PHYSICS 149: Lecture 26 Chapter 14: Heat 14.1 Internal Energy 14.2 Heat 14.3 Heat Capacity and Specific Heat 14.5 Phase Transitions 14.6 Thermal Conduction 14.7 Thermal Convection 14.8 Thermal Radiation Lecture 26 Purdue University, Physics 149 1
Final Exam Wednesday, December 15, 8:00 10:00 AM Place: MSEE B012 Chapters 1 15 (only the sections we covered) The exam is closed book. The exam is a multiple-choice test. There will be 30 multiple-choice problems. Each problem is worth 10 points. You may make a single crib sheet. you may write on both sides of an 8.5 11.0 sheet. Review session on Monday Course Evaluation: courseval.itap.purdue.edu/etw/ets/et.asp?nxappid=wcq&nxmid=start&s=8 open until 12/12/2010 Lecture 10 Purdue University, Physics 149 2
ILQ 1 The intensity of a sound wave is directly proportional to A) the frequency B) the square of the speed of sound C) the amplitude D) the square of the amplitude Lecture 26 Purdue University, Physics 149 3
ILQ 2 An open pipe (open at both ends) has a length of 50 cm. What is the wavelength of the second harmonic frequency? A) 25 cm B) 50 cm C) 75 cm D) 100 cm Lecture 26 Purdue University, Physics 149 4
Internal Energy The internal energy of a system is the total energy of all of the molecules in the system except for the macroscopic kinetic energy (kinetic energy associated with macroscopic translation or rotation) and the external potential energy (energy due to external interactions). Internal energy includes Translational kinetic energy of the molecules the average translational kinetic energy of the molecules of an ideal gas <K tr > = (3/2) k T SI (Note: T SI in K) Rotational and vibrational kinetic energy of the molecules Potential energy between molecules Chemical and nuclear binding energy of the molecules Internal energy does not include any energy related to outside or macroscopic sources or motions, like overall translational energy of the system potential energy due to external fields such as gravity. Lecture 26 Purdue University, Physics 149 5
Energy Conversion: Joule s Experiment As the two masses fall, they cause paddles to rotate. Gravitational potential energy is converted into kinetic energy of the paddle wheel. As the paddles agitate the water, it causes water s temperature rise. Kinetic energy of the paddle wheel is converted into internal energy. Lecture 26 Purdue University, Physics 149 6
Heat Definition: Flow of energy between two objects due to difference in temperature Note: similar to WORK Object does not have heat (it has energy) Units: calorie Amount of heat needed to raise 1g of water 1ºC 1 Calorie = 1 kcal = 1000 cal = 4186 Joules Heat flows from a system at higher temperature to one at lower temperature Lecture 26 Purdue University, Physics 149 7
The Cause of Thermal Expansion Objects expand when their temperatures increase because the vibrational energy of their molecules increases; this makes the average distance between molecules increase. Example: liquid-in-glass thermometer relies on thermal expansion of the mercury or alcohol. Lecture 26 Purdue University, Physics 149 8
Thermal Expansion When temperature rises molecules have more kinetic energy they are moving faster, on the average consequently, things tend to expand Amount of expansion depends on change in temperature original length Temp: T Temp: T+ΔT coefficient of thermal expansion L 0 + ΔL = L 0 + α L 0 ΔT ΔL = α L 0 ΔT (linear expansion) ΔA = 2α A 0 ΔT (area expansion) ΔV = β V 0 ΔT (volume expansion) L 0 ΔL Lecture 26 Purdue University, Physics 149 9
Expansion Coefficients Lecture 26 Purdue University, Physics 149 10
Thermal Expansion Lecture 26 Purdue University, Physics 149 11
Amazing Water ρ (kg m -3 ) Water is very unusual in that it has a maximum density at 4 degrees C. That is why ice floats, and we exist! T (C) Lecture 26 Purdue University, Physics 149 12
Stuck Lid ILQ A glass jar (α = 3x10-6 K -1 ) has a metal lid (α = 16x10-6 K -1 ) which is stuck. If you heat them by placing them in hot water, the lid will be A) Easier to open B) Harder to open C) Same Copper lid expands more, making a looser fit, and easier to open! Lecture 26 Purdue University, Physics 149 13
Jar ILQ A cylindrical glass container (β = 28x10-6 k -1 ) is filled to the brim with water (β = 208x10-6 k -1 ). If the cup and water are heated 50C what will happen? A) Some water overflows B) Same C) Water below rim Water expands more than container, so it overflows. Lecture 26 Purdue University, Physics 149 14
