot coffee • True! • Why? • Because if you added the motion of all the molecules in the iceberg together and compared it to the motion of all the molecules in the coffee, it would be much greater!

  16. Let’s Review: • Matter contains thermal energy • Heatis thermal energy in transit • Hot objects contain thermal energy • Cooler objects lack thermal energy • We feel cold when we lose thermal energy

  17. 9.5 Heat Units are Energy Units • Heat energy is measured in joules (J) • 4.2 J of heat raise the temp of 1 g of water 1° C = calorie (c) • Food calories are actually kilocalories; 1000 calories; Calorie (C)

  18. So why is it that we aren’t severely burned when we hold a sparkler on the 4th of July and the 2000˚C sparks touch our hand? • Even though the energy per molecule is high, the total energy that is transferred is low because of the size of the spark

  19. 9.6 The Laws of Thermodynamics First Law of Thermodynamics Whenever heat flows into or out of a system, the gain or loss of thermal energy equals the amount of heat transferred

  20. An example illustrating the First Law of Thermodynamics • If you were to pour a liter of water at 80˚C into a liter of water at 40˚C, what would be the final temperature of the water after combining them? • The average of the two which is 60˚ C • 80 + 40 = 120 ÷ 2 = 60 • The mathematics work because the amount of heat lost by one is gained by the other!

  21. Another example illustrating the First Law of Thermodynamics • If you were to place a 1kg block of iron at 100°C into a liter of water at 60°C ? • The average of the two which is 80˚ C • 100 + 60 = 160 ÷ 2 = 80 • Again, the amount of heat lost by the block of iron is gained by the water

  22. Remember, heat energy travels from an object with a high temperature to an object with lower temperature!

  23. The Laws of Thermodynamics The Second Law of Thermodynamics Heat never spontaneously flows from a cold substance to a hot substance

  24. The Laws of Thermodynamics The Third Law of Thermodynamics No system can reach absolute zero

  25. 9.7 Specific Heat Capacity – A Measure of Thermal Inertia • Some substances get hot faster than others • Some substances remain hot longer than others • This phenomenon is explained by specific heat capacity

  26. Specific Heat Specific Heat – the amount of heat necessary to raise a unit mass of that particular substance 1°C Every substance has a unique specific heat capacity

  27. Look at the Specific Heat chart: Which of the following would warm up faster when heat is applied: water, iron, glass or wood? • Iron • Its specific heat is lower than the others, so it warms up faster AND cools off faster too!

  28. Specific Heat of Water • Water has a relatively high specific heat • For example: Water heats up more slowly than sand (so it takes more energy to raise its temperature 1°C), but it also cools down much more slowly

  29. Desert sand is very hot in the day and very cold at night • Does this mean sand has a high specific heat or a low specific heat • Low!

  30. Materials with high specific heat capacities are able to absorb large amounts of energy in the form of internal vibrations and rotations

  31. Specific Heat of Water Because of the high specific heat of water, coastal regions experience milder climates

  32. Moderate Temperature Temp difference = 9o C (16.2o F) Temp difference = 27o C (48.6o F)

  33. If the specific heat capacity of water were lower than it is, would ponds be more or less likely to freeze in the winter? Freezing would be more likely! This would be bad of any aquatic life 

  34. 9.8 Thermal Expansion • Most substances expand when heated and contract when cooled • Examples: telephone wires, railroad tracks, roads, bridges • Application: Thermostats http://www.youtube.com/watch?v=V0ETKRz2UCA&feature=related

  35. Bimetallic Strips • If the bimetallic strip in your thermostat at home contains iron and copper strips, the bar will bend toward iron. • This is because the copper expands more than the iron. • This is what turns your furnace on and off.

  36. A bar made from two different metals is placed over a flame. The different thermal expansion rates cause the bar to bend.

  37. When heated, gases will expand more than liquids (they will also contract more when cooled) And liquids will expand more than solids Water (and ice) are the exceptions! When water at 4°C is heated, it expands When water at 4°C is cooled, it still expands Expansion Rates of Substances

  38. When an iron ring is heated, will the hole become larger or smaller? Larger. Why? Every part of the ring increases by the same amount Also, as the piece of metal with a hole in it cools, the diameter of the hole decreases at the same rate

  39. See the next slide for answer!

  40. Expansion of Water • Why does ice float in a glass of water or on the surface of a pond? • Because ice is less dense than water • Why • Because ice is made of open-structured hexagonal crystals

  41. Expansion of Water • Water is a unique substance • When water at 4°C is heated, it expands • When water at 4°C is cooled, it still expands! • If you continue cooling it though, it will eventually contract • Ice cubes

  42. Before ice can form on a lake, all the water in the lake must be cooled to 4°C At this temperature, water is most dense Because water is most dense at 4°C, it sinks to the bottom of the pond and remains unfrozen and at 4°C Expansion of Water

  43. If the water temperature falls below 4°C closer to the surface, then the water begins to expand This is because microscopic slush in the water makes the water less dense This is when ice forms at the surface

  44. Part of an iceberg extends above the surface of the water This is because the ice is less dense than the water The volume that extends above the surface is equal to the volume of the number of open spaces in the hexagonal ice crystals! For example:

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