Lecture 9: Temperature & Heat

Lecture 9: Temperature & Heat

Chapter 17Temperature & Thermal Expansion

Units of Chapter 17 • Atomic Theory of Matter • Temperature and Thermometers • Thermal Equilibrium and the Zeroth Law of Thermodynamics • Thermal Expansion • Ideal Gas Temperature Scale—a Standard

17-2 Temperature and Thermometers Temperature is a measure of how hot or cold something is. Most materials expand when heated.

17-2 Temperature and Thermometers Thermometers are instruments designed to measure temperature. In order to do this, they take advantage of some property of matter that changes with temperature. Early thermometers:

17-2 Temperature and Thermometers Common thermometers used today include the liquid-in-glass type and the bimetallic strip.

17-2 Temperature and Thermometers Temperature is generally measured using either the Fahrenheit or the Celsius scale. The freezing point of water is 0°C, or 32°F; the boiling point of water is 100°C, or 212°F.

Temperature conversion • TF = 9/5 TC + 32 • TK = TC + 273.15

17-2 Temperature and Thermometers Example 17-2: Taking your temperature. Normal body temperature is 98.6°F. What is this on the Celsius scale?

17-2 Temperature and Thermometers A constant-volume gas thermometer depends only on the properties of an ideal gas, which do not change over a wide variety of temperatures. Therefore, it is used to calibrate thermometers based on other materials.

17-3 Thermal Equilibrium and the Zeroth Law of Thermodynamics Two objects placed in thermal contact will eventually come to the same temperature. When they do, we say they are in thermal equilibrium. The zeroth law of thermodynamics says that if two objects are each in equilibrium with a third object, they are also in thermal equilibrium with each other.

17-4 Thermal Expansion Linear expansion occurs when an object is heated. Here, α is the coefficient of linear expansion.

17-4 Thermal Expansion

17-4 Thermal Expansion Example 17-3: Bridge expansion. The steel bed of a suspension bridge is 200 m long at 20°C. If the extremes of temperature to which it might be exposed are -30°C to +40°C, how much will it contract and expand?

17-4 Thermal Expansion Conceptual Example 17-4: Do holes expand or contract? If you heat a thin, circular ring in the oven, does the ring’s hole get larger or smaller?

17-4 Thermal Expansion Example 17-5: Ring on a rod. An iron ring is to fit snugly on a cylindrical iron rod. At 20°C, the diameter of the rod is 6.445 cm and the inside diameter of the ring is 6.420 cm. To slip over the rod, the ring must be slightly larger than the rod diameter by about 0.008 cm. To what temperature must the ring be brought if its hole is to be large enough so it will slip over the rod?

17-4 Thermal Expansion Conceptual Example 17-6: Opening a tight jar lid. When the lid of a glass jar is tight, holding the lid under hot water for a short time will often make it easier to open. Why?

17-4 Thermal Expansion Volume expansion is similar, except that it is relevant for liquids and gases as well as solids: Here, β is the coefficient of volume expansion. For uniform solids, β ≈ 3α.

17-4 Thermal Expansion Example 17-7: Gas tank in the Sun. The 70-liter (L) steel gas tank of a car is filled to the top with gasoline at 20°C. The car sits in the Sun and the tank reaches a temperature of 40°C (104°F). How much gasoline do you expect to overflow from the tank?

17-4 Thermal Expansion Water behaves differently from most other solids—its minimum volume occurs when its temperature is 4°C. As it cools further, it expands, as anyone who leaves a bottle in the freezer to cool and then forgets about it can testify.

17-10 Ideal Gas Temperature Scale—a Standard This standard uses the constant-volume gas thermometer and the ideal gas law. There are two fixed points: Absolute zero—the pressure is zero here The triple point of water (where all three phases coexist), defined to be 273.16 K—the pressure here is 4.58 torr.

17-10 Ideal Gas Temperature Scale—a Standard Then the temperature is defined as: In order to determine temperature using a real gas, the pressure must be as low as possible.

Summary of Chapter 17 • Temperature is a measure of how hot or cold something is, and is measured by thermometers. • There are three temperature scales in use: Celsius, Fahrenheit, and Kelvin. • When heated, a solid will get longer by a fraction given by the coefficient of linear expansion.

Summary of Chapter 17 • The fractional change in volume of gases, liquids, and solids is given by the coefficient of volume expansion.

Chapter 19Heat

Units of Chapter 19 • Heat as Energy Transfer • Internal Energy • Specific Heat • Calorimetry—Solving Problems • Latent Heat • Molar Specific Heats for Gases • Heat Transfer: Conduction, Convection, • Radiation

19-1 Heat as Energy Transfer We often speak of heat as though it were a material that flows from one object to another; it is not. Rather, it is a form of energy. Unit of heat: calorie (cal) 1 cal is the amount of heat necessary to raise the temperature of 1 g of water by 1 Celsius degree. Don’t be fooled—the calories on our food labels are really kilocalories (kcal or Calories), the heat necessary to raise 1 kg of water by 1 Celsius degree.

19-1 Heat as Energy Transfer If heat is a form of energy, it ought to be possible to equate it to other forms. The experiment below found the mechanical equivalent of heat by using the falling weight to heat the water: 4.186 J = 1 cal 4.186 kJ = 1 kcal

19-1 Heat as Energy Transfer Definition of heat: Heat is energy transferred from one object to another because of a difference in temperature. • Remember that the temperature of a gas is a measure of the kinetic energy of its molecules.

