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Heat and Temperature
Section 1: Temperature
•What does temperature have to do with energy?
•What three temperature scales are commonly used?
•What makes things feel hot or cold?
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Intro: Discussion
A person from Seattle tells his friend from Florida that the weather in Seattle is somewhat warm. When the friend arrives for a visit, he finds that he is uncomfortably cool wearing the shorts he packed. What would be a more effective way for the person from Seattle to explain the weather?
Temperature
- The degree of “hotness” or “coldness” of an object,
- related to the average kinetic energy of an object’s atoms or molecules
•What makes something hot?•Particles that make up matter are in constant motion
•They have kinetic energy
•When you heat something the particles move faster So, what kind of energy
does temperature measure?
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Kinetic Energy
•transferred as particles collide
•Particles are always in motion no matter what state they are in.
Kinetic Energy Temperature
What is happening to the coffee?What else is being affected?
• Thermometers- mechanical or electrical device for measuring temperature.
• Early thermometer was invented by Galileo.
• How thermometer works:• Most materials expand when heated.
• Liquid thermometers have a large bulb hooked to a narrow tube.
• When the liquid expands it rises up the tube.
• Won’t work if too hot or cold
Measuring Temperature
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•Metals expand when heated
•Different metals expand different amounts
•Hook two metals together to make a thermometer that turns
•Bimetallic Thermometers•Used in thermostats
IronCopper Hot
Cold
Measuring Temperature
Limits of Temperature
-Upper Limit
- No upper limit exists.
- Plasma found in starts= millions of degrees C
-Lower Limit
- Definite limit called absolute zero.
- Molecules will slow down SO much, they will essentially stop
moving
- Out of energy, so they can’t get any colder.
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•Three different scales:1. Fahrenheit- the one we use
2. Celsius- metric standard
3. Kelvin- starts at absolute zero but same degree size as Celsius
FahrenheitFahrenheitFahrenheitFahrenheit scale.In the US we know at 32 ⁰F water freezes and 212 ⁰F water boils.
Temperature Scales
- The absolute temperature
scale is called the Kelvin
scale.
-Absolute zero is 0 K.
-The melting point of ice is
273 K, and the boiling point
of water is 373 K.
-There are no negative
numbers on the Kelvin
scale.
Measuring Temperature
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0⁰C 32⁰F 273 K
0⁰C 32⁰F 273 K
100⁰C 212⁰F 373 K
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0⁰C 32⁰F 273 K
100⁰C 212⁰F 373 K
-273⁰C -492⁰F 0 K
Notice anything else about the scales?
Heat, T and Energy Video
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Converting Temperature
Fahrenheit to Celsius 0C = (0F – 32)1.8
Celsius to Fahrenheit 0F = (1.8 x 0C) + 32
Celsius to Kelvin K = 0C + 273
1. Water freezes at 32°F, what is this in Celsius? In Kelvin?
0C = (0F – 32)1.8 0C = (320F – 32)1.8 0 0C K = 0C + 273 K = 0 0C + 273 273 KPractice Problems: Temperature Scales
2. Water boils at 100°C. What is this in Fahrenheit?
In Kelvin?
0F = (1.8 x 0C) + 320F = (1.8 x100 0C) + 32 0F = 180 + 32 212 0FK = 0C + 273K =100 0C + 273
373 K
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Practice Problems: Temperature Scales
3. Lead melts at 600 K, what is this in Celsius?
In Fahrenheit?
K = 0C + 2730C= K - 273 0C= 600 - 273 327 0C0F = (1.8 x 0C) + 320F = (1.8 x327 0C) + 320F = 588.6 + 32620.6 0F
4. Body temperature is 98.6°F, what is this in Celsius?
In Kelvin?
5. Methanol boils at 75°C, what is this in Fahrenheit?
Practice Problems: Temperature Scales
In Kelvin?
0C = (0F – 32)1.80F = (1.8 x 0C) + 32K = 0C + 273
K = 0C + 273
0C = (98.60F – 32)1.8 37 0CK =37 0C + 273 310 K0F = (1.8 x 75 0C) + 32167 0F
K =75 0C + 273 348 K
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Heat• We know that temperature is the hot or cold nature of
something (based on the kinetic energy of its molecules),
So what is heat? • Heat is the thermal energy transferred from one thing to Heat is the thermal energy transferred from one thing to Heat is the thermal energy transferred from one thing to Heat is the thermal energy transferred from one thing to
another due to a temperature difference. another due to a temperature difference. another due to a temperature difference. another due to a temperature difference. • Always moves from high
temperature to low temperature.
• Faster molecules (high
temperature) hit slower molecules
(low temperature) and speed them
up
- If you touch a hot stove,
thermal energy enters your hand
because the stove is warmer than
your hand. When you touch a
piece of ice, thermal energy
passes out of your hand and into
the colder ice.
Heat
Clip
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Section 14.2 Energy Transfer
•How does energy transfer happen?
