Chapter 21 Heat & Temperature

1. Chapter 21 Heat & Temperature Conceptual Physics Chapter 21

2. Temperature • Temperature is a measure of how hot or cold something is relative to a standard. • Typically, the temperature of a substance will be measured with a thermometer. Conceptual Physics Chapter 21

3. Thermometer • A thermometer measures temperature by showing the expansion or contraction of the liquid inside it. • The two most commonly used temperature scales are the Celsius scale and the Fahrenheit scale. Conceptual Physics Chapter 21

4. The Kelvin Scale • The temperature scale used most commonly in scientific research is the Kelvin scale. • The Kelvin temperature of a substance is directly proportional to the average kinetic energy of the molecules that make up the substance. Conceptual Physics Chapter 21

5. Absolute Zero • Zero on the Kelvin scale (absolute zero) corresponds to the lowest possible temperature – this is the point when the molecules of a substance would have no remaining kinetic energy to give up. • Absolute zero is only a theoretical temperature that corresponds to -273 °C. Conceptual Physics Chapter 21

6. Common Temperatures Conceptual Physics Chapter 21

7. Temperature Conversions • A Celsius temperature can be converted to a Kelvin temperature by adding 273. • A temperature change of 1 °C is equivalent to a temperature change of 1 K. Conceptual Physics Chapter 21

8. Temperature and Kinetic Energy • In the simplest case of an ideal gas, temperature is proportional to the average kinetic energy of molecular translational motion. • In solids and liquids, where molecules are more constrained and have potential energy, temperature is more complicated. Conceptual Physics Chapter 21

9. Temperature and Kinetic Energy • Temperature is not a measure of total kinetic energy. • Although a swimming pool full of water has far more total kinetic energy than a bucketful of the same water, the two have the same kinetic energy per molecule and therefore have the same temperature. Conceptual Physics Chapter 21

10. Temperature and Kinetic Energy Which has more molecular kinetic energy, the bucketful of warm water or the small cupful of higher-temperature water? Conceptual Physics Chapter 21

11. Internal Energy • Internal energy or thermal energy is a measure of the total energy of a body; this includes translational kinetic energy, rotational kinetic energy (due to the spinning motion of molecules) and potential energy (due to the forces between molecules). Conceptual Physics Chapter 21

12. Heat When two bodies of different temperature are in thermal contact with one another, thermal energy will flow from the body with the higher temperature to the body with the lower temperature – this process is called heat or heat flow. Conceptual Physics Chapter 21

13. Heat • It is incorrect to suggest that heat is contained in a body, just as it is incorrect to suggest that work is contained in a body. • Work can be done by one object on another; heat can transfer from one object to another. Conceptual Physics Chapter 21

14. Heat • Heat transfer is driven solely by temperature differences. • When a hot metal sample is placed in a bucket of room-temperature water, heat flows from the metal (which has less internal energy) to the water (which has more internal energy). • Heat always flows from hot to cold. Conceptual Physics Chapter 21

15. Thermal Equilibrium • Heat flows between objects in thermal contact until the objects reach the same temperature. At this point the objects are said to be in thermal equilibrium. • Two objects in thermal equilibrium do not necessarily have the same internal energy. Conceptual Physics Chapter 21

16. Thermal Equilibrium A thermometer reaches thermal equilibrium with the substance that it measures. Conceptual Physics Chapter 21

17. Measurement of Heat • The most commonly used unit for measuring heat transfer is the calorie. • The calorie is defined as the quantity of heat required to raise the temperature of one gram of water by one degree Celsius. • The kilocalorie is equal to 1000 calories; this is the unit used for rating foods. 1 Calorie = 1000 calories Conceptual Physics Chapter 21

18. Measurement of Heat • Since heat is a form of energy, it can also be measured in the SI unit for energy, the Joule. 1 calorie = 4.184 Joules Conceptual Physics Chapter 21

19. Question Suppose the same quantity of heat is added to two samples of water, each placed on a hot stove. The larger sample undergoes a temperature change of 15 °C. What is the temperature change of the smaller sample? Conceptual Physics Chapter 21

