Temperature Heat and Expansion Chapter 21 Temperature n

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Temperature, Heat, and Expansion Chapter 21

Temperature, Heat, and Expansion Chapter 21

Temperature n Temperature – the quantity that tells how hot or cold something is

Temperature n Temperature – the quantity that tells how hot or cold something is compared with a standard n A common thermometer measures temperature by showing the expansion and contraction of a liquid in a glass tube using a scale n Celsius Scale – most widely used temperature scale, 0ºC is the point at which water freezes and 100ºC the point at which water boils, the gap between is divided into 100 equal parts

Temperature n Fahrenheit Scale – commonly used in the U. S. , has 180ºF

Temperature n Fahrenheit Scale – commonly used in the U. S. , has 180ºF between freezing and boiling (32ºF and 212º) n Kelvin Scale – used in scientific research, degrees are the same size as Celsius + 273º, denoted K n Absolute Zero – the lowest possible temperature on the Kelvin scale, substance has no kinetic energy

Thermometers

Thermometers

Scale Comparison

Scale Comparison

Temperature and Kinetic Energy n In an ideal gas, temperature is proportional to the

Temperature and Kinetic Energy n In an ideal gas, temperature is proportional to the average kinetic energy n The heat that you feel when you touch a hot surface is the kinetic energy transferred by molecules in the surface to molecules in your fingers n Temperature is not a measure of the total kinetic energy

Heat n n n Heat – the energy that transfers from one object to

Heat n n n Heat – the energy that transfers from one object to another because of a temperature difference between them Matter does not contain heat, but contains energy in several forms Heat is energy in transit Internal Energy – the energy resulting from heat flow When heat flows from one object or substance to another it is in contact with, the objects are said to be in thermal contact Heat flows from the higher-temperature substance into the lower-temperature substance

Heat Flow Between Two Gases

Heat Flow Between Two Gases

Thermal Equilibrium n Thermal Equilibrium – objects in thermal contact with each other reach

Thermal Equilibrium n Thermal Equilibrium – objects in thermal contact with each other reach the same temperature, no heat flows between them n When reading a thermometer, we wait until thermometer has reached thermal equilibrium with the object we want the temperature of

Thermal Equilibrium

Thermal Equilibrium

Internal Energy n Internal Energy – the total of all energies inside a substance

Internal Energy n Internal Energy – the total of all energies inside a substance n A substance does not contain heat – it contains internal energy n When a substance takes in or gives off heat, any of these energies can change

Internal Energy

Internal Energy

Measurement of Heat n n n To quantify heat, we have to specify the

Measurement of Heat n n n To quantify heat, we have to specify the mass and kind of substance affected Calorie – the most commonly used unit for heat; the amount of heat required to raise the temperature of 1 gram of water by 1ºC Kilocalorie – the heat required to raise 1 kilogram of water by 1ºC (1000 calories) The heat unit for rating foods is actually the kilocalorie (to distinguish from calorie, it is often written as Calorie) Remember that a calorie is a measure of ENERGY! The relationship between calories and joules is: 1 calorie = 4. 184 Joules

Calories

Calories

Specific Heat Capacity Different substances have different capacities for storing internal energy n We

Specific Heat Capacity Different substances have different capacities for storing internal energy n We find that specific materials require specific quantities of heat to raise the temperature of a given mass of the material by a specified number of degrees n Specific Heat Capacity – the quantity of heat required to raise the temperature of a unit mass of a substance by 1ºC Q = mcΔT Q = quantity of heat; m = mass of substance; c = specific heat capacity of substance; ΔT = change in temperature n We can think of specific heat capacity as thermal inertia (an object’s resistance to change) n

Specific Heat Capacities

Specific Heat Capacities

Thermal Expansion When the temperature of a substance increases, the molecules “jiggle” faster and

Thermal Expansion When the temperature of a substance increases, the molecules “jiggle” faster and move further apart, causing an expansion of the substance n Gases generally expand contract more than liquids, which expand contract more than solids n In concrete sidewalks and highways this expansion and contraction is taken into account when it is being built. The surface is laid down in small sections with a gap in between, that is usually filled with a substance such as tar. n

