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Heat & Temperature Chapter 5

Student Learning Objectives •Relate kinetic energy to temperature

•Compare temperature scales

•Analyze systems to determine methods & affects of heat transfer

https://www.youtube.com/watch?v=w1aVqKDAx6o

https://www.youtube.com/watch?v=CDTZoFGmZoc

What does temperature measure?

Temperature is a measure of the average kinetic energy of the atoms and molecules in a substance.

We perceive the KE of atoms and molecules as being hot or cold.

https://www.youtube.com/watch?v=o2viSF0PjlY

https://www.youtube.com/watch?v=Akd7MMRKD wc

Materials at different temperatures reach thermal equilibrium when placed in contact.

Examples

Thermometer + Person

Person + Metal Chair

Most materials expand when the temperature is increased, thermal expansion

Examples

Metal lids

Bridge Joints

Bimetallic strips

Hot Air Balloons

https://www.youtube.com/watch?v=hX2Y2IDjuGI&feature=fvw

https://www.youtube.com/watch?v=ZgTTUuJZAFs

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Bimetallic Strip and Thermal Expansion

• Brass expands more than iron.

• The degree ofdeflection is proportional to the temperature.

• A/C thermostat and dial-typethermometers are based on bimetal coils.

Section 5.1

Liquid-in-glass Thermometer

• Thermometers are calibrated to two reference points (ice point & steam point.) – Ice point – the temperature of a mixture of pure ice

and water at normal atmospheric pressure

– Steam point – the temperature at which pure water boils at normal atmospheric pressure

• Usually contains either mercury or red (colored) alcohol

Section 5.1

Temperature Scales Celsius, Kelvin, Fahrenheit

Section 5.1

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Temperature Scales Celsius, Kelvin, Fahrenheit

Scale Absolute Zero

Ice Point

Steam Point

Diff. (Boil – Ice)

Fahrenheit - 460o 32o 212o 180

Celsius -273o 0o 100o 100

Kelvin 0o 273o 373o 100

Section 5.1

Temperature Fahrenheit is based on boiling and freezing points of

water.

320 freezing at sea level 212o boiling at sea level

Celsius is based on boiling and freezing points of water.

0 o freezing at sea level 100o boiling at sea level

Kelvin is based the average KE of atoms in a gas.

F = (9/5)C + 32 C = (5/9)(F – 32)

K = C + 273

0 Kelvin ൌ ─273o C ൌ ─460o F

Object Kelvin Fahrenheit Human Body 310.15 K 98.6 ºF

Sun 5800 K 9,980 ºF Blue Star 35000 K 62,540 ºF

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Practice 1) Predict the surface temperature of a red star.

2) What is a cold day on Earth in Fahrenheit? What is a hot day on Earth in Fahrenheit? Convert these temperatures to Celsius and Kelvin.

3 Why do we rub our hands together to warm them? What is the physics behind this practice?

4) Do all substances expand when heated?

What is heat?

Heat is energy that moves from one object to another if there is a temperature difference.

The larger the temperature difference, the faster heat energy flows.

Hot → Cold

Practice 1) When ice and a warm

liquid are combined

a. The ice cools the liquid b. The liquid warms the ice

2) Where would be the fastest place to cool a very hot pizza?

a. On the counter b. In the refrigerator c. In the freezer

Heat Unit SI - Calorie

• Since heat is energy, it has a unit of joules. (J) • A more common unit to measure heat is the

calorie. • Calorie - the amount of heat necessary to raise

one gram of pure water by one Celsius degree at normal atmospheric pressure

• 1 cal = 4.186 J (or about 4.2 J) • Kilocalorie – heat necessary to raise 1kg water by

1oC • 1 food Calorie = 1000 calories (1 kcal) • 1 food Calorie = 4186 J (or about 4.2 kJ)

Section 5.2

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Expansion/Contraction with Changes in Temperature

• In general, most matter, solids, liquids, and gases will expand with an increase in temperature (and contract with a decrease in temperature.)

• Water is an exception to this rule – (ice floats!)

Section 5.2

Thermal-Expansion Joints in a Bridge

These joints allow for the contraction and expansion of the steel girders during the winter and summer seasons.

Section 5.2

Behavior of Water Strange!

• The volume of a quantity of water decreases with decreasing temperature but only down to 4oC. Below this temperature, the volume increases slightly.

• With a minimum volume at 4oC, the density of water is maximum at this temperature and decreases at lower temperatures.

Most dense point

Section 5.2

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a) An illustration of the open hexagonal (six-sided) molecular structure of ice.

b) This hexagonal pattern is evident in snowflakes.

Behavior of Water: Structure of Ice Solid water takes up more volume

Section 5.2

(Ebbing, Darrell D., General Chemistry, Sixth Edition. Copyright 1999 by Houghton Mifflin Company)

Lakes freeze from the top down – Yellowstone Lake

Photo Source: Copyright © Bobby H. Bammel. All rights reserved.

Section 5.2

Specific Heat

• If equal quantities of heat are added to equal masses of two metals (iron and aluminum, for example) – would the temperature of each rise the same number of degrees? -- NO!

• Different substances have different properties.

