1. Temperature, Heat, Specific Heat, & Heat Transfer
THERMAL ENERGY
Temperature, Heat, Specific Heat, & Heat Transfer

2. Thermal Energy Examples? “Kinetic-Molecular Theory”
Hot object – particles move faster, & have more KE
When average KE of atoms & molecules increases, they need more room to move around
Objects expand when they get hot
If average KE decreases, object contracts
Examples?

3. Thermal Energy How do we determine “hotness” of an object?
“Thermal Energy” – the overall energy of motion of the particles that make up an object
Hot object = more thermal E, & vice versa
How do we determine “hotness” of an object?
TEMPERATURE!
Body heat or heat lamps are part of the infrared spectrum.

4. Temperature Temperature – a measure of the AVERAGE molecular KE
3 commonly used temp scales:
Fahrenheit (United States) (98.6 ° F = avg body temp)
Celsius (most of the rest of the world)
Kelvin (scientific community – i.e., us!)
Both C and K scales are “metric” scales because Freezing Point and Boiling Point are 100 degrees apart on each

5. Kelvin Temperature Scale
K = °C + 273
“Absolute Zero” = 0 K
i.e., no molecular motion, therefore no KE on the molecular level
0 K is the lowest you can go! You can’t stop motion more than 0

6. Temperature

7. Heat & Temperature
Heat (Q) and temperature are NOT the same thing!
Heat – refers to total amount of thermal energy in a substance
Temp – refers only to average kinetic energy
Which has more heat: 2L of boiling water or 1L of boiling water?
2L has more heat, even though temp is the same!
Can a cup of hot water have the same heat as a cup of cold water?
YES! If you have a larger mass of cold water and a smaller mass of hot

8. Specific Heat (c)
Heat Units: Q can be measured in Joules
Specific Heat (c) – the amount of energy needed to raise the temp of a substance
1 calorie = amount of Q needed to raise 1g (1mL) of water by 1°C
1 Calorie = amount of Q needed to raise 1 kg (1L) of water by 1°C
The higher the specific heat of a substance, the longer it takes to change temperature

9. Specific Heat
The higher the specific heat of a substance, the longer it takes to change temperature
High specific heat  Slow temp change
Low specific heat  Fast temp change
Water has a very HIGH specific heat (c = 4186 J / kg K)
It is good to use as a coolant!

10. Specific Heat - Examples
Beach sand – LOW specific heat
Hot in the day and cold at night
Ocean water – HIGH specific heat
Temp at day and night roughly the same
Desert – huge swings in temp due to low specific heat of sand
British Isles – temperate climate because the water surrounding them holds in the heat
Weather in Austin more temperate than Dallas because they are closer to the coast

11. Heat Transfer Heat Transfer
Heat will transfer between objects if they are not in thermal equilibrium (at the same temp), and if they are in thermal contact (touching).
Ex: thermometer, metal spoon in a bowl of hot soup

12. Heat Transfer How is heat transferred?
Warmer molecules have higher KE – they collide with colder molecules and transfer some of their KE to the colder molecules.
“Cold” is not a physical quantity!
“Cold” cannot move from one object to another – only “heat” (thermal energy) moves!

13. Methods of Heat Transfer
Conduction
Convection
Radiation

14. Conduction
Conduction – Q transferred through collisions of electrons, atoms, & molecules
i.e., through direct contact between objects
Ex: Thermometer – particles in your body hit the glass, transferring their KE to the mercury & raising its KE

15. Conduction, cont’d
Conductor – good conductors of Q are also good conductors of electricity (ex: most metals)
Insulators – bad conductors of Q
Examples: styrofoam, wood, rubber, GASES (many times it is the air trapped in the substance that is actually doing the insulating!)

16. Convection
Convection – Warm, less-dense fluid (gas or liquid) rises and cool, more-dense fluid sinks
“Convection Current”
Examples:
Sea breeze & land breeze
Heating & cooling vents in your home

17. Radiation Radiation Examples:
Infra-red electromagnetic wave that can travel through empty space
Created by vibrating electrons
Examples:
Night vision goggles
Infra-red satellite images
“Heat-seeking” missiles

18. “Conduction, Convection, or Radiation?” wkst

19. Now what do you think?
Suppose you have two cups of water. One is hot and the other is cold.
How is the cold water different from the hot water?
Describe the motion of the molecules in each.
What changes would occur if the hot water was changed into steam?
What are the common scales used to measure temperature?
When is each scale generally used?
All scales use degrees to measure temperature. Which scale has the largest degrees? Explain.
The molecules move faster in the hot cup, on the average. Some of the molecules in the hot cup may be moving slower than some of those in the cold cup, so we must talk about average molecular speed. The particles are farther apart when the water changes into a gas.
Scientists use Celsius and Kelvin temperature scales, depending on the circumstances. Fahrenheit is used in the U.S. for meteorology. The Celsius and Kelvin degrees, which are equal in size, are both 1.8 times larger than a Fahrenheit degree. Have students count the number of degrees between freezing water and boiling water.

