1 Hewitt Chapter 14 Reading, Temperature

Please read this linked chapter on Temperature from the Conceptual Physics text by Hewitt. Also, review the linked notes on temperature. Based on the reading, please answer the following prompts.

  • How would you explain the difference between heat, temperature and internal energy?
  • Based on your experience, can you think of a material with high or low heat capacity?
  • How would you explain the importance of the difference between relative and absolute temperature conversions?

2 Hewitt Chapter 15 Reading, Heat Transfer

Read the linked chapter from Hewitt Chapter 15 Conceptual Physics and answer the following.

  • What examples of convection do you see in your daily life?
  • How about in the weather in the Rohnert Park area?

3 Temperature Concepts

  1. Two objects start at the same temperature. One raises its temperature by one degree kelvin, the other by one degree celsius. Which is hotter?
  1. Two objects start at the same temperature. One raises its temperature by one degree fahrenheit, the other by one degree celsius. Which is hotter?
  1. A thermometer reads 40 in Celsius, what does it read in Kelvin?
  1. A thermometer shows a temperature increase of 20 degrees Celsius, what would the same temperature increase be on a Fahrenheit thermometer?
  1. What are the equivalent temperature values in Kelvin and Celsius for 95 degrees Fahrenheit?

3.1 Temperature Conversion Graphs

  1. Draw a plot for converting between the absolute temperature for Celsius and Fahrenheit
  2. Draw a plot for converting between a temperature change in Celsius and Fahrenheit

Use the range of weather temperatures in our area for your graphs.

3.2 Temperature Conversion Scales

  1. Draw a linear scale with the absolute Celsius and Fahrenheit
  2. Draw a linear scale for temperature changes in Celsius and Fahrenheit

Use the range of weather temperatures in our area for your graphs.

4 Heat Transfer Concepts

For each of these questions, defend your answer in a sentence.

  • Which method of heat transfer is most important to consider when designing a beer can “Koozie”?
  • Which method of heat transfer is most important when using an electric toaster?
  • Which method of heat transfer is important when you are using a heat gun?

5 Heat Transfer Examples

Come up with two examples for the following heat transfer mechanisms:

  • Conduction
  • Convection
  • Radiation

6 Temporal vs Spatial

It is common to confuse temperature differences (deltas or \Delta T) between two points in time and two points in space.

Why is this difference important and how will you keep these concepts distinct in your work?

7 EM-notes on Heat Capacity

Read the linked section of the EM-notes on heat capacity and answer the following questions.

  • How do the equations in the notes relate to the Hewitt Chapter 15 reading?
  • In your own words, what questions does the Q = mc\Delta T equation allow us to answer?

8 Heat Capacity Conceptual

What are the correct dimensions for mc in Q = mc \Delta T?

  1. energy per unit temperature
  2. energy per unit mass
  3. temperature per unit energy
  4. mass per unit energy

What are the units of energy if you use pounds for the mass, BTU per F per pound for the heat capacity, and F for the temperature?

What are the units of energy if you use grams for the mass, joules per gram per Kelvin for the heat capacity, and C for the temperature?

9 Viking Stove Boil

A Viking stove has an 18,500 BTU burner. This means that it creates 18,500 BTU of heat per hour. About how long should it take to bring ten pounds (about a gallon) of water to a boil? Assume the tap water is at 60 degrees F.

m = 10 * u.pound
c = 4186 * u.J / u.kg / u.degK
delta_T = (212-60) / 9  * 5 * u.degK
#delta_T = 85 * u.degK
#t = (30 * u.minute).to(u.second)
#power = (m * c * delta_T / t).to(u.watt)
energy = m * c * delta_T

3534844 joule * pound / kilogram

10 Crawfish Boil

You have been put in charge of a crawfish boil. This means you need to buy a burner that can bring a 20 gallon pot of water to a boil in 30 minutes. The water starts at 15C. How many watts does this burner need to be? We make all the simplifying assumptions (100% efficient heating, negligible heat loss of the water, etc.)

