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Chapter 7 Heat and Phase Transitions

November 4: Woodstoves − Thermal energy

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Thermal energy

•Thermal energy of an object is the internal kinetic energy and internal potential energy of its atoms and molecules.

•Thermal energy is disordered within an object.

•Thermal energy is responsible for the temperature of an object.

•Thermal energy does not include ordered energies: kinetic energy of an object moving or rotating, potential energy from outside interactions.

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Energy of your car

•Your car has kinetic energy:The faster it moves, the more kinetic energy it has.

•Your car has gravitational potential energy:The higher it goes up a mountain, the more gravitational potential energy it has.

•You car has elastic potential energy:The more its surface is dented, the more elastic potential energy it has.

•Your car also has thermal energy:The hotter your car is, the faster its atoms and molecules move about, and the more thermal energy it has.

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Chemical bonds

•Atoms interact with each other via electromagnetic forces.

•The forces between two atoms are

Attractive at long distances.

Repulsive at short distances.

Zero at a specific equilibrium separation.

•When two atoms are brought together, they give up some chemical potential energy.

•Atoms at the equilibrium separation are bound together because of an energy deficit.

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Chemical reactions in burning wood

•When burning wood, the reactants are carbohydrates and oxygen. The products are water and carbon dioxide.

•Breaking old bonds requires energy, so you need a match to activate the burning.

•Although breaking old bonds takes some work, forming new bonds does much more work.

•When burning wood, the new bonds are stronger than the old bonds. Therefore chemical potential energy is transferred into thermal energy.

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Heat and temperature

•Heat is the energy that flows between objects because of the difference of their temperatures.

•Heat always flows from hotter objects to colder objects.

•When two objects are at thermal equilibrium, their temperatures are equal. No heat flows between them.

•Temperature measures the hotness and coldness of an object. The absolute temperature of an object is proportional to the average thermal kinetic energy per particle.

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Different temperature scales

325

9C)( re temperatuCelsiusF)( re temperatuFahrenheit

273.15 C)( re temperatuCelsius(K) re temperatuAbsolute

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Questions:

1.If you drive your car faster, the thermal energy of a cup of water placed on your car will (choose one)A) increaseB) decreaseC) stay unchanged.

2.Approximately how much is our room temperature when measured by the absolute temperature scale?

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Read: Ch7: 1Homework: Ch7: E3Due: November 13

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November 6: Woodstoves − Heat transfer

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Heat transfer mechanisms

There are three ways for heat to transfer:

1)Conduction: Heat flow through a stationary material.

2)Convection: Heat flow via a moving fluid.

3)Radiation: Heat flow via electromagnetic waves.

All three mechanisms transfer heat from hot objects to cold objects.

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Conduction

Heat transfer through a stationary material. Heat flows from a hot region to a cold region, but the molecules do not flow.

Thermal conduction is caused by the exchange of kinetic energy between colliding molecules or electrons.

Mobile electrons, if exist, can carry heat quickly through long distances.

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Thermal conductivity

Thermal conductivity measures how rapidly heat flows through a material when it is exposed to a difference in temperature.

Metals (copper, silver, aluminum, gold, …) are good conductors of heat. They are good conductors of electricity also.

Plastic, rubber, and glasses are poor conductors of heat. They are poor conductors of electricity also.

Diamond is a very good conductor of heat, but a poor conductor of electricity.

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Convection

Heat flow via a moving fluid.

•Fluid warms up near a hot object.

•Hotter fluid has a smaller density. Natural buoyancy drives convection. Warmed fluid rises from the hot object.

•Fluid cools down near a cold object. Cooled fluid descends.

•Overall, heat flows from hot objects to cold objects.

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Radiation

Heat flow via electromagnetic waves.

•Electromagnetic waves include radio waves, microwaves, light, x-rays, etc.

•The type of the emitted waves depends on the temperature of the objects.

Cold objects: radio waves, microwaves, infrared light

Hot objects: infrared, visible, and ultraviolet light

•Objects at higher temperature emit more heat.

•Black-colored objects emits and absorbs light best.

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Specific heat

Specific heat: Heat needed to increase one kilogram of substance by 1 Kelvin of temperature.

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Questions:

1.How much heat (energy) is needed to increase 2 kilogram of water from 10 °C to 20 °C?

2.The specific heat of water is 4190 J/kg·°C. We also know that the amount of heat required to raise the temperature of 1 gram of water by 1 °C is called a calorie.1 calorie = ? Joule

TmcQ re temperatuin Change Massheat Specific neededHeat

Heat needed in warming up a substance:

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Read: Ch7: 1Homework: Ch7: E6,9Due: November 13

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November 9: Water, Steam and Ice −Phase transition

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Phases of water

Water has three forms or phases:•Ice is a solid. It has fixed volume and fixed shape. Ice is typically present below 0 °C (32 °F).•Water is a liquid. It has fixed volume but variable shape. Water is typically present between 0 °C (32 °F) and 100 °C (212 °F).•Steam is a gas. It has variable volume and variable shape. Steam is typically present above 100 °C (212 °F).

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Anomalous properties of water

The volume of a typical substance increases when it is changed from solid to liquid, and from liquid to gas.

Water is special: solid ice is slightly less dense than liquid water. Therefore ice floats on water.

This unusual behavior makes life possible to exist on earth.

