Physics 161 Fall Announcements First quiz is next Monday (10/23) and covers chapters 1-4, homework #1 and #2. You can find a cheat sheet here - this contains formulas for the quiz and will be the first page of the quiz. First quiz is next Monday (10/23) and covers chapters 1-4, homework #1 and #2. You can find a cheat sheet here - this contains formulas for the quiz and will be the first page of the quiz.here Please have HW#2 complete no later than 2 days late, that is, by Sunday at 5:00... It is due on Friday at 5:00pm, and you will need to request an extension if want to to do it over the weekend. Please have HW#2 complete no later than 2 days late, that is, by Sunday at 5:00... It is due on Friday at 5:00pm, and you will need to request an extension if want to to do it over the weekend.

Physics 161 Fall Heat Pumps and Refrigerators: more entropy Heat Pumps provide a means to very efficiently move heat around, and work both in the winter and the summer

Physics 161 Fall Heat Pump Diagram

Physics 161 Fall Heat Pumps and Refrigerators: Thermodynamics ThTh QhQh QcQc W = Q h – Q c TcTc Hot entity (indoor air) Cold entity (outside air or refrigerator) heat energy delivered heat energy extracted delivered work: conservation of energy Just a heat engine run backwards… efficiency = = W work done Q h heat delivered (heat pump) efficiency = = W work done Q c heat extracted (refrigerator)

Physics 161 Fall Heat Pump/Refrigerator Efficiencies Can work through same sort of logic as before to see that: Can work through same sort of logic as before to see that: heat pump efficiency is: T h /(T h – T c ) = T h /  T in ºK heat pump efficiency is: T h /(T h – T c ) = T h /  T in ºK refrigerator efficiency is: T c /(T h – T c ) = T c /  T in ºK refrigerator efficiency is: T c /(T h – T c ) = T c /  T in ºK Note that heat pumps and refrigerators are most efficient for small temperature differences Note that heat pumps and refrigerators are most efficient for small temperature differences hard on heat pumps in very cold climates hard on heat pumps in very cold climates hard on refrigerators in hot settings hard on refrigerators in hot settings

Physics 161 Fall Example Efficiencies A heat pump maintaining 20 ºC when it is –5 ºC outside has a maximum possible efficiency of: A heat pump maintaining 20 ºC when it is –5 ºC outside has a maximum possible efficiency of: 293/25 = /25 = note that this means you can get almost 12 times the heat energy than you are supplying in the form of work! note that this means you can get almost 12 times the heat energy than you are supplying in the form of work! this factor is called the C.O.P. (coefficient of performance) this factor is called the C.O.P. (coefficient of performance) A freezer maintaining –5 ºC in a 20 ºC room has a maximum possible efficiency of: A freezer maintaining –5 ºC in a 20 ºC room has a maximum possible efficiency of: 268/25 = /25 = called EER (energy efficiency ratio) called EER (energy efficiency ratio)

Physics 161 Fall Example Labels (U.S. & Canada)

Physics 161 Fall Heat, Heat Transfer, Problems Heat vs. temperature: heat is energy in motion and is related to temperature change by heat capacity; temperature is a measure of the average kinetic energy the atoms inside something have Heat vs. temperature: heat is energy in motion and is related to temperature change by heat capacity; temperature is a measure of the average kinetic energy the atoms inside something have Heat is energy in motion - we do not ask ‘how much heat does something have in it’ (at least physicists don’t); it goes into the energy balance described by the first law - the ‘conservation of energy’ Heat is energy in motion - we do not ask ‘how much heat does something have in it’ (at least physicists don’t); it goes into the energy balance described by the first law - the ‘conservation of energy’ Controlling heat transfer is a key ingredient of ‘energy conservation’ and cooking. How is heat transferred? Controlling heat transfer is a key ingredient of ‘energy conservation’ and cooking. How is heat transferred?

