Heat

1. Heat A Form of Energy (Thermal energy)

2. 12.1 Temperature & Thermal Energy Objectives • Explain heat using the Kinetic-Molecular Theory • Define temperature • Understand the process of thermal equilibrium • Describe and use the Celsius & Kelvin Temperature Scales

3. Kinetic-Molecular Theory It was once common belief that heat was an invisible substance called “caloric.” It was believed that it could be transferred between objects. To heat up an object this caloric had to flow into it. This, they thought, explained why objects expanded when heated. (But……could not explain heat coming from a cold log when it was burned) The Kinetic-Molecular Theory replaced the “Caloric Theory” in the 19th century. It stated that all matter is made up of atoms/molecules in constant motion. The faster they move, the hotter an object will be.

4. Molecules and Motion • The motion of molecules produces heat • The more motion, the more heat is generated

5. Thermal Energy Thermal Energy (also called Internal energy) is the energy an object or substance has due to the kinetic and potential energies associated with the random motions of all the particles that make it up. The hotter something is, the faster its molecules are moving/vibrating, the higher its temperature.

6. Temperature and Heat • Kinetic energy is the energy of motion • Temperature is the measure of the average kinetic energy of an object

7. Heat Transfer • The movement of heat from a warmer object to a colder one

8. Forms of heat transfer • Three forms of heat transfer: • Conduction • Convection • Radiation

9. water molecule iron atom zoomed in view Conduction • Conduction involves the transfer of heat through direct contact • Heat conductors conduct heat well, insulators do not

10. Convection • Takes place in liquids and gases as molecules move in currents • Heat rises and cold settles to the bottom

11. Radiation • Heat is transferred through space • Energy from the sun being transferred to the Earth

12. Questions • What are the three types of heat transfer? • How is conduction different from radiation?

13. What type of heat transfer is involved? • Heating a room with a fireplace • Egg cooking in a frying pan • Roof of a house becoming hot

14. What type of heat transfer? • Warm air mass bringing a change in the weather • Wire getting hot from an electric appliance

15. Question • How is kinetic energy related to heat production?

16. Thermal Equilibrium Two bodies are said to be at thermal equilibrium if they are at the same temperature. If they are at different temperatures they are not in thermal equilibrium, and energy is flowing from the hot side to the cold side. hot cold heat KEhot > KEcold 26°C 26°C KE = KENo net heat flow The two purple objects are at the same temp and, therefore are in thermal equilibrium. There is no net flow of heat energy here.

17. Thermometer • A instrument used to measure temperature • Thermometers commonly have alcohol (with dye) or mercury • Digital thermometers have replaced older ones • A thermometer must be allowed to come to thermal equilibrium with the object it is in contact with to determine its temperature

18. Temperature Scales • Fahrenheit: water freezes at 32 °F; boils at 212 °F • Celsius: water freezes at 0 °C; boils at 100 °C • Kelvin: water freezes at 273.15 K; boils at 373.15 K A change of 100 °C corresponds to a change of 180 °F. This means 1 C° change = 1.8 F° change Since these scales are linear, and they’re offset by 32 °F, we get the conversion formula: F = 1.8C + 32

19. Celsius Scale • Celsius is the metric scale for measuring temperature • Based on water freezing at 0ºC and boiling at 100ºC

20. Kelvin scale • The Kelvin scale is a metric temperature scale measured in Kelvin units (K) where 0K is absolute zero (or no kinetic energy) • Formula (273+ºC)= Kelvin One step on the Kelvin scale is the same as one step on the Celsius scale. These scales are off by 273 K, so:K = C + 273Room temperature is around 293 kelvins, which is 20°C, or 68°F.

21. Absolute zero • The temperature in which all molecular motion stops (0 K)

22. Questions • What is the formula for converting a Celsius temperature to a Kelvin temperature? • What is the boiling point of water on the Kelvin scale? • What is the freezing point of water on the Kelvin scale?

23. Questions • Describe absolute zero. • What is absolute zero on the Celsius scale? -273 0C

24. Practice Problems • Practice Problems 1-3 pg 247 • Due Monday 3/31

25. 12.1 Heat and Thermal Energy Objectives • Distinguish heat from thermal energy • Define Specific Heat • Calculate temperature changes due to heat transfer

26. Measuring Heat • Addition of heat • Causes an increase in temperature • Removal of heat • Causes a decrease in temperature

27. Calories • Unit for measuring heat • The amount of heat needed to raise 1 gram of water one degree Celsius

28. Temperature Changes • Joule is another unit for measuring amount heat (heat is just energy) • Mass and type of substance determine the amount of temperature change

29. Units for Measuring Heat Flow The calorie is also related to the Joule, the SI unit of heat and energy named after James Prescott Joule 4.184 J = 1 cal Heat Capacity - the amount of heat needed to increase the temperature of an object exactly 1 oC Depends on both the object’s mass and its chemical composition 29

30. Specific Heat • The ability of a substance to absorb heat energy (specific heat) • Different substances absorb heat at different rates • The greater the mass of the object the more heat is absorbed

31. Heat Capacity and Specific Heat Specific Heat Capacity(abbreviated “C”) - the amount of heat it takes to raise the temperature of 1 gram of the substance by 1 oC often called simply “Specific Heat” Water has a HUGE value, when it is compared to other substances 31

