Temperature and Heat Transfer

1. Define temperature. The average kinetic energy of the particles in an object.

2. Define thermal equilibrium. Objects are in thermal equilibrium when they are at the same temperature.

3. Convert temperatures from C to K and vice versa. K = C + 273

4. Describe the 3 forms of heat transfer.

   • conduction: between stationary masses in contact with each other--directly proportional to contact area and temperature difference between them.

   • convection: between a mass and a fluid moving relative to the mass--directly proportional to contact area and temperature difference between them.

   • radiation: between a mass and its surroundings using electromagnetic radiation--directly proportional to area of the object, its emissivity (roughly speaking its color), and the difference between (object's temp)^4 and (surroundings temp)^4.

5. State the relative rate of heat transfer for the 3 different forms of heat transfer assuming areas and temperature differences are the same. radiation < conduction < convection
6. State that the sensation of hot or cold depends primarily on heat loss or gain and is not directly a measure of temperature.

   For example:

   • Wind chill makes the air temperature seem much lower.

   • A piece of metal at the same temperature as a pice of Styrofoam will feel colder

   • Standing in the sunlight will make the air temperature seem warmer than standing in the shade.

7. Name the type of heat transfer that does not require matter to mater contact. Radiation

8. Explain why running a fan in a room actually raises the temperature slightly. The energy used to move the air eventually ends up as heat. Running a fan slightly increases room temperature.

9. Explain why a fan blowing on a person makes them feel cooler. It increases a person's heat loss.

10. Explain steps that can be taken to make a room feel warmer without raising air temperature.

    Reduce drafts or air movement.

    Insulate outside walls or ceiling so that they become warmer on the inside.

    Heat the floor instead of directly heating the air.

    Use radiant heaters that heat people more directly than just heating the air.

     

Relevance: By understanding heat transfer it is possible to create comfortable spaces inside buildings without wasting energy.

Homefun (formative/summative assessment): Read sections 12.1

Formative Assessment: Physics Investigation

Follow up Questions

Specific Heat

7. Define specific heat. The amount of heat required to increase a unit of mas by a Unit of temperature
8. State the relationship of the amount of heat transferred to an object and its increase in temperature.

   Q = mC DT

9. Solve problems related to the change in temperature caused by heat transfer

   Formative Assessments

   Bullet problems: A bullet traveling at 300 m/s hits an object and all the kinetic energy inside it instantly changes to heat. By how much will it increase in temperature. The melting temperature of lead is 327.5 degrees C. Why would the bullet not melt?

   Cost of heating water: A typical 40 gallon hot water tank holds 151 kg of water at 50 degrees C with an inlet temperature of about 20 degrees C. In SC 1Joule of energy costs $ 2.9 (10^-8). Estimate the energy cost of taking a shower. Considering that around 20% of residential energy use is consumed by water heating, What are some of the ways to reduce the energy use for water heating?

   Meteor problems: A meteor traveling at 25,000 mph (11,200 m/s) hits Earth and all the kinetic energy in the meteor changes into heat that remains inside the meteor. By how much will it increase in temperature? Assume that it's made of iron with specific heat = 444 J /( kg K). Decribe what would likely happen to the meteor.

10. Solve calorimeter problems.

    find the specific heat of a substance

11. Describe the basic elements of a passive solar house and how they relate to thermodynamics.

    South facing windows -- these allow radient heat from the Sun to enter and warm the house

    Thermal mass -- the Sun needs to shine on some form of thermal mass to store heat which is released at night.

    High specific heat is a desirable property of thermal mass.

    Proper overhanges on Southern windows -- this allows light to come in in the winter but blocks light in the summer.

Homefun (formative/summative assessment) problems 6, 7, 8 p. 321.

Changes of State

12. Define

    heat of fusion: the amount of heat required to melt a unit of mass of a solid

    heat of vaporization: the amount of heat required to vaporize a unit of mass of a liquid

13. State that under typical conditions the melting points and freezing points of a material are the same, however, there are special conditions when this is not true, such as with supercooled liquids.

14. Assuming the normal condition where freezing and melting points are the same, describe the condition that determine if a material will freeze or melt.

