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Chapter 17. Temperature and Heat. Goals for Chapter 17. To delineate the three different temperature scales To describe thermal expansion and thermal stress To consider heat, phase changes, and calorimetry To study how heat flows with convection, conduction, and radiation. Introduction.
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Chapter 17 Temperature and Heat
Goals for Chapter 17 • To delineate the three different temperature scales • To describe thermal expansion and thermal stress • To consider heat, phase changes, and calorimetry • To study how heat flows with convection, conduction, and radiation
Introduction • Growing up in Pittsburgh, molten steel was a common sight. Still it is imposing at 1500oC. • The worst common burns you can imagine are steam burns. You have not only water heated to its boiling point but gaseous steam carrying the heat of vaporization. It’s a great deal of energy in a small space.
Measuring temperature • There are many ways to measure temperature, but the two devices mentioned below take advantage of a gas or liquid sample which expands if heat is added and contracts if heat is removed. • A cylinder of gas will show pressure rise if volume is kept constant. • A small container of liquid will see the liquid increase in volume as temperatures rise. Mercury was chosen “early on” because it’s so dense, a small volume can record large temperature ranges.
The zeroth law of thermodynamics • Simply stated? “Heat will always travel from a hot reservoir to a cold one without outside energy forcing an unnatural transfer.”
Thermometers? Just our way of trying to see where the heat is • The measure of temperature is a way of expressing how much heat one object is holding relative to another. • There are several examples shown at right. You can base a thermometer on thermal expansion of a gas, differential expansion of bimetal strips, even on something as wild as laser-doppler shift.
The coldest we can ever get? • Early experiments observed changes in pressure or volume as temperature changed. • It was noticed that the linear trends lead to a consistent lowest temperature that we call “absolute zero”—labeled 0K after Lord Kelvin. • Refer to Example 17.1.
Conversions are expected • Values on the temperatures scales (Fahrenheit, Centigrade/Celsius, and Kelvin) may be readily interconverted. Physics professors will want values to eventually be in Kelvins because that’s the form in SI units. • See Figure 17.7 below.
Thermal expansion—linear • A change in length will accompany a change in temperature. The size of the change will depend on the material.
Changing temperature changes atomic spacing • Molecules can be visualized as bedsprings and spheres. More heat (higher temperatures) is reflected by the motion of the atoms relative to each other. • See Figure 17.9 below.
Thermal changes in material length and volume • Refer to Problem-Solving Strategy 17.1. • Consult Example 17.2 (change in length). • Consult Example 17.3 (change in length II). • Consult Example 17.4 (change in volume).
Thermal expansion we see constantly • Water is interesting. There are no other liquids that expand to become less dense as a solid than they are as a liquid. This is fortunate, if lakes were to freeze and dense ice sink to the bottom, everything in the water would die as the liquid became solid from the bottom up. • Thermal expansion joints allow roads to expand and contract without any stress to the material used to build. • Refer to Example 17.5.
James Joule and the mechanical equivalent of heat • Joule knew a mass above the ground had potential energy. He dropped an object on a cord, turning a paddle in water monitored by a very accurate thermometer. • His conclusion was to connect energy conservation (potential and kinetic) to heat as a third form observed.
Specific heat • A specific heat value reveals how much temperature will change when a given amount of a substance absorbs a given amount of heat. • Water is a “benchmark” as one ml of water will absorb 1 cal of heat to raise its temperature by 1oC. • Refer to Example 17.6 and Example 17.7.
Phase changes and temperature behavior • A solid will absorb heat according to its heat capacity, becoming a hotter solid. • At the melting point, a solid will absorb its heat of fusion and become a liquid. An equilibrium mixture of a substance in both its liquid and solid phases will have a constant temperature. • A cold liquid will absorb heat according to its heat capacity to become a hotter liquid. • At the boiling point, a liquid will absorb its heat of vaporization and become a gas. An equilibrium mixture of liquid and gas will have a constant temperature. • A cold gas can absorb heat according to its heat capacity and become a hotter gas.
Using well-behaved water to measure other systems • Because water is a good thermal sink, is readily available, and reproducibly absorbs 4.184 J for every gram to rise in temperature by 1oC, it is often used to measure another object’s change in heat energy by comparison. • For example, an unknown metal might be massed, raised to a known temperature (say to 100oC in a boiling water bath), then added to a known amount of cold water. The resulting change in the temperature of the water will allow heat absorbed to be calculated and then the heat capacity of the unknown metal.
Heat calculations • Follow Problem-Solving Strategy 17.2. • Refer to Example 17.8 (no phase change). • Refer to Example 17.9 (changes in both temperature and phase). • Refer to Example 17.10 (an example that could be done in a kitchen). • Refer to Example 17.11 (combustion, temperature change, and phase change).
Why, and how well, do materials transfer heat? • Figure 17.23 illustrates the model. • Table 17.5 lists thermal conductivities. They are dramatically different, from very large values for conductors likemetals to very small values for insulatorslike styrofoam or wood. • Consider Problem-Solving Strategy 17.3.
Conduction of heat I • Consider Example 17.12. • What makes a picnic cooler effective? • Figure 17.25 at right illustrates the problem.
Conduction of heat II • Consider Example 17.13. • This is a good reason not to pick up a metal frying pan by its bare handle. • Figure 17.26 below illustrates the problem.
Conduction of heat III • Consider Example 17.14. • There are variations of the metal bar problem. • Figure 17.27 below illustrates the problem.
Convection of heat • Heating by moving large amounts of hot fluid, usually water or air. • Figure 17.28 at right illustrates heat moving by convection.
Radiation of heat • Infrared lights, hot metal objects, a fireplace, standing near a running furnace … these are all objects heating others by broadcast of EM radiation just lower in energy than visible red. • Consider Example 17.15. • Consider Example 17.16.
