Presentation on theme: "Question An ice cube is removed from the freezer and put in a cup of hot chocolate. Which statement is most accurate? A) Cold flows from the ice cube."— Presentation transcript:
1QuestionAn ice cube is removed from the freezer and put in a cup of hot chocolate. Whichstatement is most accurate?A) Cold flows from the ice cube into the hot chocolate.B) Heat flows from the hot chocolate into the ice cube.C) The cold from the ice cube mixes with the heat from the hot chocolate.
2QuestionIf the kinetic energy of an average ideal gas molecule in a sample at 20ºC doubles, its final temperature must be _______.A) 10ºCB) 40ºCC) 313ºCD) none of the above
3Heat and Heat TransferWhere would you rather go for break, Montreal or Southern France?
4Heat CapacityWhy do you burn your mouth (but not your hand) when you eat hot pie or pizza?
5Why do we go to the beach to cool off? All because water has a high heat capacity!
6Which heats up more quickly? Put a pot of water on the stove ~ 15 minutes to boilPut same mass of iron ~ 2 minutes to reach 100o CSubstances have different ability to absorb or lose heat.
8Heat Capacity and Specific Heat For many substances, under normal circumstances TQ. Example: heat pot of waterOr Q = CT where C is the heat capacity.Takes more Q to boil 2 cups than 1 cupThe specific heat capacity, or just specific heat, of a substance is the heat capacity per unit mass.Could incorporate personal response system questions from the College Physics by G/R/R 2E ARIS site (www.mhhe.com/grr), Instructor Resources: CPS by eInstruction, Chapter 14, Questions 3, 4, and 12.or
9Which has a higher specific heat iron or water? A) Iron B) water Which has a higher specific heat water or sand? A) water B) sand
12Example (text problem 14. 12): If 125. 6 kJ of heat are supplied to 5 Example (text problem 14.12): If kJ of heat are supplied to 5.00102 g of water at 22 C, what is the final temperature of the water?
13Example (text problem 14. 19): A 0 Example (text problem 14.19): A kg aluminum teakettle contains 2.00 kg of water at 15.0 C. How much heat is required to raise the temperature of the water (and kettle) to 100 C?
14The heat needed to raise the temperature of the water to Tf is Example (text problem 14.19): A kg aluminum teakettle contains 2.00 kg of water at 15.0 C. How much heat is required to raise the temperature of the water (and kettle) to 100 C?The heat needed to raise the temperature of the water to Tf isThe heat needed to raise the temperature of the aluminum to Tf isThen Qtotal = Qw + QAl = 732 kJ.
15Can we predict any specific heats? Specific Heat of Ideal Gases The average kinetic energy of a molecule in an ideal gas isAnd the total kinetic energy of the gas is
16Define the molar specific heat at constant volume; this is the heat capacity per mole.Heat is allowed to flow into a gas, but the gas is not allowed to expand. If the gas is ideal and monatomic, all the heat goes into increasing the average kinetic energy of the particles.
17The amount of added heat is If the gas is diatomic:
18Rotational motions of a 2-atom molecule: Internal energy will be distributed equally among all possible degrees of freedom (equipartition of energy). Each degree of freedom contributes ½kT of energy per molecule and ½R to the molar specific heat at constant volume.
19Example (text problem 14.26): A container of nitrogen gas (N2) at 23 C contains 425 L at a pressure of 3.5 atm. If 26.6 kJ of heat are added to the container, what will be the new temperature of the gas?T1 = 23 V1 = 425 L P1 = 3.5 atm Q = 26.6 T2 = ?Change to standard units Absolute T, V in m3 andP in N/m2T =296oK V = 425 x 10-3 m3 P = 3.5 x 1.01 x105 N/m2
20The number of moles n is given by the ideal gas law Example (text problem 14.26): A container of nitrogen gas (N2) at 23 C contains 425 L at a pressure of 3.5 atm. If 26.6 kJ of heat are added to the container, what will be the new temperature of the gas?For a diatomic gas,The number of moles n is given by the ideal gas law
21The final temperature of the gas is Tf = Ti + T = 317 K = 44 C. The change in temperature isThe final temperature of the gas is Tf = Ti + T = 317 K = 44 C.
23Thermal ConductionThrough direct contact, heat can be conducted from regions of high temperature to regions of low temperature. Energy is transferred by collisions between neighboring atoms or molecules.Could incorporate personal response system questions from the College Physics by G/R/R 2E ARIS site (www.mhhe.com/grr), Instructor Resources: CPS by eInstruction, Chapter 14, Questions 7, 16, 17, and 19.