Thermal Expansion Joint

Thermal Expansion Joint

Expansion of Water n Almost all liquids will expand when they are heated, ice-cold

Expansion of Water n Almost all liquids will expand when they are heated, ice-cold water instead contracts to go from ice to a liquid n When the water reaches a temperature of 4ºC, it will stop contracting and begin expanding n This has to do with the crystal structure of water, its solid state has an open structure that takes up more volume and is therefore less dense

Expansion of Water

Expansion of Water

Heat Transfer Chapter 22

Heat Transfer Chapter 22

Conduction n n n Conduction – energy transfer from particle to particle within certain

Conduction n n n Conduction – energy transfer from particle to particle within certain materials, or from one material to another when the two are in direct contact Conductors – materials that conduct heat well Metals are the best conductors (silver, copper, aluminum, and iron) Materials composed if atoms with “loose” outer electrons are good conductors of heat Insulators – materials which delay the transfer of heat (wood, wool, straw, paper, cork, and Styrofoam) Cold is simply the absence of heat, only heat is transferred through a conductor or insulator

Convection n Convection – a means of heat transfer by movement of the heated

Convection n Convection – a means of heat transfer by movement of the heated substance itself, such as by currents in a fluid n Convection occurs in all fluids, whether liquid or gas n Convection is occurring all around you, the atmosphere, the ocean, Earth, the sun! n As warm air rises, it expands and cools n When the air has been cooled, it will sink back down again and warm up …

Convection

Convection

Radiation n Radiation – Energy transmitted by electromagnetic waves (i. e. the sun) n

Radiation n Radiation – Energy transmitted by electromagnetic waves (i. e. the sun) n Radiant Energy – any energy, including heat, that is transmitted by radiation n All objects continually emit radiant energy in a mixture of wavelengths n The lower the temperature, the longer the wavelength

Three Mechanisms of Heat Transfer

Three Mechanisms of Heat Transfer

Absorption of Radiant Energy n Absorption and reflection are opposite processes, a good absorber

Absorption of Radiant Energy n Absorption and reflection are opposite processes, a good absorber of radiant energy will reflect only a little amount of radiant energy n A perfect absorber will reflect no radiant energy and appear black n Good reflectors are poor absorbers of radiant energy

Emission of Radiant Energy n Good absorbers are also good emitters; poor absorbers are

Emission of Radiant Energy n Good absorbers are also good emitters; poor absorbers are poor emitters n If a good absorber was not also a good emitter, then black objects would remain warmer than lighter colored objects and never come to thermal equilibrium with them n Each object is emitting as much energy as it is absorbing

Absorption and Emission of Radiant Energy

Absorption and Emission of Radiant Energy

Newton’s Law of Cooling n The rate of cooling of an object depends on

Newton’s Law of Cooling n The rate of cooling of an object depends on how much hotter the object is than the surroundings The rate of cooling of an object – whether by conduction, convection, or radiation – is approximately proportional to the temperature difference ΔT between the object and its surroundings Rate of cooling ~ ΔT n Newton’s law of cooling also holds for heating

Global Warming n n Greenhouse Effect – The warming effect whose cause is that

Global Warming n n Greenhouse Effect – The warming effect whose cause is that short-wavelength radiant energy from the sun can enter the atmosphere and be absorbed by Earth more easily than longwavelength energy from Earth can leave Earth absorbs the energy from the sun through the atmosphere As the atmosphere gets thicker from carbon dioxide, it will not allow as much energy to escape into space Terrestrial Radiation – Energy that Earth radiates

The Greenhouse Effect

The Greenhouse Effect

Assignment n Read Chapters 21 & 22 (pg. 307 -336) n Do Chapter 21

Assignment n Read Chapters 21 & 22 (pg. 307 -336) n Do Chapter 21 #25 -48 (pg. 323 -324); Appendix F #1 -7 (pg. 680) n Do Chapter 22 #21 -30 (pg. 338)