0203Notes - Temperature and Heat What are they? What’s...

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Unformatted text preview: Temperature and Heat What are they? What’s the difference? Heat is the transfer of energy from one object to another— specifically, the kinetic energy of the random motions of particles in the objects. Temperature is a measurement that compares the average kinetic energy per particle (atom or molecule) in any two objects. If one object has a higher average than the other, the hotter object can impart some of that energy to the other—via heat flow. Thus temperature tells us which way heat will flow between two objects that are in thermal contact. 2/3/10 Oregon State University PH 202, Lecture 13 1 How do we quantify temperature? It’s an indicator of the average kinetic energy of the particles within any substance. But we can’t put a stopwatch on a molecule or atom to see how fast it’s bumping around (and besides, that speed varies). We need a temperature scale—a set of reference temperatures (using reliable natural phenomena) against which we measure all others. We choose convenient reference points and arbitrarily assign those temperature values: The boiling point of pure water at standard atmospheric pressure is assigned a temperature of 100°C (100 hundred degrees Celsius) or 212°F (212 degrees Fahrenheit). The freezing point of pure water at standard atmospheric pressure is assigned a temperature of 0°C or 32°F. Notice that the units of temperature are not energy. 2/3/10 Oregon State University PH 202, Lecture 13 2 How do we reconcile these different scales? We know that 1°C is the same change in temperature as 1.8°F. But since the Fahrenheit scale doesn’t start at zero like the Celsius scale, there’s also an addition factor of 32 degrees: TF = 1.8TC + 32 or TC = (TF – 32)/1.8 Q: What temperature would be the same on both the Celsius and Fahrenheit temperature scales? 1. –100° 2. –40° 3. –16° 4. Absolute Zero. 5. There’s no such temperature. 2/3/10 Oregon State University PH 202, Lecture 13 3 Of course, we can devise any such scale we want to, of course— using properties of water or any other readily available reference material. What temperature scale is used in SI? An absolute scale—one whose zero temperature (called absolute zero) signifies zero molecular kinetic energy. That scale is the Kelvin scale. A Kelvin (not “degree Kelvin”) is the same increment in temperature as a degree Celsius. But the Kelvin scale’s zero point corresponds to –273.15°C. 2/3/10 Oregon State University PH 202, Lecture 13 4 Thermometers It’s not convenient to carry around pots of boiling or freezing water. We need other devices to measure along (and subdivide) a temperature scale. These are thermometers, most of which use some variation on the following two phenomena: Thermal expansion: Increased temperature—at the same pressure —generally increases the volume of a given solid, liquid or gas. In other words, the mass expands; its density decreases. Examples: Simple liquid (tube) thermometer Coil thermostat Galileo’s thermometer Thermal compression: Increased temperature—at the same volume —increases the pressure in a given mass of gas. (This is how the Kelvin scale zero point was determined: extrapolating from known temperatures back to a theoretical zero pressure point.) 2/3/10 Oregon State University PH 202, Lecture 13 5 Thermal Expansion As things get hotter, they generally expand—true for solids, liquids and gases (at constant pressure). That is, they become less dense— they take up more space per kg. So a given object made of a given material will get larger (longer, wider, deeper, etc.). Why? Consider what holds the particles in a substance together in the first place. Chemical attraction forces overcome the “restlessness” of the particles’ thermal energy—their random-motion collisions and kinetic energies (which we assess by measuring temperature). But as the temperature (the average random kinetic energy per particle) rises, the particles’ additional energies can take them farther and farther afield—much as a more energetic projectile can go higher in its trajectory—or as two objects can bounce farther apart after a more energetic collision. 2/3/10 Oregon State University PH 202, Lecture 13 6 So, how much does an object expand as it warms up? That depends on the specific material in the object and how much hotter it gets— the change, T, in its temperature. Each substance has some characteristic coefficients of thermal expansion: Length (1-D): Area (2-D): Volume (3-D): Lf = Li(1+ Af = Ai(1+ Vf = Vi(1+ T) T) T) or or or L= Li T A = Ai T V= Vi T Fortunately, there’s an easy approximate relationship among , and for any substance: 2 3 (We can prove these, too—with a little algebra.) 2/3/10 Oregon State University PH 202, Lecture 13 7 What are the units of , and —the coefficients of thermal expansion? 1. K 2. °C 3. K-1 4. °C-1 5. These coefficients are unitless. 2/3/10 Oregon State University PH 202, Lecture 13 8 Of course, fluids expand only in the volumetric sense; they don’t maintain rigid dimensions in the sense of length, width or height. In a solid material, though, each of its dimensions (length, width, height) expands according to the coefficient of the material: L = Li T Moreover, a geometric analysis shows that any gap in a solid material expands just as if it were made of that material. So when two different solid materials undergo the same temperature change, they expand by different proportions; they don’t “fit together” anymore. Examples: A hot car radiator overflows. The lid of a sealed jar loosens when heated. 2/3/10 Oregon State University PH 202, Lecture 13 9 ...
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This note was uploaded on 03/27/2010 for the course PH 202 taught by Professor Staff during the Spring '08 term at Oregon State.

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