24The rate of energy transfer by conduction is where is the thermal conductivity, A is the cross-sectional area, and T/d is the temperature gradient, the temperature change per unit distance. depends on the material. Some materials conduct heat better than others
25Alsowhere R is the thermal resistance.This is convenient when heat is conducted through multiple layers because
26For each slab, calculate the thermal resistance per square meter Example (text problem 14.51): For a temperature difference of 20 C, one slab of material conducts 10.0 W/m2; another of the same shape conducts 20.0 W/m2. What is the rate of heat flow per m2 of surface when the slabs are placed side by side with a total temperature difference of 20 C?For each slab, calculate the thermal resistance per square meter
27When the materials are placed in series, Example continued:When the materials are placed in series,When the materials are placed in series, the rate of heat flow is
28FinalExam IV just like all exams. Covers all material since Thanksgiving including material covered only in lecture.Final-Separate exam. Comprehensive, all material until Thanksgiving.8 MC + 3 problems. Only from text materialRecommend studying: HW most important +….Curve exams and many lab grades +Promod’s lab – (3-5%)I am in my office T&W 2-5
29QuestionYou are making macaroni and cheese for dinner, again. Which is the best choice for stirring the noodles in the pot of boiling water?A) a wooden spoonB) a metal spoonC) any kind of spoonD) your finger, but really fast
30Example (text problem 14. 48): A metal rod with a diameter of 2 Example (text problem 14.48): A metal rod with a diameter of 2.30 cm and a length of 1.10 m has one end immersed in ice at 0 C and the other end in boiling water at 100 C. If the ice melts at a rate of 1.32 grams every 175 s, what is the thermal conductivity of the metal? Assume no heat loss to the surrounding air.
31We are given geometrical information, A and d Rate of ice melting which is related to rate ofheat transferFind κ
32Heat is conducted to the ice at a rate of Example (text problem 14.48): A metal rod with a diameter of 2.30 cm and a length of 1.10 m has one end immersed in ice at 0 C and the other end in boiling water at 100 C. If the ice melts at a rate of 1.32 grams every 175 s, what is the thermal conductivity of the metal? Assume no heat loss to the surrounding air.Heat is conducted to the ice at a rate ofQc is the heat necessary to melt the ice.The heat conducted to the ice in a time period t isThe heat needed to melt a given mass of ice isLf = 333 x 103 J/kg
33Since all the heat conducted by the rod is absorbed by the ice, Example continued:Since all the heat conducted by the rod is absorbed by the ice,
34QuestionYou are trying to transfer heat from a hot reservoir to a cold reservoir. You have at your disposal an aluminum rod and a copper rod of the same size. Which should you choose to have the highest rate of energy transfer?A) The copper rod aloneB) The aluminum rod onlyC) The rods in parallel.D) The rods in series.
35QuestionYou are in the doctor's office and notice that the metal instrument tray feels much colder than the exam table you are sitting on. This is becauseA) the tray actually is colder than the table.B) the exam table has a cushion inside, which is a good insulator.C) the metal conducts heat away from your hand more quickly than does the table.
36Thermal ConvectionHot air rises. We can see the ripples in the air above ahot road. The rising air transfers heat.Convection is the movement of heat by fluid currents. Material is transported from one place to another.
43Thermal Radiation The most important source of heat on Earth is the Sun.How can the heat get here?There is nothing (vacuum between us and Sun)Not by conduction or convection.Heat gets here by radiationCould incorporate personal response system questions from the College Physics by G/R/R 2E ARIS site (www.mhhe.com/grr), Instructor Resources: CPS by eInstruction, Chapter 14, Questions 6 and 15.
44Radiation is a an electromagnetic (EM) wave. Light is one example of EM radiationEven if I turn out the lights this room is full of EM radiation
45Examples of radiationAll bodies emit electromagnetic (EM) radiation. The perfect absorber and emitter of EM radiation is called a blackbody. The amount and type of radiation emitted depends on the temperature of the object.
47QuestionA wood-burning fireplace has a chimney which allows the heated air to rise and escape thehouse. How then does a fireplace then heat the room?A) convectionB) conductionC) radiationD) all of the above
48The rate of energy emission by a blackbody is (Stefan’s Law) where A is the surface area of the emitting body,T is its temperature, and = 5.670108 W/m2 K4 is the Stefan-Boltzmann constant.
49Since an ideal blackbody does not exist, Stefan’s law is written as where e is the emissivity; e = 0 for a perfect reflector of EM radiation and e = 1 for perfect blackbody.
50A spectrum shows the amount of radiation emitted at a particular wavelength. For a blackbody, the peak of the spectrum is determined only by its temperature.Wien’s law
51The net energy gained or lost by a blackbody at a temperature T is where Ts is the temperature of the surroundings.
52Example (R&S 14): A sphere with a diameter of 80 cm is held at a temperature of 250 C and is radiating energy. If the intensity of the radiation detected at a distance of 2.0 m from the sphere’s center is 102 W/m2, what is the emissivity of the sphere?The power emitted by a point source isThis is the total power passing through a sphere of radius dWas chapter 14 problem 69 (2nd ed.) and now is review and synthesis problem 14 for this section.The emissivity isNotice absolute T
53Thermos bottle and heat transfer VacuumSilvered surface
54Thermos and heat transfer No Heat transfer by conduction. (Vacuum doesn’t conduct). Small losses through stopper.No convection (Vacuum)No heat transfer by radiationSilvered surface reflectsheat radiation back intobottle
55Question Which of the following do not emit radiant energy? A) Sun B) EarthC) Cup of hot chocolateD) Ice cubeE) all of the above emit radiation
56Good luck on all your finals! Especially Physics