Thermodynamics

Thermodynamics Chapters: 21, 22, and 24 Temperature:  Temperature tells us how warm or cold an object is relative to some standard.  A measure of the average kinetic energy of a substance Several scales are used to measure temperature:  Celsius  Fahrenheit F = 1.8C + 32  Kelvin K = C + 273 Freezing point of water: 00C, 273K, 320F Boiling point of water: 1000C, 373K, 2120F  Kinetic-molecular theory: Matter is made up of many tiny particles that are always in motion. In a hot body, the particles move faster, and therefore have a higher energy than particles in a cooler body.  Atoms and molecules all have energy of motion (vibrational, rotational, translational).  Energy of motion is called Kinetic Energy.  The more kinetic energy present in the molecules of a substance, the higher the temperature (which is simply a measure of the average kinetic energy).  At Absolute Zero (0K), no more energy can be removed from a substance. This is the lower limit on temperature.  There is no upper limit on temperature.  There is no such thing as “cold”. In order to cool an object, energy (or heat) must be removed. o Cold is simply the absence of heat (or energy). ? Can two containers of water contain the same amount of energy but have different temperatures? Temperature is not a measure of the total Kinetic Energy. ? If the same amount of heat energy is added to a cup of water and a gallon of water, which one will end up having a higher temperature? Heat: The energy that flows as a result of a difference in temperature.  A substance or object does NOT contain heat.  Once heat is transferred to an object or a substance, the object or substance contains a raised level of internal (or kinetic) energy.  Units of heat are Joules (metric) or calories (English).  Heat flows from substances with high temperature to substances with low temperature o Heat does NOT necessarily flow from substances with greater total energy to substances with lesser total energy. Why?  Heat will continue to flow until equilibrium is reached. The Laws of Thermodynamics The First Law of Thermodynamics: Whenever heat is added to a system, it transforms to an equal amount of some other form of energy. Heat added to a system can:  Increase the internal energy (temperature) of that system.  Do external work and leave the system (steam engine, piston) The Second Law of Thermodynamics: Heat will NEVER of itself flow from a cold object to a hot object.  Heat can only be made to flow from cold to hot by imposing external effort (heat pumps, refrigerators). Newton’s Law of Cooling: The rate of heat exchange between two regions at different temperatures is approximately proportional to the temperature difference between the two regions.  The rate of cooling of an object, regardless of the method by which the heat is lost (conduction, convection, radiation), is proportional to the temperature difference. o A house will lose heat at a greater rate when the temperature difference between the inside and the outside of the house is greater. The rate at which an object warms up is proportional to the temperature difference between the object and its surroundings. o Frozen food will thaw more quickly on the counter than in the refrigerator.  Thermal Expansion As the temperature of a substance increases, molecular movement increases, and the substance expands. Most substances expand when they are heated, and contract when they are cooled. ? Why might this pose a problem for engineers? Because of thermal expansion:  Bridges and railroad tracks have expansion joints  Sidewalks are built in sections  You can loosen a lid on a glass jar by running it under hot water. ? Why might it be that you can use hot water to loosen a metal lid on a glass jar, but you cannot use the same technique to loosen a metal lid on a metal jar?  Bimetallic strips can be used in thermostats and other appliances that regulate temperature. Water  Water is the most notable exception to the rule of thermal expansion.  When water freezes, it expands, becomes less dense, and floats.  Because water floats, bodies of water do not freeze solid each winter; this allows fish to survive under the layer of ice that forms at the top of water. Heat Transfer Conduction: Energy transfer from particle to particle within a certain material OR from one material to another when they are in direct contact with each other.  Conduction requires physical contact between atoms.  When atoms are in contact, the high energy atoms collide with low energy atoms causing them to speed up and have a greater amount of kinetic energy (or a higher temperature).  The process of conduction is very similar to collisions and the transfer of momentum between larger objects such as tennis balls. ? When a fast moving tennis ball collides with a slow moving tennis ball, what is the result? o The slow moving tennis ball speeds up and the fast moving tennis ball slows down. o When a hot object is placed in contact with a cool object, the cool object warms up, and the warm object cools down until equilibrium is reached.  Any substance with loosely bound outer electrons is a good conductor. o Metals are very good conductors  Air is a very poor conductor of heat; this is why insulating materials contain substances with many small air pockets.  Insulators: any substance that is not a good conductor is called an insulator. An insulator slows the flow of heat from one region to another. ? Why is it that when you are barefoot on a cold day a floor feels much colder than the carpet if they are both at the same temperature? Why do metal objects feel colder than non metal objects? Because the floor and metal objects are better conductors and transfer heat away from your body more quickly, making them feel cooler. ? How does a blanket make you warm? After all, it does not create heat. The reactions in your body constantly produce heat. If you cover your body with a blanket (which is an insulator), you will prevent the heat from being lost to the atmosphere as quickly. The heat will stay near your body, and you will feel warm. Convection: Heat transfer through motion of a fluid (a fluid can be a gas or a liquid).  