11 Conduction Conceptual

11.1 What are the correct dimensions for UA in q = UA \Delta T?

  1. energy per unit temperature per unit time
  2. power per unit temperature per unit time
  3. temperature per unit power
  4. temperature per unit energy

11.2 What are the dimensions of an R-value?

  1. power per area per temperature difference
  2. area times temperature difference per power
  3. power per area per temperature difference per time
  4. area times temperature difference

11.3 If you double the rate at which fuel is burned in a homes heater does the

  1. inside temperature double
  2. temperature difference between inside and outside double
  3. inside temperature decreases
  4. temperature difference between inside and outside decreases

11.4 Imagine a simple home with walls made of a single insulating material. You double the thickness of the walls. You hold other things the same: the weather, the rate of fuel being burned in the heater, the area of the home. Does the

  1. inside temperature double
  2. temperature difference between inside and outside double
  3. inside temperature decreases
  4. temperature difference between inside and outside decreases

12 Conduction Wall Metric

How much thermal power will conduct through a 10 square meter section of wall that has a U value of 10 watts per meter squared per Kelvin. The temperature outside is 5C and the temperature inside is 15C.

  • Draw a picture of the wall and label the relevant quantities.
  • Show your calculation of the power.

13 Conduction Wall US

How much power will travel through a 100 square foot section of wall that is rated with a total US R-value of 25 square feet-degrees F-hour per BTU on a cold day of 40F when the inside temperature is 70F? Give your answer in BTU/hour.

14 Pohl Heat Flow Reading

Read section 4.3 of the [linked chapter] and answer the following questions.

  • What does “steady-state” mean for the Q = U A \Delta T equation?
  • What parts of this section are most and least clear to you?

15 Reinventing Fire Heat Capacity

Read Chapter 3 of Reinventing Fire.

  • List two examples of heat capacity mentioned in the reading

16 Reinventing Fire Conduction

Read Chapter 3 of Reinventing Fire.

  • List two examples of heat transfer by conduction mentioned in the reading

17 Power Unit Conversion

A Viking stove has an 18,500 BTU burner. This means that it creates 18,500 BTU of heat per hour. How many watts is this? Show the unit conversion including BTU to joules and hours to seconds.

18 R-Value Conversion

If you have an US R-value of 25 hr-square feet-F/BTU for a type of wall, what is the SI R-value in units of square meter-kelvin per watt? Show the steps of the full conversion from BTU, feet, hours, and Fahrenheit to the metric pieces.

19 Passivehaus Cabin

What is US R-value so cabin is at comfy temp only with occupant heating?

You are designing a very small single room cabin that won’t have a heater and will be only heated by the two occupants. What does the overall UA value need to be in order to maintain the room at 65 degrees Fahrenheit when the temperature is 20 degrees Fahrenheit outside? Assume that a person radiates 100 W of heat energy.

20 Refrigerator

A freezer operates in a warm climate with a temperature of 35 degrees Celsius. The inside of the freezer is at a temperature of 2 degrees Celsius. What is the maxiumum COP possible for this temperature range?

  1. 8.33
  2. 9.33
  3. 1.06
  4. 0.061

21 Heat Engine

A coal plant is able to create steam at 600 degrees Celsius and uses cooling water at 25 degrees Celsius. What is the maximum possible efficiency of this plant?

22 Ton of Cooling

One ton of cooling is defined as the power to melt a one-ton (2000 pounds) block of ice in a 24 hour period. What is this power expressed in kilowatts?

23 Melting Ice

You have 10 kilograms of ice in a container. The ice is at a temperature of -5C. How much energy will it take to turn this ice into water that is at a temperature of 5C? Give your answer in MJ.

24 Ice Business cooling or freezing

Say you want to start an ice business. You start with water at 25C cool it to 0C and then freeze it. What takes more energy per gram, cooling the water to 0C or freezing it?

