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Temperature, Heat, and Expansion. Chapter 21. Temperature. Temperature – the quantity that tells how hot or cold something is compared with a standard A common thermometer measures temperature by showing the expansion and contraction of a liquid in a glass tube using a scale
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Temperature, Heat, and Expansion Chapter 21
Temperature • Temperature – the quantity that tells how hot or cold something is compared with a standard • A common thermometer measures temperature by showing the expansion and contraction of a liquid in a glass tube using a scale • 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 • Fahrenheit Scale – commonly used in the U.S., has 180ºF between freezing and boiling (32ºF and 212º) • Kelvin Scale – used in scientific research, degrees are the same size as Celsius + 273º, denoted K • Absolute Zero – the lowest possible temperature on the Kelvin scale, substance has no kinetic energy
Temperature and Kinetic Energy • In an ideal gas, temperature is proportional to the average kinetic energy • 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 • Temperature is not a measure of the total kinetic energy
Heat • 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
Thermal Equilibrium • Thermal Equilibrium – objects in thermal contact with each other reach the same temperature, no heat flows between them • When reading a thermometer, we wait until the thermometer has reached thermal equilibrium with the object we want the temperature of
Internal Energy • Internal Energy – the total of all energies inside a substance • A substance does not contain heat – it contains internal energy • When a substance takes in or gives off heat, any of these energies can change
Measurement of Heat • 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
Specific Heat Capacity • Different substances have different capacities for storing internal energy • 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 • 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 • We can think of specific heat capacity as thermal inertia (an object’s resistance to change)
Thermal Expansion • When the temperature of a substance increases, the molecules “jiggle” faster and move further apart, causing an expansion of the substance • Gases generally expand and contract more than liquids, which expand and contract more than solids • 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.
Thermal Expansion Expansion Joint
Expansion of Water • Almost all liquids will expand when they are heated, ice-cold water instead contracts to go from ice to a liquid • When the water reaches a temperature of 4ºC, it will stop contracting and begin expanding • 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
Heat Transfer Chapter 22
Conduction • 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 • Convection – a means of heat transfer by movement of the heated substance itself, such as by currents in a fluid • Convection occurs in all fluids, whether liquid or gas • Convection is occurring all around you, the atmosphere, the ocean, Earth, the sun! • As warm air rises, it expands and cools • When the air has been cooled, it will sink back down again and warm up …
Radiation • Radiation – Energy transmitted by electromagnetic waves (i.e. the sun) • Radiant Energy – any energy, including heat, that is transmitted by radiation • All objects continually emit radiant energy in a mixture of wavelengths • The lower the temperature, the longer the wavelength
Absorption of Radiant Energy • Absorption and reflection are opposite processes, a good absorber of radiant energy will reflect only a little amount of radiant energy • A perfect absorber will reflect no radiant energy and appear black • Good reflectors are poor absorbers of radiant energy
Emission of Radiant Energy • Good absorbers are also good emitters; poor absorbers are poor emitters • 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 • Each object is emitting as much energy as it is absorbing
Newton’s Law of Cooling • 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 • Newton’s law of cooling also holds for heating
Global Warming • 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 long-wavelength 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
Assignment • Read Chapters 21 & 22 (pg. 307-336) • Do Chapter 21 #25-48 (pg. 323-324); Appendix F #1-7 (pg. 680) • Do Chapter 22 #21-30 (pg. 338)