20. Specific Heat Capacity • Different substances have different abilities to store internal energy. • Absorbed energy can cause an increase in rotational kinetic energy or it could be stored as potential energy. • If the absorbed energy causes an increase in translational motion of the molecules, the substance experiences a temperature increase. Conceptual Physics Chapter 21

21. Specific Heat Capacity • Equal masses of different substances will experience different increases in temperature given the same heat transfer to the bodies. • A substance that can absorb large quantities of thermal energy and undergo only small temperature changes is said to have a high specific heat capacity. Conceptual Physics Chapter 21

22. Q c = m·ΔT Specific Heat Capacity • We can find the specific heat capacity, c, of a substance experimentally from: where Q is the heat transfer in Joules, m is the mass in kg, and ΔT is the temperature change in Kelvins. Conceptual Physics Chapter 21

23. Specific Heat Capacity Conceptual Physics Chapter 21

24. Question Suppose a red-hot piece of iron is put into a bucket of cool water. In what direction does the heat transfer occur? Which is greater, the heat lost by the hot iron or the heat gained by the water? Upon reaching thermal equilibrium, which undergoes the greater change in temperature, the iron or the water? Conceptual Physics Chapter 21

25. Thermal Inertia of Water • Water has a very high thermal inertia – it resists changes in temperature. • Water is a good coolant. • Water moderates the local climate. Conceptual Physics Chapter 21

26. Question Why does the sand at the beach get much hotter than the ocean water during the day and yet get much cooler than the ocean water at night? Conceptual Physics Chapter 21

27. Question Why is the apple filling in a slice of hot pie far more likely to burn your mouth than the crust? Conceptual Physics Chapter 21

28. Thermal Expansion The atoms or molecules that make up a substance (even a solid substance) are in constant motion. When the substance is heated, the atoms vibrate more rapidly and the amplitude of the vibrations increases. This results in expansion of the substance. Conceptual Physics Chapter 21

29. Thermal Expansion • Nearly all substances, regardless of its state (solid, liquid or gas) will expand when heated and contract when cooled. • Gases will generally expand or contract more than liquids, and liquids will expand or contract more than solids. • Water at or near the freezing point is a notable exception to this rule. Conceptual Physics Chapter 21

30. Thermal Expansion Expansion joints are used in concrete sidewalks, bridges and other applications to avoid potential damage to these structures. Without expansion joints, the results can be disastrous. Conceptual Physics Chapter 21

31. Thermal Expansion The amount of expansion of a body, ΔL, is dependant on the initial amount of the material, L0 the change in temperature of the body, ΔT and the type of material, specifically the coefficient of linear expansion of the material, α ΔL = L0 α ΔT ΔL can represent any linear dimension of a body. Conceptual Physics Chapter 21

32. Thermal Expansion • When liquids expand, they will expand equally in all dimensions – liquids undergo a volumetric expansion. ΔV = V0 β ΔT where β is the coefficient of volumetric expansion. Conceptual Physics Chapter 21

33. Thermal Expansion In a bimetallic strip, two different metals are welded together back-to-back. This difference is also apparent when the device is cooled. When the device is heated, the different rates of expansion are apparent. Conceptual Physics Chapter 21

34. Thermal Expansion When the temperature of a metal ring increases, does the size of the hole increase, decrease or remain the same? Conceptual Physics Chapter 21

35. Thermal Expansion of Water • Most substances, when heated, will expand, but water at the freezing point actually contracts when heated! • This unusual behavior of water is due to the open-celled, hexagonal structure of ice crystals. • Water molecules will occupy a smaller volume in the liquid state than in a solid state. • Once the water is heated above 4°C, it begins to expand with further heating just like most substance do. Conceptual Physics Chapter 21

36. Biological Impacts • Because of the unusual behavior of ice-cold water, lakes must freeze from the top down and will very rarely freeze completely to the bottom. Conceptual Physics Chapter 21