20. What do you think?
Internal energy is the energy due to the kinetic and potential energy of the particles.
Does the ice water or an equal quantity of hot chocolate have greater internal energy? Why?
Which has more internal energy, a gallon of cold water or a drop of hot chocolate?
How will the internal energy of the water and hot chocolate change over time?
How will this change occur?
When asking students to express their ideas, you might try one of the following methods.
(1) You could ask them to write their answers in their notebook and then discuss them.
(2) You could ask them to first write their ideas and then share them with a small group of 3 or 4 students. At that time you can have each group present their consensus idea. This can be facilitated with the use of whiteboards for the groups.
The most important aspect of eliciting student’s ideas is the acceptance of all ideas as valid. Do not correct or judge them. You might want to ask questions to help clarify their answers. You do not want to discourage students from thinking about these questions and just waiting for the correct answer from the teacher. Thank them for sharing their ideas. Misconceptions are common and can be dealt with if they ar125125e first expressed in writing and orally
Students will probably agree that the hot chocolate has the greater amount of internal energy. It is important that they eventually realize that the internal energy does depend on the quantity. If students are unclear about this now, be sure to return to this point on the concluding slide.
Students may not have a clear understanding of the process that causes one substance to lose energy while the other substance gains energy. Try to get them to explain the process on a molecular level. That is what we are measuring with internal energy.

21. Internal Energy and Heat
Internal Energy (U) is the energy contained within the particles of a substance.
Heat (Q) is the internal energy transferred between objects.
Heat always moves from a higher-temperature object to a lower-temperature object.
The rate of transfer depends on the difference in temperature.
The greater the temperature difference, the greater the rate of energy transfer (if other factors are equal).
Point out to students that heat is not the same as internal energy; heat is internal energy that moves from a hotter substance to a colder substance.

22. Internal Energy and Heat
Which way does heat flow if you place a warm canned beverage in cold water?
How does this occur, on a molecular level?
Answers:
from the beverage into the water
Molecules in the beverage are moving faster, so they strike the molecules in the can. This causes the molecules in the can to move faster. The molecules in the beverage slow down as they lose energy. The molecules in the can then strike the molecules of the liquid, causing them to move faster. Eventually, the beverage, can, and water will reach the same temperature. The can and beverage will be colder, and the water will be warmer.

23. Temperature and Heat Click below to watch the Visual Concept.

24. Heat Transfer
In what three ways can internal energy be transferred from a hot object to a colder object?
Conduction is the transfer of heat through a substance by molecule to molecule contact.
Metals are good conductors.
Styrofoam is a good insulator.
Convection is the transfer of energy by the movement of a fluid.
Hot air in a room rises and cold air moves in to replace it.
Radiation is the transfer of energy by electromagnetic waves.
No matter is transferred, only energy.
Use the following examples to probe students about each method of energy transferring before revealing the related bullet points.
First, ask students how heat gets from their hand into the liquid in a can of soda they are holding. See if they come up with the term conduction.
Next ask students how the heat from a campfire can warm your hand if it is held high above the flame. There is no contact, so conduction is not the answer. They may say radiation. This would be true above or beside the fire. Above the flame, you also have the warm air rising toward your hand.
Finally, ask students how the sun’s energy reaches Earth. Radiation is heat transfer through electromagnetic waves. The sun’s energy is transferred to Earth in this way. The sun loses energy as the radiation is sent out into space, and Earth gains energy as it absorbs this radiation.

25. Comparing Convection, Conduction, and Radiation
Click below to watch the Visual Concept.
Visual Concept

26. Heat and Work Work can be changed into internal energy.
Rub your hands together and you’ll feel the increase in internal energy produced by your work.
Pull a nail from a piece of wood and the nail is hot.
Mechanical energy (PE + KE) is conserved when there is no friction.
Total energy, including internal energy, is always conserved.

27. Now what do you think?
Does the ice water or an equal quantity of hot chocolate have greater internal energy? Why?
Which has more internal energy, a gallon of cold water or a drop of hot chocolate?
How will the internal energy of the water and hot chocolate change over time?
How will this change occur?
For equal quantities, the hot chocolate has greater internal energy because the particles have greater kinetic and potential energy. However, a gallon of water has more internal energy than a drop of hot chocolate, because internal energy also depends on quantity.
The hot chocolate will lose internal energy while the ice water gains internal energy. The hot chocolate will transfer energy to the air when the molecules in the liquid strike the air molecules and transfer KE to them. Similarly, the air molecules are moving faster than the ice-water molecules. Thus, collisions between the two will cause the ice-water molecules to move faster and the air molecules to slow down.

