Heat and Temperature

1. Heat and Temperature

2. Heat and modern technology are inseparable. These glowing steel slabs, at over 1,100OC (about 2,000OF), are cut by an automatic flame torch. The slab caster converts 300 tons of molten steel into slabs in about 45 minutes. The slabs are converted to sheet steel for use in the automotive, appliance and building industries.

3. The Kinetic Molecular Theory.

4. Introduction. • Ancient Greeks knew that matter was made up of very small particles. • Democritus wrote that matter was made up of tiny indivisible particles he called atoms. • We now know that atoms are not indivisible, but are themselves made up of even smaller particles. • We have identified more that 200 smaller particles that make up atoms.

5. Molecules. • Basic assumption is that matter is made up of tiny units of structure called atoms. • Atoms are neither created or destroyed during any type of chemical or physical change. • Arrangements of atoms determines type of entity of matter. • Elements are pure substances made up of only one type of atom. • Compounds are made up of one type of atom, but have more complex structures.

6. Pure substances are composed of 2 or more elements in defined proportions. • A molecule is the smallest particle of a compound in which all of the atoms maintain their identity. • maintains all of the chemical and physical properties of the compound. • Some atoms naturally form molecules called diatomic molecules: O2, F2, Cl2, N2, Br2,

7. Metal atoms appear in the micrograph of a crystal of titanium niobium oxide, magnified 7,800,000 times by an electron microscope.

8. Molecules Interact. • Cohesion. • Some solids and liquids attract each other and cling to each other. • Cohesion is when this attractive force is between like molecules. • Adhesion. • Some molecules are attracted to other molecules.

9. Phases of Matter. • Solids. • Defined shapes. • Defines volumes. • Molecules are fixed distances apart and have strong cohesive forces.

10. Liquids. • Close together. • Cohesive forces not as strong as in a solid. • Defined volume, but not a defined shape. • Gases • Weak cohesive forces. • High kinetic energy. • Molecules far apart and move in random motion • No fixed shape or volume. • Vapor is a gas that is above a liquid phase.

11. (A) In a solid, molecules vibrate around a fixed equilibrium position and are held in place by strong molecular forces. (B) In a liquid, molecules can rotate and roll over each other because the molecular forces are not as strong. (C) In a gas, the molecules move rapidly in random free paths.

12. Molecules Move. • All molecules have kinetic energy due to movements. • This kinetic energy can be in the form of: • Vibrational energy. • Rotational energy. • Translational energy where the entire molecule has motion.

13. The basic forms of kinetic energy of molecules. (A) Translational motion is the motion of a molecule as a whole moving from place to place. (B) Rotational motion is the motion of a turning molecule. (C) Vibrational motion is the back-and-forth movement of a vibrating molecule.

14. The kinetic energy of a substance is measured as the temperature of that substance. • Temperature is actually a measure of the average kinetic energy and has nothing to do with heat until there is a transfer of energy.

15. The number of oxygen molecules with certain velocities that you might find a sample of air at room at temperature. Notice that a few are barely moving and some have velocities over 1,000 m/s at a given time, but the average velocity is somewhere around 500 m/s.

16. Temperature.

17. Thermometers. • Conceptually a thermometer is used to measure the hotness or coldness of an object. • What a thermometer really measures is the average kinetic energy of an object. • There is a physical transfer of kinetic energy to the thermometer which them responds due to the increase in its kinetic energy. • Mercury. • Ethylene glycol.

18. (A) A bimetallic strip is two different metals, such as iron and brass, bonded together as a single unit, shown here at room temperatures. (B) Since one metal expands more than the other, the strip will bend when it is heated. In this example, the brass expands more than the iron, so the bimetallic strip bends away from the brass.

19. This thermostat has a coiled bimetallic strip that expands and contracts with changes in the room temperature. The attached vial of mercury is tilted one way or the other, and the mercury completes or breaks an electric circuit that turns the heating or cooling system on or off.