•What do Conductors and Insulators do?
•What makes something a good conductor of heat?
Energy Transfer
Heat can be transferred 3 different ways:1. Conduction2. Convection3. Radiation
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Energy Transfer: Conduction
• Transferred of energy by direct contact• Works well in some solids, then liquids, and least in gases. • ConductorsConductorsConductorsConductors- materials that
allow heat to pass through them
• Most metals
• InsulatorsInsulatorsInsulatorsInsulators- materials that don’t let heat pass through them well
• Rubber, plastics, glass, air
Energy Transfer: Convection
• Transferring energy by moving fluids• Liquids and gases are fluids• When heated they expand, become less
dense• They rise, replaced by cooler denser fluids• Make a circular flow called a convection
current
Clip
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Convection Current
Hot Air Rises
Cool Air Sinks
Same in waterClipConvection Current
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Why is it windy at the seaside?
The third method of heat transfer
How does heat energy get
from the Sun to the Earth?There are no particles
between the Sun and the
Earth so it CANNOT
travel by conduction or
by convection.
?RADIATION
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Energy Transfer: Radiation
• Energy transferred by electromagnetic waves• Ex: infrared radiation, visible light, ultraviolet
rays• Can travel through empty space• When wave hit object they make the molecules
move faster.
Radiation
Radiation travels in straight lines
True/False
Radiation can travel through a vacuum
True/False
Radiation requires particles to travel
True/False
Radiation travels at the speed of light
True/False
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Specific Heat
• Some materials heat up easily• Others require a large amount of energy
to change their temperature• Specific heat measures the amount of
energy required to raise the temperature 1 kg of a substance by 1 degree Kelvin
• Table on page 485 (Learn to use it)• Water has a high specific heat of
4186 J/kg K
Specific Heat
• The amount of energy required to change the temperature of substance identifies type of conductor.
• Metals have a low specific heat
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Calculating Specific Heat
• Energy = Specific heat x mass x change in temp• Energy (Q) = cm∆T• mass= kg• energy= Joules• Temperature= K• c= specific heat
Q
c m ∆T
Specific heat is the “price” to change 1 kg of a substance.
1. How much energy must be transferred as heat to 200 kg of water in a bathtub to raise the water’s temperature from 25 °C to 37 °C?energy = cm∆T
Practice Problems: Specific Heat
∆T= 37 °C- 25 °C ∆T= 12 Km= 200 kgc= 4,186 J/kg x Kenergy = ?
energy = 4186 x 200 kg x 12 kenergy = 10,000,000 J or 1.0 x 107 J
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2. How much heat does it take to change the temperature of 3 kg of water by 75 K?
Practice Problems: Specific Heat
energy = cm∆T∆T= 75 Km= 3 kgc= 4,186 J/kg x Kenergy = ?
energy = 4186 x 3 kg x 75 kenergy = 900000 J or 9.0 x 105 J
3. How much energy is needed to increase the temperature of 0.755 kg of iron from 283 K to 403 K?
Practice Problems: Specific Heat
energy = cm∆T∆T= 403 K -283 K∆T= 120 Km= 0.755 kgc= 449 J/kg x Kenergy = ?
energy = 449 x 0.755 kg x 120 kenergy = 40,700 J
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Adding energy either raises T or changes state, not both at the same time.
Temperature vs. Time
Section 3 Using Heat
•Laws of Thermodynamics
•First Law of Thermodynamics: the total Energy used in any process is conserved whether that energy is transferred as a result of work, heat, or both.
•Second law of Thermodynamics: energy transferred as heat always moves from an object at a higher temperature to an object at a Lower temperature.
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1st Law of Thermodynamics
-The word thermodynamics stems from Greek for
“movement of heat.”
• When thermal energy transfers as heat, it does so without
net loss or gain. The energy lost from one place is gained
by the other.
• Whenever heat flows into or out of a system, the gain or
loss of thermal energy equals the amount of heat
transferred. (You can’t get something from nothing
because energy and matter are always conserved).
First Law of Thermodynamics:First Law of Thermodynamics:First Law of Thermodynamics:First Law of Thermodynamics:
2nd Law of Thermodynamics
Energy transfers as heat always moves from a hot to cold
objects. (You can’t break even; you can’t return to the
same energy state because entropy always increases).
• Entropy is the tendency for natural
systems to become disorganized over
time.
• Any system that is left to itself will fall
apart. This means Entropy increases.
• Usable energy decreases with every
transfer or energy transformation.
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Thermodynamics
• Work increases average kinetic energy• Mechanical Process is the process in
which energy is transferred by work.
Bow drill
Heat Engines
Automobile Cylinder Animation
• Heat engines convert chemical energy to mechanical energy through the process of combustion.
• Internal combustion engines• Fuel burns inside the engine• A carburetor is the part of the engine in which liquid
gasoline becomes vaporized.• 4 strokes: Intake, Compression, Power, and exhaust