25 Melting Ice Mult Choice

How much energy must be added to 1 kg of ice at 0C to melt it and produce water at 0C?

  1. 334 kJ

  2. 334 J

  3. 33.4 kJ

  4. 3.34 kJ

26 Warming up ice

You place a cold (-5C) 100 gram ice cube in a cup in a warm room. How much energy does it take to bring the melt the ice and create water at 10C?

  1. 38.6 kJ

  2. 33.4 kJ

  3. 39.7 kJ

  4. 6.2 kJ

27 Melting Ice

You have a 500 gram block of ice at 0 C. How much energy must be added to the ice to melt it?

a) 0.0165 kJ
b) 0.165 kJ
c) 165 kJ
d) 16.5 kJ

28 Heat Pump and UA

We want to know the average power consumed by a heat pump for a small home. If UA value of a small home is 50 W/K and heat pump COP is 5, and temperature difference is 20 Celsius, what is electrical power needed?

29 Hewitt Change of State Reading

Please read the linked chapter and answer the following questions.

  • Why does sweating lower human body temperature?
  • What is the difference between evaporation and boiling?

30 Hewitt Thermodynamics Reading

Please read the linked chapter and answer the following questions.

  • How does the law of conservation of energy relate to the first law of thermodynamics?
  • What thermodynamic law influences the ideal efficiency of an automobile?

31 Refrigeration Cycle Video

Please watch these videos:

and answer the following questions:

  • Why doesn’t a refrigerator violate the second law of thermodynamics by moving heat from a cold mass to a hot mass?
  • What do you think the mechanisms of heat transfer are in the evaporator and condenser?
  • How is latent heat used in the refrigeration cycle?
  • How is the heat pump operation changed from heating to cooling?

32 Heat Pump Efficiency

What is the maximum possible efficiency for an air-source heat pump in the following conditions? The inside temperature is 70F, the outside temperature is 30F.

33 HSPF Energy Needs

You need to deliver 50 MMBTU of heating to a building with a heat pump with an HSPF of 10 BTU/Wh. How many kilowatt-hours of electricity are required?

34 SEER Energy Needs

You need to deliver 20 MMBTU of cooling to a building with a heat pump with a SEER rating of 20 BTU/Wh. How many kilowatt-hours of electricity are required?

35 HSPF and UA Energy Needs

You have a building with a UA value of 400 BTU/hr/degF in a climate with 2500 degF-days of heating needs. How many kWh of electricity are needed if you have a heat pump with an HSPF of 10?

36 SEER and Weather

Why do we need to know the weather temperature data to determine the SEER?

37 OpenStax Conduction Reading

Please read the linked page from OpenStax and answer the following questions:

  • Why is conductive heat transfer important to understand?
  • What properties of a material reduce or increase the rate of energy flow?

38 Tankless Water Heater

Assume you have a shower that consumes 1.5 gallons per minute and you’ll supply all of this water from a tankless on-demand water heater.

  • What is the power that must be delivered to the water to raise it from the tap temperature of 50 degrees F to the shower temperature of 100 degrees F?
  • If the water heater is 90% efficient, how much energy does it consume?

39 Radiant Floor Plot

Assume you have a concrete heated floor area of about 100 square meters, that the emissivity of the floor is 0.9, and that the room is at 70 degrees F.

Create a plot of the net power delivered to the room in a spreadsheet.

Using your plot, if that floor is heated to 90 degrees Fahrenheit and the surroundings are 70 degrees F, what do you estimate the net radiant heat from the floor to be in units of watts?

40 Equivalent Temperature for Electric Motor

As a thought experiment, what hot side temperature would be needed to get the 90% efficiency typical of an electric motor from a heat engine?

41 UA Box Resistor Temperature

42 Rate of Melting

A bar connecting a bath of ice water and a bath of boiling water will move thermal energy with a 100 C temperature difference. If the bar has a cross sectional area of 1 square centimeter and is 10 centimeters long, what is the rate of ice melting in grams per minute?