20. Thermometer Scales. • Fahrenheit scale • Sets boiling point of water at 212 OF and freezing point of water at 32 OF. • 180 divisions between these two. • Like most English measures is quite cumbersome.

21. Celsius scale. • Sets boiling point of water at 100 OC and freezing point of water at 0 OC. • 100 divisions between these two points. • Kelvin or absolute scale. • Begins at absolute zero, the temperature at which all kinetic energy is changed into potential energy. • ie, all molecular motion ceases. • Boiling point of water is 373 K and freezing point of water is 273 K. • Divisions are same as for Celsius scale

22. The Fahrenheit, Celsius, and Kelvin temperature scales

23. Conversions. • TF = 1.8 TC + 32 OC • TC = (TF - 32 OF) • 1.8 • 1.8 accounts for the divisions between freezing point of water and boiling point of water. • There are 1.8 divisions in the F scale for every 1 division in the C scale. • TK = TC + 273.

24. Example. • The temperature of Lake Superior in August averages 34 OF. What is the temperature in OC. • Use: TC = (TF - 32 OF) • 1.8 • TC = (34 OF - 32 OF) • 1.8 • TC = (2 OF) • 1.8

25. Example: • What is the equivalent Celsius temperature of 400.0 K? The equivalent Fahrenheit temperature? • Use: TK = TC + 273 • Rearrange to : TC = TK - 273 • TC = 400.0 K - 273 = 127.0 OC • TF = 1.8 (127.0 OC) + 32 OC =

26. Heat.

27. Internal and External Energy. • External energy is total potential and kinetic energy of everyday sized objects. • Internal energy is the total kinetic and potential energy of an object molecules.

28. One theory about how friction results in increased temperatures: Molecules on one moving surface will catch on another surface, stretching the molecular forces that are holding it. They are pulled back to their home position with a snap, resulting in a gain of vibrational kinetic energy.

29. External energy is the kinetic and potential energy that you can see. Internal energy is the total kinetic and potential energy of molecules. When you push a table across a floor, you do work against friction. Some of the external mechanical energy goes into internal kinetic and potential energy, and the bottom surface of the legs becomes warmer.

30. Heat as Energy Transfer. • Temperature is a measure of the average kinetic energy of an object. • Heat is a measure of the internal energy that has been absorbed or transferred from one body to another. • Increasing the internal energy is called heating. • Decreasing the internal energy is called cooling.

31. Two ways to increase temperature: • From a temperature difference, with energy moving from a region of higher temperature to a region of lower temperature. • From an object gaining energy by way of a temperature conversion.

32. Heat and temperature are different concepts, as shown by a liter of water (1,000 mL) and a 250 mL cup of water, both at the same temperature. You know the liter of water contains more heat since it will require more ice cube to cool it, say, 25OC than will be required for the cup of water. In fact, you will have to remove 48,750 additional calories to cool the liter of water

33. Measures of Heat. • The metric unit of measuring work, energy, or heat is the joule. • The metric unit of heat is the calorie. • A calorie is the amount of energy needed to increase the temperature of 1 gram of water 1 OC (from 14.5 OC to 15.5 OC. • A kilocalorie is the amount of energy needed to increase the temperature of 1 kg of water 1 OC.

34. The Calorie value of food is determined by measuring the heat released from burning the food. If there is 10.0 kg of water and the temperature increased from 10OC to 20OC the food contained 100 Calories (100,000 calories). The food illustrated here would release much more energy than this.

35. Joule worked with the English system of measurement used during his time. When a 100 lb object falls 7.78 ft, it can do 778 fl?lb of work. If the work is done against friction, as by stirring 1 lb of water, the heat produced by the wok raises the temperature 1OF.