15. State that while the boiling point of s liquid typically has the same value for a given pressure, liquids can be superheated under special conditions beyond their boiling points without boiling.

16. State that the point at which a liquid condenses depends heavily on both solubility and temperature. Water, for instance is highly soluble in air and in air has to be cooled well below it's boiling point to condense.

17. Draw and label the phase temperature vs. heat diagram for water.

18. Solve problems using the temperature vs. heat diagram of water.

    Specific Heat Air: 1 J/(gm K)

    Specific Heat Water: 4.18 J/(gm K)

    Heat of fusion Water: 334 J/gm

    Heat of vaporization Water: 2260 J/gm

     

Homefun (formative/summative assessment):Section Review Problems 19 to 21; page 325

1st Law of Thermodynamics

Relevance:Heat engines based on the 1st law power out cars while heat pumps both heat and cool our buildings.

16. State the purpose of a heat engine. Convert thermal energy or heat into mechanical energy or work.

17. Draw a diagram of a heat engine and explain how it obeys the 1st Law of Thermodynamics.

18. State the efficiency of a heat engine. efficiency = (work out) / (heat in)

19. State that even a perfect friction-free heat engine cannot achieve 100% efficiency. Some amount of heat always has to be rejected to the cold reservoir.

Where the energy goes in cars

Energy Flow

20. Solve heat engine problems.

21. Explain that a refrigeration system is a heat engine that's run backwards. Running a heat engine bacwards makes it possible to transfer heat backwards or in other words transfer heat from a low temperature to a high temperature region.

22. State how a heat pump system (a type of refrigeration system) can be used for heating or cooling.

23. Describe the possible advantages of a heat pump used for heating. In a typical heating application the energy available for heating is always less than the energy supplied to the heater. With a heat pump, the energy available for heating is greater than the energy supplied to run the heat pump assuming that the temperature increas created by the heat pump is relatively low.

     

Homefun (formative/summative assessment): problems 22 to 24, page 328

2nd Law of Thermodynamics

Relevance: The 2nd Law makes it clear that many things cannot happen, such as perpetual motion, driving a car with the back wheels while generating the electricity to do so with the front wheels, having a cold object spontaneously heat up, etc.

24. State the implications of the 2nd Law with respect to heat engines

    Even if friction free, no heat engine operating in a cycle could convert 100% of its heat input into work output.

    The maximum possible efficiency of a heat engine is called Carnot efficiency

    Carnot efficiency = 1 - T_c / T_h

    where:

    T_c = the cold temperature where the heat flows out of the heat engine

    T_h = the hot temperature where the heat flows into the heat engine

     

25. State the implications of the 1st and 2nd Law with respect to perpetual motion machines.

    A perpetual motion machine cannot work in the real world.

    The 1st Law of Thermo says such a system would have a finite amount of mechanical energy which would slowly decline as it was converted to heat by friction. When the system reached zero mechanical energy it would stop.

    The 2nd Law of Thermo delivers the final blow by saying that mechanical energy lost as heat could never be completely recovered and converted back into mechanical energy.

26. Define entropy 2 ways.

    • A system property that is a measure of the degree of disorder of a thermodynamic system.

    • An indication of how much thermal energy is unavailable for doing work at a given temperature. Entropy times absolute temperature can be  interpreted as a measure of the unavailable thermal energy in a system.

27. State the 2nd Law with respect to entropy.

    The entropy of the universe can be increased but not decreased. Likewise, the total entropy for an isolated system can be increased but not decreased.

     

Homefun (formative/summative assessment): problems 57, 61, 67 page 337

Review for the Test

Formative Assessments:

1. Work review problems at the board

2. Work practice problems.

Metacognition Problem Solving Question: Can I still work the problems done in class, several hours or days later? Some amount of repetition on the exact same problems is necessary to lock in learning. It is often better to thoroughly understand a single example of a problem type than to work example after example understanding none of them completely.

Relevance: Good test preparation is essential to performance in physics class.

Homefun (formative/summative assessment): turn in on the day stapled to the back of the test.

Summative Assessment: Unit exam objectives 1-23