Chapter 17 Temperature and Heat © 2016 Pearson Education Inc.

Learning Goals for Chapter 17 Thermal equilibrium - what thermometers REALLY measure. Absolute (Kelvin) temperature scale. How dimensions change as a result of temperature changes. Calculating heat flow, temperature changes, & changes of phase. Heat transfer by conduction, convection, and radiation. © 2016 Pearson Education Inc.

Introduction Does molten iron at 1500°C contain “heat”? What about at 150°C? Or 15°C? Or -150°C?? “Temperature” & “heat” have very different meanings, even though most people use them interchangeably. Focus on large-scale, or macroscopic, objects! (but in next chapter look at microscopic scale….) © 2016 Pearson Education Inc.

Temperature & Thermal Equilibrium Adding (or removing) heat energy can visibly change materials… Volume of liquid in a thermometer changes with temperature change. Definition: Two systems are in thermal equilibrium if and only if they have same temperature. Qin © 2016 Pearson Education Inc.

Temperature and Thermometers Common thermometers used today include liquid-in-glass type and bimetallic strip. Figure 17-5. (a) Mercury- or alcohol-in-glass thermometer; (b) bimetallic strip. Figure 17-6. Photograph of a thermometer using a coiled bimetallic strip.

The “Zeroth” law of thermodynamics If C is initially in thermal equilibrium with both A & B, then A & B are in thermal equilibrium with each other. © 2016 Pearson Education Inc.

Temperature scales Celsius (or centigrade) temperature scale: 0°C = freezing point of pure water 100°C = boiling point. Fahrenheit temperature scale, 32°F = freezing point of pure water 212°F = boiling point. To convert from Celsius to Fahrenheit: © 2016 Pearson Education Inc.

Absolute zero At −273.15°C, absolute pressure of ANY (and EVERY) gas would become zero. © 2016 Pearson Education Inc.

Temperature scales On Kelvin (absolute) temperature scale, 0 K is extrapolated temperature at which gas would exert no pressure. To convert from Celsius to Kelvin: © 2016 Pearson Education Inc.

Temperature conversions Temperatures have been rounded off to the nearest degree. © 2016 Pearson Education Inc.

Linear expansion 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? © 2016 Pearson Education Inc.

Linear thermal expansion Increasing temperature of a material can causes it to expand. For moderate changes in temperature, change in length is: © 2016 Pearson Education Inc.

Molecular basis for thermal expansion Model atoms as being held together by springs. When temperature increases, average distance between atoms also increases. As atoms get farther apart, every dimension increases. © 2016 Pearson Education Inc.

Molecular basis for thermal expansion Graph of “spring” potential energy versus distance between neighboring atoms is not symmetrical. As energy increases & atoms oscillate with greater amplitude, average distance increases. © 2016 Pearson Education Inc.

Add heat to something with a hole…? © 2016 Pearson Education Inc.

Expanding holes & volume expansion If an object has a hole in it, hole also expands with object! Hole does not shrink. Change in volume due to thermal expansion is is the coefficient of volume expansion = © 2016 Pearson Education Inc.

Linear expansion © 2016 Pearson Education Inc.

Linear expansion © 2016 Pearson Education Inc.

Coefficients of linear expansion © 2016 Pearson Education Inc.

Coefficients of volume expansion © 2016 Pearson Education Inc.

Example of thermal expansion Railroad track gap between segments for thermal expansion. On hot days, segments expand & fill in gap. If no gaps, track could buckle under very hot conditions. © 2016 Pearson Education Inc.

Example of thermal expansion 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? © 2016 Pearson Education Inc.

Example of thermal expansion 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? Is it less expensive to buy gas in the summer or winter, assuming it is the same price at the pump?? © 2016 Pearson Education Inc.

Thermal expansion of water Water is interesting! But between 0°C & 4°C, water decreases in volume with increasing temperature. Lakes freeze from top down instead of from bottom up! © 2016 Pearson Education Inc.

Thermal stress Change temperature of rod but prevent it from expanding or contracting, thermal stress develops. Expansion joints on bridges are needed to accommodate changes in length that result from thermal expansion. © 2016 Pearson Education Inc.

Quantity of heat Sir James Joule (1818–1889) studied how water can be warmed by vigorous stirring with a paddle wheel. © 2016 Pearson Education Inc.