• Specific Heat – the amount of heat necessary to raise the temperature of one kilogram of the substance 1oC

Section 5.3

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The amount of heat it takes to increase the temperature of amaterial by 1o C is the specific heat capacity of the material.

c=specific heat capacity, m=mass, T=temperature (K or C) A substance with a high specific heat capacity will resist changes in

temperature. See page 117 for different specific heat capacities

Q = cmT

Practice

1) Does a swimming pool lose heat quickly or slowly during the night?

2) In a standard shower, 120 kg of hot water is used during a 10 minute shower. What is the heat energy required to raise the temperature of the 120 kg of water from 15°C to 61°C?

3) A solid copper penny is dropped from a height of 10 meters. If all energy is conserved within the penny, what is the temperature change of the penny when it lands on the ground?

More Practice

4) Where would you expect a greater temperature change between day and night, in a humid location or a dry location? Why?

5) Are day to night temperature changes larger when the sky is clear or cloudy?

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What is the greenhouse effect? What is global warming?

The greenhouse effect traps heat near the surface of a planet.

Greenhouse gasses in the atmosphere do not create heat. Greenhouse gasses slow heat loss to space.

Questions

1) What contributes most to the CO2 in our atmosphere?

2) Is the greenhouse effect “bad”?

3) What is global warming?

https://www.youtube.com/watch?v=Hi3ERes0h84

What happens to temperature & energy during a phase change?

A substance that has reached the temperature required for a phase change maintains its temperature during the phase change.

All energy lost or gained is used to change the phase of the substance.

Evaporation (Liquid to gas)

Sublimation (Solid to gas)

Condensation (Gas to liquid)

https://www.youtube.com/watch?v=1MiQCBIx1mM

The amount of heat energy per kilogram that must be added or removed for a substance to change phase is called latent heat (L).

IceWaterSteam

Q = mL

T (°C)

Joules

0

100

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Graph of Temperature vs. Heat for Water

• Latent heat of fusion – heat necessary to go from A to B

• Latent heat of vaporization – heat necessary to go from C to D

Section 5.3

A=100% solid at 0oC

B=100% liquid at 0oC

C=100% liquid at 100oC

D=100% gas at 100oC

Graph of Temperature vs. Heat for Water

Section 5.3

Graph of Temperature vs. Heat for 1 kg of Pure Water

540 kcal 80 kcal 0.5 kcal/k g.Co

1.0 kcal/kg.Co

0.5kcal kg.Co

Section 5.3

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Other phase changes

• Sublimation – when a substance changes directly from solid to gas (dry ice CO2 gas, mothballs, solid air fresheners)

• Deposition – when a substance changes directly from gas to solid (ice crystals that form on house windows in the winter)

Section 5.3

Practice

1) Describe the physical processes which occur when –20ºC ice is added to +25º soda.

2) A 1 kg piece of 0o C ice is dropped 5 meters. Assume all energy is conserved within the ice, melting some of the ice upon impact. How much ice will melt?

3) A 50 gram piece of steel is brought into full contact with a large block of 0o C ice, and the system of steel and ice is isolated. If the steel has an initial temperature of 95o C, how much of the ice will melt?

Heat Transfer Occurs by Conduction, Convection, and Radiation

Section 5.4

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What are the three methods of heat transfer?

Conduction is the transfer of heat energy from one place to another by direct collisions.

Solids tend to conduct heat better than liquids or gases. Why?

Insulators slow or impede the rate of heat flow.

Styrofoam

Fiberglass

Freely moving electrons = High thermal conductivity

Convection is the transfer of heat energy from one place to another by the bulk motion of groups of molecules in a fluid. (sinking/floating)

Radiation is the transfer of heat energy from one place to another by electromagnetic waves (light).

Practice

1) Does something that has a high thermal conductivity have a high or low specific heat capacity?

2) Why does hot fluid rise above cool fluid?

3) Why do people tend to wear darker colored clothes in the winter and lighter colored clothes in the summer?

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How are temperature and pressure related in a gas?

In a gas, pressure is determined by temperature, volume, and number of particles.

Boyle's Law: P1V1 = P2V2

Charles, Law: P1/T1 = P2/T2

PV = nkT

Practice 1) A certain number of gas molecules are contained in a

volume, like a tire. What happens to the pressure if temperature increases?

2) A certain number of gas molecules are contained in a volume, like a balloon. What happens to the pressure if the container is compressed (volume decreases)?

3) On a hot day (101ºF), you measure the tire pressure to be 38 psi. The next day, the temperature only reaches 81 ºF. What would be the tire pressure on the cooler day?

What are the 3 laws of thermodynamics?

1st Law: The amount of heat energy transferred is equal to the change in internal energy plus work.

Heat Energy IE + Work

2nd Law: Heat cannot flow spontaneously from a cold substance to a hot substance. Entropy (disorder) cannot spontaneously decrease.

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3rd Law: The entropy of a perfect crystal is zero when the temperature of the crystal is equal to absolute zero. KE → 0

https://www.youtube.com/watch?v=dxVlFf4DYU4

https://www.youtube.com/watch?v=VtEqn-5XHpU

1) If a balloon is filled with air, sealed, and placed in the freezer, will the entropy increase, decrease, or remain constant? What would happen to the motion, KE, and temperature of the air trapped in the balloon?

2)

Practice

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