Unit 5 Thermodynamics

Transcription

1 Unit 5 Thermodynamics Unit 13: Heat and Temperature Unit 14: Thermal Expansion /Heat Exchange/ Change of Phase Test: Units 13-14

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3 Thermal Energy The total kinetic and potential energy of all the molecules of a body due to their motion. (Internal Energy) Heat Thermal energy that is transferred from one object to another because of the difference in temperature.

4 Temperature: The physical quantity that tells how warm or cold a body is with respect to some standard body. 1. Proportional to the average kinetic energy of the particles of a body. 2. Determines the direction in which thermal energy flows between substances.

5 Temperature and Heat Measurements / Units Temperature is measured by an instrument called a Thermometer. 4 Types of Thermometers: 1. Solid 2. Liquid 3. Gas 4. Thermocouple

6 Solid Thermometers (Metal) Contains a bi-metal coil (2 metals bonded together with different expansion rates) which winds or unwinds when temperature changes. (Typically brass and steel/iron) The bimetallic strip thermometer, because it is made of metal, is good at controlling very sensitive things. (House furnace, ovens, refrigerator, etc.) The longer the bimetal element the better accuracy.

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13 Liquid Thermometer Simply put, alcohol and mercury expand when heated. In a standard bulb thermometer, this means the liquid will rise as the temperature increases. Mercury (silver): Used in hotter temps (-40ºC to boiling Point) Alcohol (red or blue dye): Used in colder temps (-129ºC to 78ºC)

14 Glass tube filled with liquid (often mercury or alcohol) that expands/contracts with air temperature

15 Gas Thermometer A Gas filled bulb to which a pressure gauge is attached. Usually this gas is hydrogen or helium. This is used on a large range of temperatures. Gas thermometers measure temperature by changes in the pressure of a gas kept at constant volume

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17 Thermocouple A device consisting of two different conductors (usually metal alloys) that produce a voltage, proportional to a temperature difference, between either end of the two wires. Applications include temperature measurement for large gas ovens, diesel engines, and other industrial processes. Temperatures range from -270ºC to 700ºC.

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19 Note: The three types of thermometers (solid, liquid and gas) all operate on the principle that substances expand when heated and contract when cooled. Making of a thermometer:

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21 Four major Temperature scales: (We will only deal with three) Celsius Fahrenheit Kelvin Rankine

22 Temperature is a physical quantity which gives us an idea of how hot or cold an object is. The temperature of an object depends on how fast the atoms and molecules which make up the object can shake, or oscillate. As an object is cooled, the oscillations of its atoms and molecules slow down. For example, as water cools, the slowing oscillations of the molecules allow the water to freeze into ice. In all materials, a point is eventually reached at which all oscillations are the slowest they can possibly be. The temperature which corresponds to this point is called absolute zero. (Note that the oscillations never come to a complete stop, even at absolute zero.)

23 Most people are familiar with either the Fahrenheit or the Celsius scales, with temperatures measured in degrees Fahrenheit (ºF) or degrees Celsius (ºC) respectively. The Fahrenheit and Celsius scales coincide only at one temperature: 40 degrees (i.e. 40 F and 40 C describe the same temperature).

24 Fahrenheit scale: Water Freezes: 32º F Water Boils: 212º F Absolute zero: -459º F Celsius scale: Water Freezes: 0º C Water Boils: 100º C Absolute zero: -273º C

25 The Kelvin (symbol: K) is a unit increment of temperature and is one of the seven SI base units. The Kelvin scale is a thermodynamic (absolute) temperature scale where absolute zero, the theoretical absence of all thermal energy, is zero (0 K). This definition also exactly relates the Kelvin scale to the Celsius scale. Absolute zero the temperature at which nothing could be colder and no heat energy remains in a substance is, by definition, exactly 0 K and 273 C.

26 Scientists - especially those who study what happens to things when they become very, very cold - commonly use the Kelvin scale, with temperatures measured in Kelvin (K). This scale uses the same temperature steps as the Celsius scale, but is shifted downward. On this scale, water freezes at 273 K and boils at 373 K. Kelvin scale: Water Freezes: 273 K Water Boils: 373 K Absolute zero: 0 K

27 FROM Celsius TO Celsius Fahrenheit F = ( C 9 5 )+ 32 C = ( F 32) 5 9 Kelvin K = C C = K 273 For temperature intervals rather than specific temperatures: 1 C = 1 K 1 C = 1.8 F

28 FROM Fahrenheit TO Fahrenheit Celsius C = ( F 32) 5 9 F = ( C 9 5 ) + 32 Kelvin K = ( F + 459) 5 9 F = (K 9 5 ) 459 For temperature intervals rather than specific temperatures: 1 F = 5 9 C

29 FROM Kelvin TO Kelvin Celsius C = K 273 K = C Fahrenheit F = (K 9 5 ) K = ( F +459) 5 9

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31 Throughout the world, except in the USA and a few other countries use the Celsius temperature scale for practically all purposes. The only exceptions are some specialist fields (e.g., low-temperature physics, astrophysics, light temperature in photography) where the closely related Kelvin scale dominates instead. Even in the U.S., almost the entire scientific world and most engineering fields, especially high-tech ones, use the Celsius scale. The general U.S. population, however, remains more accustomed to the Fahrenheit scale, which is therefore the scale that most U.S. broadcasters use in weather forecasts. The Fahrenheit scale is also commonly used in the U.S. for body temperatures. The UK has almost exclusively used the Celsius scale since the 1970s (but it is often called centigrade). A notable exception is that some broadcasters and publications still quote Fahrenheit air temperatures in weather forecasts (especially during summer), for the benefit of generations born before about 1950, and air-temperature thermometers sold still show both scales for the same reason.

32 Convert the following to the Celsius and Kelvin scale: 68 ºF 5 ºF 176 F Celsius Kelvin 20 ºC 293 K -15 ºC 258 K 80ºC 353 K

33 Convert the following to the Fahrenheit and Kelvin scale: 30ºC 5ºC -20 C Fahrenheit Kelvin 86 ºF 303 K 41 ºF 278 K -4 ºF 253 K

34 Ethyl alcohol boils at 78.5 C and freezes at -117 C. Convert these temperatures to the (a) Fahrenheit scale and (b) Kelvin scale. Fahrenheit scale: Kelvin scale: Boiling 173 F Freezing -179 F K 156 K

35 How do we measure heat? CALORIE (cal) The calorie is the amount of heat needed to increase the temperature of 1 gram of water by 1 C. British Thermal Unit (B.T.U) The amount of heat needed to raise the temperature of 1 lb. of water 1 F.

36 Food Calories are measured in kcal 1 Cal = 1000 cal = 1kcal

37 Conversions: 1 cal. = 4.19 Joules 1 B.T.U. = 250 cal. 1 Calorie (food) = 1000 cal. = 1 kcal.

38 Mechanical Equivalent of Heat: S.I.: 4.19 J of work converts to 1 calorie of heat. B.E.: 778 ft-lbs of work converts to 1 B.T.U. of heat.

39 Formula for work: W = J Q W = Work (Joules) J= 4.19 J/cal (constant) Q = Heat (calories)

40 a. How much work is required for a 1,500 Calories per day person to be a couch-potato? 6,285,000 J b. What is their Power consumption? 6,285,000 J / 86,400 seconds = W We re like light bulbs, constantly putting out heat