Physics 161 Fall 2006 1 Announcements First quiz is next Monday (10/23) and covers chapters 1-4, homework #1 and #2. You can find a cheat sheet here -

1

Physics 161 Fall 2006

AnnouncementsAnnouncements First quiz is next First quiz is next Monday (10/23)Monday (10/23) and covers and covers

chapters 1-4, homework #1 and #2. You can chapters 1-4, homework #1 and #2. You can find a cheat sheet here - this contains formulas find a cheat sheet here - this contains formulas for the quiz and will be the first page of the quiz.for the quiz and will be the first page of the quiz.

Please have HW#2 complete no later than 2 days Please have HW#2 complete no later than 2 days late, that is, by Sunday at 5:00 . . . It is due on late, that is, by Sunday at 5:00 . . . It is due on Friday at 5:00pm, and you will need to request Friday at 5:00pm, and you will need to request an extension if want to to do it over the weekend.an extension if want to to do it over the weekend.

2


Heat Pumps and Refrigerators:Heat Pumps and Refrigerators:more entropymore entropy

Heat Pumps provide a means to very efficiently move heataround, and work both in the winter and the summer

3


Heat Pump DiagramHeat Pump Diagram

4


Heat Pumps and Refrigerators: Heat Pumps and Refrigerators: ThermodynamicsThermodynamics

Th

Qh

Qc

W = Qh – Qc

Tc

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 runbackwards…

efficiency = =W work doneQh heat delivered

(heat pump)

efficiency = =W work doneQc heat extracted

(refrigerator)

5


Heat Pump/Refrigerator Heat Pump/Refrigerator EfficienciesEfficiencies

Can work through same sort of logic as before Can work through same sort of logic as before to see that:to see that: heat pump efficiency is: heat pump efficiency is: TThh//((TThh – T – Tcc) = ) = TThh//TT

in ºK in ºK refrigerator efficiency is: refrigerator efficiency is: TTcc//((TThh – T – Tcc) = ) = TTcc//TT

in ºK in ºK Note that heat pumps and refrigerators are Note that heat pumps and refrigerators are

most efficient for small temperature most efficient for small temperature differencesdifferences hard on heat pumps in very cold climateshard on heat pumps in very cold climates hard on refrigerators in hot settingshard on refrigerators in hot settings

6


Example EfficienciesExample Efficiencies

A heat pump maintaining 20 ºC when it is –5 ºC A heat pump maintaining 20 ºC when it is –5 ºC outside has a maximum possible efficiency of: outside has a maximum possible efficiency of:

293/25 = 11.72293/25 = 11.72 note that this means you can get almost 12 note that this means you can get almost 12

times the heat energy than you are supplying times the heat energy than you are supplying in the form of work!in the form of work!

this factor is called the C.O.P. (coefficient of this factor is called the C.O.P. (coefficient of performance)performance)

A freezer maintaining –5 ºC in a 20 ºC room has a A freezer maintaining –5 ºC in a 20 ºC room has a maximum possible efficiency of:maximum possible efficiency of:

268/25 = 10.72268/25 = 10.72 called EER (energy efficiency ratio)called EER (energy efficiency ratio)

7


Example Labels (U.S. & Canada)Example Labels (U.S. & Canada)

8


Heat, Heat Transfer, Heat, Heat Transfer, ProblemsProblems

Heat vs. temperature: heat is energy in motion and is Heat vs. temperature: heat is energy in motion and is related to temperature change by heat capacity; related to temperature change by heat capacity; temperature is a measure of the average kinetic energy temperature is a measure of the average kinetic energy the atoms inside something havethe atoms inside something have

Heat is energy in motion - we do not ask ‘how much Heat is energy in motion - we do not ask ‘how much heat does something have in it’ (at least physicists heat does something have in it’ (at least physicists don’t); it goes into the energy balance described by the don’t); it goes into the energy balance described by the first law - the ‘conservation of energy’first law - the ‘conservation of energy’

Controlling heat transfer is a key ingredient of ‘energy Controlling heat transfer is a key ingredient of ‘energy conservation’ and cooking. How is heat transferred?conservation’ and cooking. How is heat transferred?

9


Heat Transfer: ConductionHeat 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?

10


Heat Conduction and Insulation Heat Conduction and Insulation R-factorsR-factors

Ever shop for insulation? If so, you’ve run up against R-factors or Ever shop for insulation? If so, you’ve run up against R-factors or R-values. These are essentially d/R-values. These are essentially d/, which is a normalized , which is a normalized efficiency for insulationefficiency for insulation

Large R means good insulation: Q/t = A x Large R means good insulation: Q/t = A x T/RT/R

Example: 6” of fiberglass insulation has an R-value of 19 in units Example: 6” of fiberglass insulation has an R-value of 19 in units of ftof ft22-hr--hr-ooF/Btu. With a temperature difference of 30 F/Btu. With a temperature difference of 30 ooF, what is the F, what is the rate of heat flow through 100 square feet of R-19 fiberglass?rate of heat flow through 100 square feet of R-19 fiberglass?