Heat Capacity, C If no mechanical work is done either on or by a system, its change in internal energy is equal to the heat energy transferred into the system. For many substances, the change in temperature is proportional to the change in heat energy; the constant of proportionality is called the heat capacity. In other words, the heat capacity of the system is the ratio of heat flow into a system to the temperature change of the system: Heat capacity depends on both the substance and also on the amount of the substance which is present. Heat capacity is a scalar quantity. Units: J/K or J/ C Q > 0 for heat flow into the system, which causes ΔT > 0 Q < 0 for heat flow out of the system, which causes ΔT < 0 Lecture 26 Purdue University, Physics 149 15
Specific Heat, c The specific heat capacity (or specific heat) of a substance is the heat capacity per unit mass. Specific heat means the amount of heat necessary to change the temperature of 1 kg of a substance by 1 C Specific heat depends only on the substance (since we divide the heat capacity by the mass). Specific heat is a scalar quantity. Units: J/(kg K) or J/(kg C) If more than two substances are in contact, heat energy is transferred until they are all in thermal equilibrium. The final temperature and the changes in temperature of the various substances depend on the specific heats and the amounts of the substances that are present. The heat required to produce a temperature change in a system is: Q > 0 for heat flow into the system, which causes ΔT > 0 Q < 0 for heat flow out of the system, which causes ΔT < 0 Lecture 26 Purdue University, Physics 149 16
Specific Heat Heat adds energy to object/system IF system does NO work then: Heat increases internal energy: Q = ΔU Heat increases temperature: Q = C ΔT Q = c m ΔT Heat required to increase T depends on amount of material (m) and type of material (c) Q = cmδt : Cause = inertia x effect (just like F=ma) cause = Q effect = ΔT inertia = cm (mass x specific heat capacity) c specific heat ΔT = Q/cm (just like a = F/m) Lecture 26 Purdue University, Physics 149 17
Phase Transitions Phase transitions occur when a substance goes from one phase (solid, liquid, or gas) to another. Phase transitions occur at constant temperature. During a phase transition, heat flow continues, but the temperature of the substance does not change. The latent heat is the heat energy required per unit mass of effect a phase change. The latent heat of fusion L f : The heat per unit mass that must flow to melt a solid or to freeze a liquid (that is, L f is for the solid-liquid transition). The latent heat of vaporization L v : The heat per unit mass that must flow to change the phase from liquid to gas or from gas to liquid (that is, L v is for the liquid-gas transition). Latent heat is a scalar quantity. Unit: J/kg Phase transitions can go in either direction, the latent heat is the same (but the heat energy flows the opposite way, of course). Lecture 26 Purdue University, Physics 149 18
Example: Ice Water Steam Phase Transitions m = 1 kg Q = m ice c ice ΔT = 1 kg 2.1 kj/kg K 25 K = 52.3 kj Q = m i+w L f = 1 kg 333.7 kj/kg = 333.7 kj Q = m water c water ΔT = 1 4.19 100 = 419 kj Q = m w+s L v = 1 2256 = 2256 kj Q = m steam c steam ΔT = 1 2 25 = 50 kj Lecture 26 Purdue University, Physics 149 19
Evaporation Liquids evaporate due to the spread in kinetic energy of their molecules; the highest-energy molecules are able to escape (because it can break loose from the molecular bonds at the surface of the water), which reduces the average energy of those that are left (thereby cooling the liquid). Lecture 26 Purdue University, Physics 149 20
Molecular Picture of Gas Gas is made up of many individual molecules Number density is number of molecules/volume N/V = ρ/m ρ is the mass density m is the mass for one molecule Number of moles n = N / N A N A = Avogadro s number = 6.022 10 23 mole -1 N A = number of molecules per mole 1 mole = amount of substance that contains as many elementary entities as there are atoms in exactly 12 grams of carbon-12 Lecture 26 Purdue University, Physics 149 21
Atoms, Molecules and Moles 1 mole = 6.022 10 23 molecules (N A = Avogadro s Number) N A = Number of atoms or molecules that make a mass equal to the substance's atomic or molecular mass in grams. 1 u = 1 atomic mass unit = (mass of 12 C atom)/12 Approximately # of neutrons + # of protons Atomic weight W 1 u = 1.66 10-27 kg = 1gram/N A Mass of 1 mole of stuff in grams = molecular mass in u E.g. 1 mole of N 2 has mass of 2 14 = 28 grams Lecture 26 Purdue University, Physics 149 22
The Ideal Gas Law P V = N k B T P = pressure in N/m 2 (or Pascals) V = volume in m 3 N = number of molecules T = absolute temperature in K k B = Boltzmann s constant = 1.38 x 10-23 J/K Note: P V has units of N-m or J (energy!) Lecture 26 Purdue University, Physics 149 23