36. The English unit of heating is the BTU. • A BTU is the amount of energy needed to increase the temperature of 1 lb of water 1 OF. • A Quad is 1 quadrillion BTU 1 X 1015 BTU. • 778 ftlb = 1 BTU • 4.184 ftlb = 1 calorie • 4,184 J = 1 kcalorie

37. Example: a 2,200.0 kg automobile is moving at 90.0 km/hr (25.0 m/s). How many kilocalories are generated when the car brakes to a stop? • KE = 1/2mv2 • KE = 1/2(2,200 kg)(25.0m/s)2 • KE = (1,100 kg)(625.0 m2/s2) • KE = 687,500 m2/s2 • KE = 687,500 J • Kcal = 687,500 J X 1 kcal/4,184J = 164 kcalories

38. Specific Heat. • Three variables that influence energy transfer. • The temperature change. • The mass of the substance. • The nature of the material being heated. • The amount of heat (Q) needed to increase the temperature (Ti) of a pot of water from the initial temperature to a final temperature (Tf) is proportional to (Tf-Ti). • Q  (Tf-Ti). • Q T.

39. The quantity of heat (Q) absorbed or given off during a certain change in temperature is also proportional to the mass (m) of the substance. • Q  m • Putting this all together we get: • Q  mcT • c is the specific heat of the substance. • Specific heat is the energy needed to increase the temperature of 1 gram of a substance 1 OC.

40. When two materials of different temperatures are involved in heat transfer and are perfectly insulated from the surroundings, the heat lost by one is equal to the heat gained by the other. • heat lost = heat gained. • Qlost = Qgained • (mcT)lost = (mcT)gained

41. Of these three metals, aluminum needs the most heat per gram per degree when warmed, and releases the most heat when cooled.

42. Example: How much heat must be supplied to a 500.0 g pan to increase its temperature from 20.0 OC to 100.0 OC if the pan is made of a) iron and b) aluminum. • Iron from table 5.2 has a specific heat of 0.11 cal/gOC. • Q = mcT • Q = (500.0g)(0.11 cal/gOC)(80.0OC) • Q = 4,400 cal or 4.40 kcalories • Aluminum from table 5.2 has a specific heat of 0.22 cal/gOC • Q = mcT • Q = (500.0g)(0.22 cal/gOC)(80.0OC) • 8,800 calories or 8.80 kcalories

43. Heat Flow. • Conduction. • Anytime there is a temperature difference; there is a natural tendency for temperature to flow from the area of higher temperature to the area of lower temperature. • Conduction is the transfer of energy from molecule to molecule. • The rate depends on the temperature difference, the area and thickness of the substance, and the nature of the material.

44. Thermometers place in holes drilled in a metal rod will show that heat is conducted from a region of higher temperature to a region of lower temperature. The increased molecular activity is passed from molecule to molecule in the process of conduction.

45. Some materials are good conductors while others are good insulators. • Conductors transfer energy very efficiently. • Insulators transfer energy very inefficiently, • The best conductors are usually metals which have very little air space between molecules. • The best insulators have a great deal of air space between molecules. • The absolute best insulator is a vacuum as there are no molecules to pass on energy.

46. Fiberglass insulation is rated in terms of R-value, a ratio of the conductivity of the material to its thickness.

47. Convection. • Large scale transfer of heat by a large scale displacement of groups of molecules with relatively higher kinetic energy. • Molecules with higher kinetic energy are moved from one place to another place. • Happens only in liquids and gases where fluid motion can carry molecules with higher kinetic energy over a distance.

48. (A) Two identical volumes of air are balanced, since they have the same number of molecules and the same mass. (B) Increased temperature causes one volume to expand from the increased kinetic energy of the gas molecules. (C) The same volume of the expanded air now contains fewer gas molecules and is less dense, and it is buoyed up by the cooler, more dense air.

49. Convection currents move warm air throughout a room as the air over the heater becomes warmed, expands, and is moved upwards by cooler air.

50. Radiation. • Radiation involves the form of energy called radiant energy that moves through space. • All objects with a temperature above absolute zero give off radiant energy. • The absolute temperature of the object determines the rate, intensity, and kinds of radiant energy emitted.