Physics 161 Fall Heat Transfer: Conduction Heat moves ‘from the hot end to the cold end’ Heat moves ‘from the hot end to the cold end’ Q = heat transferred in time t A = cross-sectional area  = thermal conductivity d = thickness of material Metals, eg., copper: high thermal conductivity Insulators, e.g., wood, low thermal conductivity What are the units of thermal conductivity?

Physics 161 Fall Heat Conduction and Insulation R-factors Ever shop for insulation? If so, you’ve run up against R-factors or R-values. These are essentially d/ , which is a normalized efficiency for insulation Ever shop for insulation? If so, you’ve run up against R-factors or R-values. These are essentially d/ , which is a normalized efficiency for insulation Large R means good insulation: Q/t = A x  T/R Large R means good insulation: Q/t = A x  T/R Example: 6” of fiberglass insulation has an R-value of 19 in units of ft 2 -hr- o F/Btu. With a temperature difference of 30 o F, what is the rate of heat flow through 100 square feet of R-19 fiberglass? Example: 6” of fiberglass insulation has an R-value of 19 in units of ft 2 -hr- o F/Btu. With a temperature difference of 30 o F, what is the rate of heat flow through 100 square feet of R-19 fiberglass? Q/t = 100 ft 2 x 30 o F/19 ft 2 -hr- o F/Btu = 158 Btu/hr = 46 Watts This ignores other forms of heat transfer... Water balloon demo... Huh?

Physics 161 Fall Heat Transfer by Radiation This is what we called Blackbody Radiation a last week: This is what we called Blackbody Radiation a last week: F =  T 4 in Watts per square meter  = 5.67  W/ºK 4 /m 2 F =  T 4 in Watts per square meter  = 5.67  W/ºK 4 /m 2 Except we did not talk about emissivity , which is a measure of how efficiently something radiates; black things radiate efficiently and have emissivity of ~1, white and shiny things do not and have emissivity below 0.1. Except we did not talk about emissivity , which is a measure of how efficiently something radiates; black things radiate efficiently and have emissivity of ~1, white and shiny things do not and have emissivity below 0.1. In reality, F =   T 4 and power = P =   T 4 A In reality, F =   T 4 and power = P =   T 4 A Black vs silver radiation demo Black vs silver radiation demo

Physics 161 Fall Transfer of heat due to the actual motion of a fluid. You’ve seen this above your toaster or a hot parking lot. Convection often entails ‘rolls’ of fluid motion, with cool fluid being warmed and thus having its density lowered in a localized region. It then rises locally, but falls elsewhere. It is often the dominant mechanism of heat transfer in everyday life. Geothermal convection drives plate tectonics as well as many astrophysical phenomena.plate tectonics astrophysical phenomena On-shore vs. off-shore breezes. Heat Transfer by Convection

Physics 161 Fall Heat Transfer Summary Controlling heat transfer requires careful design, taking into account conduction, radiation, and convection. Controlling heat transfer requires careful design, taking into account conduction, radiation, and convection. Building codes have evolved a lot over the past several decades to require better and better insulation - old houses lose a lot of heat compared to new houses Building codes have evolved a lot over the past several decades to require better and better insulation - old houses lose a lot of heat compared to new houses On a global scale, heat transfer by convection and radiation are key ingredients of atmospheric physics. So are latent heats... On a global scale, heat transfer by convection and radiation are key ingredients of atmospheric physics. So are latent heats...