Quantity of heat Same temperature change caused by stirring can also be caused by putting water in contact with hotter body. Calorie (cal) = amount of heat required to raise temperature of 1 gram of water from 14.5°C to 15.5°C. © 2016 Pearson Education Inc.

Specific heat Quantity of heat Q required to increase temperature of mass m of certain material by ΔT is: Specific heat c has different values for different materials. Specific heat of water ~ 4190 Joules/kg ∙ K Add 4190 Joules per kg of water to raise temp 1°C © 2016 Pearson Education Inc.

Molar heat capacity Quantity of heat Q required to increase temperature of n moles of certain material by ΔT is: Molar heat capacity C has different values for different materials. Molar heat capacity of water ~ 75.4 J/mol ∙ K. © 2016 Pearson Education Inc.

Specific heats and molar heat capacities © 2016 Pearson Education Inc.

Phase changes Phases (or states) of matter are solid, liquid, & gas. phase change is transition from one phase to another. Temperature does not change during a phase change. Latent heat L = heat per unit mass transferred in a phase change. © 2016 Pearson Education Inc.

Heat added to ice at a constant rate © 2016 Pearson Education Inc.

Heat of fusion Gallium is one of few elements that melts at room temperature. Melting temperature is 29.8°C Latent heat of fusion (from liquid to solid or back) is Lf = 8.04 × 104 J/kg. © 2016 Pearson Education Inc.

Heat of vaporization Water may be warm & it may be a hot day, but we will feel cold when first stepping out of swimming pool! As water evaporates from skin, it removes heat of vaporization from our bodies. It takes heat to turn liquid into gas. Latent heat of vaporization! © 2016 Pearson Education Inc.

Mechanisms of heat transfer In nature, energy naturally flows from higher temperature objects to lower temperature objects Three mechanisms of heat transfer are conduction, convection, and radiation. Conduction occurs within body or between two bodies in contact. Convection depends on motion of mass from one region of space to another. Radiation is heat transfer by electromagnetic radiation, such as sunshine, with no need for matter to be present in the space between bodies. © 2016 Pearson Education Inc.

Conduction of heat Heat flows from a higher to a lower temperature. RATE of heat flow (Joules/sec = Watts) depends upon Temperature difference DT Area connecting high & low temperature reservoirs Materials Involved (conductivity coefficient “k”) How far between hot & cold surfaces L © 2016 Pearson Education Inc.

Conduction of heat Consider a solid rod of conducting material with cross- sectional area A and length L. Left end of rod is kept @ TH Right at lower temperature TC Rate of heat transfer: © 2016 Pearson Education Inc.

Thermal conductivities of common substances k (W/m ∙ K) Silver 406 Copper 385 Aluminum 205 Wood 0.12 – 0.04 Concrete 0.8 Fiberglass 0.04 Styrofoam 0.027 © 2016 Pearson Education Inc.

Thermal conductivities of common substances © 2016 Pearson Education Inc.

Conduction of heat Re-write Heat Current as: 𝑑𝑄 𝑑𝑡 = 𝐴∆𝑇 𝐿 𝑘 𝑑𝑄 𝑑𝑡 = 𝐴∆𝑇 𝐿 𝑘 Call L/k = “R” © 2016 Pearson Education Inc.

Conduction of heat Re-write Heat Current as: 𝑑𝑄 𝑑𝑡 = 𝐴∆𝑇 𝑅 Call L/k = “R” © 2016 Pearson Education Inc.

Convection of heat Convection is transfer of heat by mass motion of fluid. Heating element in tip of submerged tube warms surrounding water, producing a complex pattern of free convection. © 2016 Pearson Education Inc.

Radiation of heat Radiation is transfer of heat by electromagnetic waves, such as visible light or infrared. False-color IR photograph reveals radiation emitted by various parts of body. Strongest emission comes from warmest areas; very little emission from bottle of cold Joules’ Stone Ale. Stefan-Boltzmann law: Heat current (Watts) in radiation is: © 2016 Pearson Education Inc.

Radiation & climate change Energy radiated by earth’s surface is mostly infrared. CO2 molecules in atmosphere readily absorb some infrared radiation & re-radiate part of it back down toward surface. © 2016 Pearson Education Inc.