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Melting ice and freezing water

•Ice has a melting temperature of 0 °C, below which solid ice is the stable phase, above which liquid water is the stable phase.

•At 0 °C ice and water can coexist.

•To melt ice, we need to add heat to it.

•Latent heat of melting: The heat needed to transform a unit mass of solid into liquid without changing its temperature.For ice the latent heat of melting is 330,000 J/kg.

•You need to add a certain amount of heat to melt ice into water. You must remove the same amount of heat (latent heat of freezing) from water to change it back into ice.

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Question:

The specific heat of ice is 2220 J/kg·°C. The latent heat of melting ice is 330,000 J/kg. The specific heat of water is 4190 J/kg·°C.How much energy is needed to change 1 kg of ice at −2 °C to 1 kg of water at 3 °C?

TmcQ re temperatuin Change Massheat Specific neededHeat

Heat needed in warming up a substance:

Heat needed in phase transition:

mLQ Massheat Latent neededHeat

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Phase equilibrium

When two phases are present:

•Molecules continually shift between the two phases.

•One phase may grow at the expense of the other phase.

At phase equilibrium:

•Two phases can coexist.

•Neither phase grows at the expense of the other when no thermal energy is added to or subtracted from the system.

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Water and steam

•Liquid water and gaseous steam can coexist over a broad range of temperatures, but the equilibrium steam density rises with temperature.

•To evaporate water, we can add heat or reduce steam density.

•Latent heat of evaporation: the heat needed to transform a unit mass of liquid into gas without changing its temperature.For water the latent heat of evaporation is 2,300,000 J/kg.

•You must remove the same amount of heat (latent heat of condensation) from steam to change it back into water.

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Phase diagram of water

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Read: Ch7: 2Homework: Ch7: E15,16,17Due: November 20

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November 11: Water, Steam and Ice −Boiling water

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Relative humidity

Liquid water and gaseous steam can coexist over a broad range of temperature. At the surface between water and steam, water molecules are continuously leaving and landing.

Relative humidity measures the water molecule’s landing rate as a percentage of its leaving rate.

• Below 100% relative humidity, water evaporates.• At 100% relative humidity, water is in equilibrium with steam.• Above 100% relative humidity, steam condenses.• Increasing temperature decreases the relative humidity.

Decreasing temperature increases the relative humidity.

Below 0 °C, ice coexists with steam. Ice molecules are leaving (sublimation) and landing (deposition) depending on the relative humidity.

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Boiling water

Saturated steam: The steam at phase equilibrium with water is called saturated steam.

Saturated vapor pressure: The pressure of saturated steam over water surface. It increases dramatically as temperature increases.

Near room temperature, the pressure of the saturated steam is only a few percentage of one atmospheric pressure.

At 100 °C, the pressure of the saturated steam is about one atmospheric pressure.

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If a saturated steam bubble is formed in the water at room temperature, the pressure inside the bubble is much lower than one atmosphere. The bubble will be smashed and the steam inside is changed into water.

Near 100 °C, a saturated steam bubble becomes stable because its pressure is comparable to the ambient pressure. Bubbles grow and rise in water. This is called boiling.

Boiling water

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Water’s boiling temperature

Water’s boiling temperature depends on its ambient pressure.

• Near the sea level (one atmospheric pressure), water boils at 100 °C.

• At 3000 m altitude, water boils at 90 °C. You need a longer time to cook an egg.

• The pressure inside a pressure cooker is more than one atmosphere. Water inside the cooker thus boils at more than 100 °C. Food cooks faster in a pressure cooker.

Demo: Water in vacuum

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More about water boiling

•Dissolved chemicals (sugar or salt) forms bonds with water molecules. They increase the boiling temperature and slow down water’s evaporation.Salt water freezes at lower temperature.Salt also tends to melt ice.

•Boiling required tiny seed bubbles. The formation of seed bubbles is called nucleation, which is usually provided by defects of the container or contaminants in water.

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More about water boiling

In a clean glass or ceramic container, clean water can be superheated without boiling due to lack of nucleation source. Superheated water is dangerous.

Superheated water:Movie 1Movie 2

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Read: Ch7: 2Homework: Ch7: E21,25Due: November 20

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November 13: Clothing, Insulation, and Climate −Thermal radiation

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Thermal radiation

Materials all emit thermal radiation because they contain electric charges. Electric charges emit electromagnetic waves when they are accelerating.

Objects at higher temperatures emit shorter wavelengths.

Types of electromagnetic radiation arranged by wavelength

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Visible light arranged by wavelength

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More about light

•The speed of light is a constant in vacuum:c =299 792 458 m/s.How many times can light circle the earth in one second?

•Light has frequency f and wavelength : f ×= c.Example: = 500 nanometer (green-blue), f =6 ×1014 Hz.

•Light travels slower in medium: v = c/n.n is the index of refraction of the medium. Blue light has a larger index of refraction than red light.

•Light can be decomposed into its frequency components using diffractive optics, such as prisms and gratings.

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Light spectrum emitted by black objects at different temperatures

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Demo: Thermal radiation from a hot bulb

Demo: Emission color from a bulb

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Stefan-Boltzmann law

)KmJ/(s1067.5

area surfaceetemperaturconstant Boltzman-Stefanemissivity

power Radiation

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ATeP

Question:How many more times of power can an object radiate if its temperature is doubled?

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Read: Ch7: 3Homework: Ch7: E35,P3Due: November 20