Physics 161 Fall Latent Heat: Heat Associated with a Phase Change Add heat to a solid, e.g., ice, and the temperature rises according to the material’s specific heat: Q = m c ice  T Add heat to a solid, e.g., ice, and the temperature rises according to the material’s specific heat: Q = m c ice  T When the ice melts, the temperature remains constant at 0 o C while a latent heat of fusion of 80 cal/g is added and the ice changes to water. When the ice melts, the temperature remains constant at 0 o C while a latent heat of fusion of 80 cal/g is added and the ice changes to water. Adding additional heat to water again raises the temperature according to the heat capacity of Q = m c water  T Adding additional heat to water again raises the temperature according to the heat capacity of Q = m c water  T At 100 o C, the water boils and the latent heat of vaporization of 540 cal/g must be added to produce water vapor At 100 o C, the water boils and the latent heat of vaporization of 540 cal/g must be added to produce water vapor Adding additional heat to water vapor will raise the temperature according to Q = m c vapor  T Adding additional heat to water vapor will raise the temperature according to Q = m c vapor  T Latent heat of vaporization of water is unusually high. This energy play a key role in powering hurricanes. You don’t need to boil the water – it just needs to evaporate from the warm Gulf of Mexico to acquire the extra latent heat, which reappears when clouds form.

Physics 161 Fall Quiz Format 10 short answer questions, similar (in some cases identical) to those in the ‘questions’ sections at the end of the chapters in the text 10 short answer questions, similar (in some cases identical) to those in the ‘questions’ sections at the end of the chapters in the text 6 multiple choice, probably some a little bit numerical (e.g., a unit change) 6 multiple choice, probably some a little bit numerical (e.g., a unit change) 2 numerical problems like those on the homework sets. 2 numerical problems like those on the homework sets. Bring a calculator. Bring a calculator. A good way to study will be to look at the ‘questions’ and ‘problems’ sections in chapters 1-4. If you can do all of those that can be done simply, you will be fine.

Physics 161 Fall A flashlight lamp connected to a battery that provides 1.4 V draws a current of 0.10 A. What power is used by the lamp? A flashlight lamp connected to a battery that provides 1.4 V draws a current of 0.10 A. What power is used by the lamp? What is the resistance of the bulb’s filament? What is the resistance of the bulb’s filament? More...

Physics 161 Fall More... How much mass is lost in the fission of the nuclear fuel in a power plant in one year if the reactor operates at 1000 MW? How much mass is lost in the fission of the nuclear fuel in a power plant in one year if the reactor operates at 1000 MW?

Physics 161 Fall More... A 100 Watt light bulb has a tungsten filament with an area of 100 mm 2. Estimate the temperature of the filament when the bulb is on. (P =  T 4 A;  = 5.67x10 -8 W/(m 2 K 4 ), A = area, T = absolute temperature) Tungsten melts at ~4000K. How much power could the light bulb handle before the filament melts?

Physics 161 Fall More liters of water is heated from 0 o C to 100 o C. How much heat does this require? (heat capacity of water is C p = 1 Cal/(kg o C); 1 Cal = 4.2 kJ, density of water is 1 kg/l) How much does the mass of the water change? If we used a 500 Watt immersion heater to heat the water, how long would it take?

Physics 161 Fall More... How much power does an adult with a surface area of 2 m 2 radiate? Assuming the heat capacity of a human is the same as that of water (1 Cal/(kg o C)) and that the only heat loss is by radiation, how long would it take for a 70 kg adult located in a very cool environment to cool by 10 o C?

Physics 161 Fall More... A river 100 m wide, 2 m deep, and flows at 1 m/s has a hydroelectric dam 100 m high. What is the maximum power this dam could produce?

Physics 161 Fall More... A conveyor belt delivers 100 kg of coal/minute to a height of 20 m. How much power is required?

Physics 161 Fall More... An alpha particle (helium nucleus with no electrons) is accelerated through a potential difference of 5000 V. What is the change in potential energy of the alpha? Give your answer in J. An alpha particle (helium nucleus with no electrons) is accelerated through a potential difference of 5000 V. What is the change in potential energy of the alpha? Give your answer in J. How fast is the alpha particle traveling? How fast is the alpha particle traveling?

Physics 161 Fall Ahem... Is it possible to cool your kitchen down by leaving the refrigerator door open? I mean, the whole kitchen and for an extended period of time. Is it possible to cool your kitchen down by leaving the refrigerator door open? I mean, the whole kitchen and for an extended period of time.