1. Unit 6
Temperature & Heat
Chapters 21, 22
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2. Temperature
The average kinetic energy within the particles of a substance or object.
Hot and cold are only sensations and not measures.
Thermometer
Galileo credited with making the 1st thermometer.
Celsius and Fahrenheit are the scales that we see in common use.
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3. Temperature
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4. Thermal Expansion
Why do your windows and doors tend to stick more in the summer time?
How do you open a stuck jar lid?
If you place a handful of ice cubes in a glass of water, will the water level go up, down, or stay the same when they melt?
Why do we allow telephone lines to sag when stringing them in the summer?
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5. Thermal Expansion What happens to materials as they are heated?
Do ALL materials expand when heated?
Water has special properties…
Blinking Christmas lights…
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6. So, What do you think?
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7. So, What do you think?
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8. How thermometers work…
What happens to the material in the thermometer as the temperature goes up?
Liquids expand in a predictable manner as temperature goes up.
Most thermometers rely on expansion of liquids.
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9. Celsius-Fahrenheit conv.
Fahrenheit-Celsius conv.
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10. The Kelvin scale or absolute scale
Based on absolute zero.
What is absolute zero?
KE in the matter is zero.
“No molecular movement”
This is a theoretical point
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11. Celsius-Kelvin conversion
K = ºC + 273
0 K = -273 ºC
0 ºC = 273 K
ºC ºF K
Freezing point of water
Boiling point of water
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12. You Try it... Convert body temperature to Celsius…and to Kelvins.
98.6ºF = 37ºC = 310K
Convert room temperature to Celsius…and to Kelvins.
70.0ºF = 21.1ºC = 294K
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13. HEAT A transfer of thermal energy between two objects or substances.
Transfers in nature are always from high energy to low.
High temperature to low.
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14. Objects can have potential, kinetic, or thermal energy.
Objects can not “have” heat or work. These are not properties.
Heat and work are energy transfers.
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15. Thermal Equilibrium
Thermal equilibrium is reached when any two or more objects in thermal contact have equal temperatures.
Objects in contact in nature seek to be at the same temperature
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16. What is a calorie? The calorie is widely used to discuss food energy.
1 calorie = 4.18 Joules
1 Calorie = 4184 J = 4.18 KJ
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17. Think about it…
What has more thermal energy, a glass of boiling water, or the pond across the road when the water is 40ºF???
What does thermal energy depend on?
Mass
Temperature
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18. So, What do you think?
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19. Specific Heat Capacity (Cp)
The amount of energy transferred as heat that will raise the temperature of 1g of a substance by 1 ºC.
Differs for each substance.
Can be used to identify a substance.
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20. Units of Specific heat
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21. Think about it…
On a hot summer day, why does the pool water stay cool?
Specific heat of water is higher than the air around it.
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22. Think about it…
What has a higher specific heat capacity, water or sand? How can you tell from every day life???
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23. Changes in thermal energy can be measured.
Change in thermal energy = Q
Q = m·Cp·ΔT
ΔT = change in temperature
m = mass
Cp = specific heat of substance
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25. Try it out…
Ex #1: How much thermal energy is needed to heat 2.00 kg of copper from 21 ºC to 35 ºC?
Ex #2: How much thermal energy is radiated off as a cup of 225 mL of hot coffee cools from 98.0 ºC to 90.5 ºC?
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26. So, What do you think?
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27. Global Warming? What does current data show?
What does historical data show?
What/Who is responsible for current trends?
Should you sell your SUV?
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28. Methods of Energy Transfer
Heat energy transfer can occur in 3 ways:
Conduction
Convection
Radiation
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29. Conduction
Transfer of energy as heat between particles as they collide.
Within a substance or between two in contact with each other.
Objects of differing temperatures in contact with each other will always experience conduction.
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30. Convection
The transfer of energy by the movement of liquids or gases with differing temperatures.
As molecules are heated, they expand and become less dense.
These particles then rise.
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31. Convection: Can only occur in liquids and gases.
Solids are too tightly packed. Molecules are not free to move.
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32. Convection currents
This is the cycle of heated (less dense) liquid or gas rising and being replaced by cooler (more dense) that falls to take its place.
Responsible for plate tectonics
Responsible for weather and ocean current patterns
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33. So, What do you think?
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34. Radiation Transfer of heat energy by electromagnetic waves.
As these waves strike molecules they result in an increase in the KE of the molecules they contact.
Radiation differs from conduction and convection.
Does not involve the movement of matter.
Radiation is energy transfer that can occur in a vacuum.
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35. Conductors and Insulators
Materials that are conductors transfer energy quickly.
Pans are made of metal because metal is a good conductor.
Gases are poor conductors, due to particles being so far apart.
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36. Insulators Solids in general are the best conductors.
Rubber and wood are solids which are poor conductors.
Metals in general are better than non-metals.
Insulators
Poor conductors of energy transfer.
We use these to prevent unwanted heat transfer.
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37. Emitters and Absorbers
Some materials absorb thermal radiation better than others?
Can you name some?
Good Absorbers are also good emitters!
What is the best color inside/outside for a cold weather sleeping bag?
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38. So, What do you think?
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39. Brownies anyone?
Why are there different cooking times for brownies if you are cooking in a metal pan vs. a glass pan? Which cooks faster? Why?
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