Q/t = 100 ftQ/t = 100 ft22 x 30 x 30ooF/19 ftF/19 ft22-hr--hr-ooF/Btu = 158 Btu/hr = 46 WattsF/Btu = 158 Btu/hr = 46 Watts

This ignores other forms of heat transfer. . .This ignores other forms of heat transfer. . .

Water balloon demo . . . Huh?Water balloon demo . . . Huh?

11


Heat Transfer by RadiationHeat Transfer by Radiation

This is what we called Blackbody Radiation a last week: This is what we called Blackbody Radiation a last week:

F = F = TT44 in Watts per square meter in Watts per square meter = 5.67 = 5.671010-8-8 W/ºK W/ºK44/m/m22

Except we did not talk about emissivity Except we did not talk about emissivity , which is a measure of , which is a measure of how efficiently something radiates; black things radiate efficiently how efficiently something radiates; black things radiate efficiently and have emissivity of ~1, white and shiny things do not and have and have emissivity of ~1, white and shiny things do not and have emissivity below 0.1. emissivity below 0.1.

In reality, In reality, F = F = TT44 and power = P = and power = P = TT44 A A

Black vs silver radiation demoBlack vs silver radiation demo

12


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.

On-shore vs. off-shore breezes.

Heat Transfer by ConvectionHeat Transfer by Convection

13


Heat Transfer SummaryHeat Transfer Summary

Controlling heat transfer requires careful design, taking Controlling heat transfer requires careful design, taking into account conduction, radiation, and convection.into account conduction, radiation, and convection.

Building codes have evolved a lot over the past several Building codes have evolved a lot over the past several decades to require better and better insulation - old decades to require better and better insulation - old houses lose a lot of heat compared to new houseshouses lose a lot of heat compared to new houses

On a global scale, heat transfer by convection and On a global scale, heat transfer by convection and radiation are key ingredients of atmospheric physics. So radiation are key ingredients of atmospheric physics. So are latent heats . . .are latent heats . . .

14


Latent Heat: Heat Associated with a Latent Heat: Heat Associated with a Phase ChangePhase Change

Add heat to a solid, e.g., ice, and the Add heat to a solid, e.g., ice, and the temperature rises according to the material’s temperature rises according to the material’s specific heat: Q = m cspecific heat: Q = m ciceice TT

When the ice melts, the temperature remains When the ice melts, the temperature remains constant at 0constant at 0ooC while a latent heat of fusion C while a latent heat of fusion of 80 cal/g is added and the ice changes to of 80 cal/g is added and the ice changes to water.water.

Adding additional heat to water again raises Adding additional heat to water again raises the temperature according to the heat the temperature according to the heat capacity of Q = m ccapacity of Q = m cwaterwater TT

At 100At 100ooC, the water boils and the latent heat C, the water boils and the latent heat of vaporization of 540 cal/g must be added to of vaporization of 540 cal/g must be added to produce water vaporproduce water vapor

Adding additional heat to water vapor will Adding additional heat to water vapor will raise the temperature according to Q = m raise the temperature according to Q = m ccvaporvapor TT

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.

15


Quiz FormatQuiz Format

10 short answer questions, similar (in some cases identical) 10 short answer questions, similar (in some cases identical) to those in the ‘questions’ sections at the end of the chapters to those in the ‘questions’ sections at the end of the chapters in the textin the text

6 multiple choice, probably some a little bit numerical (e.g., a 6 multiple choice, probably some a little bit numerical (e.g., a unit change)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 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 ‘problems’ sections in chapters 1-4. If you can do all of those that can be done simply, you will be fine.that can be done simply, you will be fine.

16


A flashlight lamp connected to a battery that A flashlight lamp connected to a battery that provides 1.4 V draws a current of 0.10 A. What provides 1.4 V draws a current of 0.10 A. What power is used by the lamp? 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. . . More. . .

17


More . . .More . . .

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

18



A 100 Watt light bulb has a tungsten filament with an area of 100 mm2. Estimate the temperature of the filament when the bulb is on. (P = T4 A; = 5.67x10-8 W/(m2 K4), A = area, T = absolute temperature)

Tungsten melts at ~4000K. How much power could the light bulb handle before the filament melts?

19



100 liters of water is heated from 0oC to 100oC. How much heat does this require? (heat capacity of water is Cp = 1 Cal/(kg oC); 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?

20



How much power does an adult with a surface area of 2 m2 radiate?

Assuming the heat capacity of a human is the same as that of water (1 Cal/(kg oC)) 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 10oC?

21



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?

22



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

23



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

How fast is the alpha particle traveling?How fast is the alpha particle traveling?

24


Ahem . . .Ahem . . .

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

Physics 161 Fall 2006 1 Announcements First quiz is next Monday (10/23) and covers chapters 1-4, homework #1 and #2. You can find a cheat sheet here -

Documents

t c t h t c

t h t h t c

q c heat

heat pump efficiency

heat engine

heat transfer

problems heat

q h heat