28. What do you think?
What property of water makes it so useful as a coolant in automobiles, nuclear reactors, and other machinery?
How does it differ from other liquids regarding its ability to cool substances?
Why do you feel cool when getting out of a warm swimming pool on a hot day?
How do you feel if it is windy out? Why?
How do you feel if it is an indoor pool? Why?
When asking students to express their ideas, you might try one of the following methods.
(1) You could ask them to write their answers in their notebook and then discuss them.
(2) You could ask them to first write their ideas and then share them with a small group of 3 or 4 students. At that time you can have each group present their consensus idea. This can be facilitated with the use of whiteboards for the groups.
The most important aspect of eliciting student’s ideas is the acceptance of all ideas as valid. Do not correct or judge them. You might want to ask questions to help clarify their answers. You do not want to discourage students from thinking about these questions and just waiting for the correct answer from the teacher. Thank them for sharing their ideas. Misconceptions are common and can be dealt with if they are first expressed in writing and orally.
Students may attribute water’s cooling properties to its ability “conduct” heat. Water, however, is not a very good conductor. See if students make statements regarding the ability of water to absorb heat without increasing its temperature a great deal. They may also discuss the cost of water and its availability, which are important factors as well. Try to lead the discussion back towards a discussion of heat and thermal energy.
Try to elicit as many ideas as possible regarding the cool sensation when getting out of a pool. Students may know that water evaporation is a cooling process. If they do, ask them why evaporating water would cool you off. Does water evaporating from a freshly-mopped floor cool the floor as well?

29. Specific Heat Capacity
Specifc heat capacity (cp) measures the amount of heat required to raise the temperature by 1°C for 1 kg of a substance.
It is different for every substance.
SI Units: J/kg•°C
Alternate form of this equation: Q = cpmT

30. Suppose each metal shown above absorbs 100 J of energy.
Which will show the greatest increase in temperature? the least?
How does water compare to iron with regard to heat capacity?
The temperature of aluminum would rise the least, and the temperature of lead would rise the most. Students need to realize that a high heat capacity means that the temperature will rise more slowly as energy is absorbed because the substance has a high capacity for absorbing heat.
Water has a heat capacity that is more than 9 times greater than iron. For a given amount of heat, the temperature of the iron will go up 9 times as much as the water. Point out that water therefore makes a nice coolant for cast-iron engine blocks, and for machinery made of aluminum and iron. It can absorb a large quantity of heat without a large increase in temperature.

31. Latent Heat Latent heat is heat gained or lost during phase changes.
When substances melt, freeze, boil, condense, or sublime, the temperature does not change during the phase change.
Heat absorbed changes the potential energy of the particles.

32. Latent Heat
Click below to watch the Visual Concept.
Visual Concept

33. Latent Heat
Heat of fusion (Lf) is the heat required to melt 1 kg of a substance.
Also equals the heat released when 1 kg freezes
Which graph segment represents this?
Heat of fusion is segment B, and heat of vaporization is segment D. The other portions show the increase in temperature (KE) as the heat is absorbed.
Segment D has a broken line because the heat of vaporization for water is so large the line would have been very long.
You can refer to Table 5 in the text to show students how the heat was evaluated for each segment.
Heat of vaporization (Lv) is the heat required to change 1 kg of a substance from a liquid to a gas.
Which graph segment represents this?

34. Latent Heats of Fusion and Vaporization
Note the high heat of vaporization for water (2.26 million J/kg). The evaporation of water requires large amounts of heat. This is why swimming pools get colder as the water evaporates from the surface. The surface molecules absorb heat from the molecules below and evaporate, leaving those molecules left behind with less energy.

35. Now what do you think?
What property of water makes it so useful as a coolant in automobiles, nuclear reactors and other machinery?
How does it differ from other liquids regarding its ability to cool substances?
Why do you feel cool when getting out of a warm swimming pool on a hot day?
How do you feel if it is windy out? Why?
How do you feel if it is an indoor pool? Why?
Water has a very high specific heat capacity. Water can absorb more heat than nearly all liquids without large increases in temperature.
Evaporation is a phase change that removes heat. The latent heat of vaporization for water is 2.26 x 106 J/kg, so every gram of water that evaporates from your skin requires 2.26 x 103 J of energy. This cools your skin. It is faster on a windy day, so you feel colder. Indoors, evaporation is slowed because of the high humidity, so you do not feel as cold as you would outdoors.