32. Heat Capacity and Specific Heat For water, C = 4.18 J/(g oC) in Joules, and C = 1.00 cal/(g oC) in calories. Thus, for water: it takes a long time to heat up, and it takes a long time to cool off! Water is used as a coolant and regulates Earths temperatures! 32

33. Heat Capacity and Specific Heat To calculate, use the formula: Q = mass (in grams) x T x C heat is abbreviated as “q” T = change in temperature C = Specific Heat Units are either: J/(g oC) or cal/(g oC) 33

34. 703 J Q = 7 kg · ·10ºC = 49210 J kg·ºC Note that the units do indeed work out to be energy units. *Use table 12-1 on p. 248 for some specific heats of different substances. Specific Heat Equation Q = mCT Q = thermal energym = massC = specific heatT = change in temp Ex: The specific heat of silicon is 703 J/(kg·ºC). How much energy is needed to raise a 7 kg chunk of silicon 10ºC? answer:

35. Questions • How can heat be measured? • What is the unit used to measure heat? • What is specific heat?

36. Homework • Practice Problems 5-8 page 248 • Due Wed. 4/2/14 *Remember DT is the same in oC or K (eg: a change of 10 oC is a change of 10K) So you can use specific heats in J/kg-K or J/Kg-C they are the same number

37. 12.1 Calorimetry: Measuring Specific Heat Objectives • Use the law of conservation of energy to determine temperature changes using calorimetry • Use Calorimetry to determine specific heats

38. What is a calorimeter? • Device used to measure changes in thermal energy • It can be used to measure heat given off during chemical reactions (measure energy required to burn off a food product)

39. Calorimetry A horseshoe at 275 ºC, is dropped into bucket of water and covered. The bucket and cover are made of an insulating material. The bucket contains 2.5 L of water originally at 25 ºC. The 1.9 kg shoe is made of iron, which has a specific heat of 448 J/(kg·ºC). Let’s find the temp of the horseshoe and water once equilibrium is reached. At thermal equilibrium the water and shoe are at the same temp. The total thermal energy in the bucket does not change, but it is redistributed. continued on next slide

40. Calorimetry (cont.) Let T = the equilibrium temperature.Qlost by iron = Qgained by water or DEiron + DEwater = 0 mironCironTiron + mwaterCwaterTwater= 0 mironCiron(Tf-Ti,iron)+ mwaterCwater(Tf-Ti,water) = 0 We’ve got a simple linear equation in which we want to solve for Tf. Isolating Tf to one side gives: mironCironTi,iron + mwaterCwaterTi,waterTf= ____________________________ mironCiron+ mwaterCwater *You can use this for determining Tf of any substance

41. Calorimetry (cont.) mironCironTi,iron + mwaterCwaterTi,waterTf= ____________________________ mironCiron+ mwaterCwater (1.9 kg)(448 J/kg·ºC)(275 ºC)+ (2.5 kg)(4186 J/kg·ºC)(25 ºC) Tf = _____________________________________________________ (1.9 kg)(448 J/kg·ºC) + (2.5 kg)(4186 J/kg·ºC) Tf = 43.8 0C We’ve got a simple linear equation in which we want to solve for Tf. Solving it gives us Tf = 43.8 ºC. This is the equilibrium temp--the final temp for both the shoe and water.

42. Practice Problems • Practice Problems 9-12, page 252 • Due Thurs 4/3

43. 12.2 Changes of State Objectives • Define heats of fusion and vaporization • Calculate heat transfers needed to cause changes of state

44. States of Matter • Solid, Liquid and Gas (won’t discuss plasma’s yet……) • States change when the bonds between particles (atoms/molecules) are over come by the thermal energy of the particles

45. Heat and Changes of State (phase change) • A phase change is a physical change that requires a change in heat energy • Addition or removal of HEAT

46. Latent Heat The word “latent” comes from a Latin word that means “to lie hidden.” When a substance changes phases (liquid  solid or gas  liquid) energy is transferred without a change in temperature. This “hidden energy” is called latent heat. For example energy is required to change 0 ºC ice into 0 ºC water. When frozen, water molecules are in a crystalline structure, and energy is needed to break this structure. The energy needed is called the latent heat of fusion. Additional energy is also needed to change water at 100 ºC to steam at 100 ºC, and this is called the latent heat of vaporization.

47. Heat in Changes of State 1. Heat of Fusion (Hf.) = the energy required for one kilogram of a substance to melt from a solid to a liquid Q = mHf.(no temperature change) Values given in Table 12-2, page 254 2. Heat of Solidification( - Hf.) = the heat lost when one kilogram of liquid solidifies (or freezes) to a solid Q = - mHf.(no temperature change) 47

48. Heat in Changes of State Heat absorbed by a melting solid is equal to heat lost when a liquid solidifies Thus, Hfus. = - Hsolid. Note Table 12-2, page 254 for the number values. There is no value listed for the heat of solidification it is just -Hf 48

49. Heats of Vaporization and Condensation When liquids absorb heat at their boiling points, they become vapors. 3. Heat of Vaporization(Hv.) = the amount of heat necessary to vaporize one kilogram of a given liquid. Q = mHv.(no temperature change) Table 12-2, page 254 49

50. Heats of Vaporization and Condensation Condensation is the opposite of vaporization. 4. Heat of Condensation(Hcond.) = amount of heat released when one kilogram of vapor condenses to a liquid Q = mHcond.(no temperature change) Hcond = - Hv . or …..Q = -mHv 50