Ideal Gas Law You inflate the tires of your car so the pressure is 30 psi, when the air inside the tires is at 20 degrees C. After driving on the highway for a while, the air inside the tires heats up to 38 C. Which number is closest to the new air pressure? A) 16 psi B) 32 psi C) 57 psi Careful, you need to use the temperature in K P = P 0 (38+273)/(20+273) Lecture 26 Purdue University, Physics 149 24
Balloon ILQ What happens to the pressure of the air inside a hot-air balloon when the air is heated? (Assume V is constant) A) Increases B) Same C) Decreases Balloon is still open to atmospheric pressure, so it stays at 1 atm Lecture 26 Purdue University, Physics 149 25
Transfer of Heat Energy There are three ways in which heat energy can be transferred: Conduction Convection Radiation For each case, we will study the rate of heat flow (that is, power). Lecture 26 Purdue University, Physics 149 26
Heat Transfer: Conduction Hot molecules have more KE than cold molecules High-speed molecules on left collide with lowspeed molecules on right energy transferred to lower-speed molecules heat transfers from hot to cold I = rate of heat transfer = Q/t [J/s] I = κ A (T H -T C )/d Q/t = κ A ΔT/Δx κ = thermal conductivity Units: J/s-m-C good thermal conductors high κ good thermal insulators low κ R = d/(aκ) = thermal resistance T H Hot Area A d = Δx T C Cold Lecture 26 Purdue University, Physics 149 27
Heat Transfer: Convection Air heats at bottom Thermal expansion density gets smaller Lower density air rises Archimedes: low density floats on high density Cooler air pushed down Cycle continues with net result of circulation of air I = Q/t = h A ΔT h = coefficient of convection Practical aspects heater ducts on floor A/C ducts on ceiling stove heats water from bottom riding the thermals Lecture 26 Purdue University, Physics 149 28
Convection Lecture 26 Purdue University, Physics 149 29
Heat Transfer: Radiation All things radiate electromagnetic energy I emit = Q/t = eaσt 4 e = emissivity (between 0 and 1) perfect black body has e=1 T is Kelvin temperature σ = Stefan-Boltzmann constant = 5.67 x 10-8 J/s-m 2 -K 4 No medium required All things absorb energy from surroundings I absorb = eaσt 0 4 good emitters (e close to 1) are also good absorbers Lecture 26 Purdue University, Physics 149 30
Thermal Radiation Thermal radiation does not (have to) travel through a material medium (unlike convection) and does not have to be in direct contact (unlike conduction). The energy is carried by electromagnetic waves that travel at the speed of light. All bodies emit energy through electromagnetic radiation. All materials radiate electromagnetic waves whose frequency depends on their temperature; some radiate in the visible spectrum, such as the Sun and glowing coals, while others radiate in the infrared. Ex) The Earth is warmed by the Sun s heat, which is transferred via radiation. Lecture 26 Purdue University, Physics 149 31
Heat Transfer: Radiation All things radiate and absorb electromagnetic energy I emit = eaσt 4 I absorb = eaσt 0 4 I net = I emit -I absorb = eaσ(t 4 -T 04 ) if T > T 0, object cools down if T < T 0, object heats up T Surroundings at T 0 Hot stove Lecture 26 Purdue University, Physics 149 32
Radiation Which of the following is an example of radiative heat transfer? A) You stir some hot soup with a silver spoon and notice that the spoon warms up. B) You stand watching a bonfire, but cant get too close because of the heat. C) Its hard for central air-conditioning in an old house to cool the attic. Lecture 26 Purdue University, Physics 149 33
Blackbody An idealized body that absorbs all the radiation incident on it is called a blackbody. A blackbody emits more radiant power per unit surface area than any real object at the same temperature. Note that a good absorber is also a good emitter of radiation, because it has to emit the same amount of radiation to keep the same temperature. It emits a unique radiation spectrum which depends only on its temperature and size, and not on the material it is made of or its shape. Lecture 26 Purdue University, Physics 149 34
Radiation Spectrum Infrared: 100 μm - 0.7 μm Visible Light: 0.7 μm - 0.4 μm Ultraviolet: < 0.4 μm Wien s Law: λ max T = 2.898 10-3 mk Lecture 26 Purdue University, Physics 149 35
Greenhouse Effect Lecture 26 Purdue University, Physics 149 36
PHYS 149 Students You guys are great! Good luck for the exam Thank you for being my students. Lecture 26 Purdue University, Physics 149 37