19-1 Heat as Energy Transfer Example 19-1: Working off the extra calories. Suppose you throw caution to the wind and eat too much ice cream and cake on the order of 500 Calories. To compensate, you want to do an equivalent amount of work climbing stairs or a mountain. How much total height must you climb?

19-3 Specific Heat The amount of heat required to change the temperature of a material is proportional to the mass and to the temperature change: The specific heat, c, is characteristic of the material. Some values are listed at left.

19-3 Specific Heat • Example 19-2: How heat transferred depends on specific heat. • How much heat input is needed to raise the temperature of an empty 20-kg vat made of iron from 10°C to 90°C? • (b) What if the vat is filled with 20 kg of water?

19-4 Calorimetry—Solving Problems Closed system: no mass enters or leaves, but energy may be exchanged Open system: mass may transfer as well Isolated system: closed system in which no energy in any form is transferred For an isolated system, energy out of one part = energy into another part, or: heat lost = heat gained.

19-4 Calorimetry—Solving Problems Example 19-3: The cup cools the tea. If 200 cm3 of tea at 95°C is poured into a 150-g glass cup initially at 25°C, what will be the common final temperature T of the tea and cup when equilibrium is reached, assuming no heat flows to the surroundings?

19-4 Calorimetry—Solving Problems The instrument to the left is a calorimeter, which makes quantitative measurements of heat exchange. A sample is heated to a well-measured high temperature and plunged into the water, and the equilibrium temperature is measured. This gives the specific heat of the sample.

19-4 Calorimetry—Solving Problems Example 19-4: Unknown specific heat determined by calorimetry. An engineer wishes to determine the specific heat of a new metal alloy. A 0.150-kg sample of the alloy is heated to 540°C. It is then quickly placed in 0.400 kg of water at 10.0°C, which is contained in a 0.200-kg aluminum calorimeter cup. (We do not need to know the mass of the insulating jacket since we assume the air space between it and the cup insulates it well, so that its temperature does not change significantly.) The final temperature of the system is 30.5°C. Calculate the specific heat of the alloy.

19-5 Latent Heat Energy is required for a material to change phase, even though its temperature is not changing.

19-5 Latent Heat Heat of fusion, LF: heat required to change 1.0 kg of material from solid to liquid Heat of vaporization, LV: heat required to change 1.0kg of material from liquid to vapor

19-5 Latent Heat The total heat required for a phase change depends on the total mass and the latent heat: Example 19-5: Will all the ice melt? A 0.50-kg chunk of ice at -10°C is placed in 3.0 kg of “iced” tea at 20°C. At what temperature and in what phase will the final mixture be? The tea can be considered as water. Ignore any heat flow to the surroundings, including the container.

19-5 Latent Heat • Problem Solving: Calorimetry • Is the system isolated? Are all significant sources of energy transfer known or calculable? • Apply conservation of energy. • If no phase changes occur, the heat transferred will depend on the mass, specificheat, and temperaturechange. • (continued)

19-5 Latent Heat 4. If there are, or may be, phase changes, terms that depend on the mass and the latent heat may also be present. Determine or estimate what phase the final system will be in. 5. Make sure that each term is in the right place and that all the temperature changes are positive. 6. There is only one final temperature when the system reaches equilibrium. 7. Solve.

19-5 Latent Heat Example 19-6: Determining a latent heat. The specific heat of liquid mercury is 140 J/kg·°C. When 1.0 kg of solid mercury at its melting point of -39°C is placed in a 0.50-kg aluminum calorimeter filled with 1.2 kg of water at 20.0°C, the mercury melts and the final temperature of the combination is found to be 16.5°C. What is the heat of fusion of mercury in J/kg?

19-5 Latent Heat The latent heat of vaporization is relevant for evaporation as well as boiling. The heat of vaporization of water rises slightly as the temperature decreases. On a molecular level, the heat added during a change of state does not go to increasing the kinetic energy of individual molecules, but rather to breaking the close bonds between them so the next phase can occur.

Heat Transfer: Conduction Q/t = k A (T1 – T2)/L Q/t = Rate of heat flow k = thermal conductivity A = cross-sectional area of the object (T1 – T2)/L = temperature gradient

19-10 Heat Transfer: Conduction Example 19-13: Heat loss through windows. A major source of heat loss from a house is through the windows. Calculate the rate of heat flow through a glass window 2.0 m x 1.5 m in area and 3.2 mm thick, if the temperatures at the inner and outer surfaces are 15.0°C and 14.0°C, respectively.

19-10 Heat Transfer: Conduction Building materials are measured using R-values rather than thermal conductivity: Here, is the thickness of the material.

19-10 Heat Transfer: Convection Convection occurs when heat flows by the mass movement of molecules from one place to another. It may be natural or forced; both these examples are natural convection.

19-10 Heat Transfer: Radiation Radiation is the form of energy transfer we receive from the Sun; if you stand close to a fire, most of the heat you feel is radiated as well. The energy radiated has been found to be proportional to the fourth power of the temperature:

19-10 Heat Transfer: Conduction, Convection, Radiation The constant σ is called the Stefan-Boltzmann constant: The emissivityε is a number between 0 and 1 characterizing the surface; black objects have an emissivity near 1, while shiny ones have an emissivity near 0. It is the same for absorption; a good emitter is also a good absorber.

19-10 Heat Transfer: Radiation Thermography—the detailed measurement of radiation from the body—can be used in medical imaging. Warmer areas may be a sign of tumors or infection; cooler areas on the skin may be a sign of poor circulation.

Lecture 9: Temperature & Heat