Hot fluids rise because they are less dense than the surrounding, cooler fluid; this is how hot air balloons work.  This is why “sea breezes” exist by the shore.  Air is a very poor conductor of heat; without convection, it would be much more difficult to heat your house. Radiation: Energy transmitted by electromagnetic radiation. (We will talk more about this later in the semester).  Does not require a medium through which to travel.  Can travel through empty space.  This is how energy from the sun travels to Earth.  When radiation strikes an object, energy from the radiation is transferred to the atoms of that object, and the temperature rises.  There are visible (light) and invisible (Infrared, X-ray, UV rays) forms of radiation.  Your body emits infrared radiation. Place a piece of aluminum foil in front of your face; what do you feel? Practical Applications of Thermodynamics How does a refrigerator work? From Howstuffworks.com ? What do you feel when you place rubbing alcohol on your finger and it evaporates? When rubbing alcohol evaporates, it absorbs and removes heat, making your finger feel cold. This is exactly what Freon and other refrigerants do, the only difference is that these refrigerants absorb a lot more heat. 1. The compressor compresses the refrigerant gas. The gas condenses to a liquid. Condensation is an exothermic reaction, and heat is released. This is why the coils at the back of the fridge feel warm. 2. The liquid refrigerant then flows through the expansion valve 3. When it flows through the expansion valve, the liquid refrigerant is allowed to move from a high-pressure zone to a low-pressure zone, so it expands and evaporates. Evaporation is an endothermic reaction that absorbs heat making the inside of the refrigerator cold. 4. The gas then returns to the coils outside of the fridge where heat is released and the process begins again. ? If your air conditioning breaks, can you use a refrigerator to cool your house? ? An air conditioner works on the same principles as the refrigerator. How is the air conditioner configured differently so that it can be used to cool your house? How does a heat pump work?  A heat pump works much like a refrigerator except that the hot coils are located inside the house and the cool coils are located outside the house.  In the summer, the flow of refrigerant can be reversed so that the inside of the house is cooled while heat is pumped outside the house.  Because air is a poor conductor of heat, there are fans located by each set of coils to move the air through the house or out into the environment.  Heat pumps are extremely efficient.  The main problem with heat pumps is that the coils outside often collect ice in winter. When this happens, the heat pump switches to A/C mode so that the outside coils can warm up and the ice can melt. To avoid pumping cold air into the house the heat pump has electric coils or burners that heat the air.  Because the coils freeze, heat pumps are not ideal for very cold climates. How does a Thermos work?  A thermos is encased in a vacuum flask which is a glass envelope from which all (or almost all) atoms have been removed.  In order for heat to be transferred by conduction or convection, air particles must be present. If all air has been removed, heat cannot be lost through conduction or convection.  The glass inside the thermos is silvered; this prevents heat loss through radiation because it reflects any infrared radiation back into the thermos.  A thermos can keep substances warm or cold for long periods of time. Screw Top tT Silvered on inside tT Contents Vacuum Outer Container tT  It is impossible to create a perfect vacuum, and the cap of the thermos provides a path of escape for heat; therefore, a thermos cannot keep a substance warm or cold forever. ? A thermos keeps hot substances hot and cold substances cold. How does a thermos “know” whether to keep a substance hot or cold? What is wind-chill?  The temperature you feel because of the wind.  Air is a poor conductor of heat. On a calm cold day, your body looses heat, but the warmed air forms a thin layer around you and is slowly conducted away from you by air molecules.  If the wind is constantly blowing heat away from your body, the rate of heat loss increases, and you feel cooler.  This is why you blow on hot food when you want it to cool.  Movement of air increases heat loss by convection How does insulation work? The purpose of insulation is to slow the rate of heat flow from one region to another.  Insulation is used in houses to keep them cool in the summer and warm in the winter.  Insulation is generally made of porous materials; the small spaces are filled with air which is a very poor conductor of heat  Fiberglass insulation contains glass fibers surrounded by many small airspaces.  Foam insulation contains many bubbles of air.  The air pockets must be encased in fiberglass, foam, or some other kind of material in order to prevent convection currents from forming within the walls and increasing the rate of heat loss.  Insulation is described by its R-value.  The higher the R-value, the greater an insulation’s resistance to heat flow.  The R-value is equal to the thickness of the insulation (L) divided by the thermal conductivity (K). o R = L/K o Units: ft2-h-oF/Btu  Increasing the thickness of insulation will increase its R-value  Decreasing the thermal conductivity of insulation will increase its R-value.  In Georgia, it is recommended that R-26 insulation be used in ceilings below a roof or unheated attic, R-19 in an exterior wall, and R-13 in floors over an unheated basement or crawl space. o Weatherproofing, Time-Life Books, 1977. Thermal Conductivities of Some Common Materials at 270C Material K(W/m-0C) Copper 398 Aluminum 297 Brick 0.4-0.8 Glass 0.72-0.86 Water 0.61 Asbestos 0.083 Fiberglass 0.046 Polystyrene foam 0.033 Air 0.026 Jones/Childers, Contemporary College Physics, 3 ed., 2001 update; p365. rd R-Values of Some Common Building Materials R-Value (ft2-h-0F/Btu) 0.45 0.62 0.6-1.2 1 2 2.9 11 19 Material Thickness (in.) Gypsum board 0.5 Plywood 0.5 Brick 3.625 Glass, single pane -Glass, double pane -Polystyrene foam 0.75 Fiberglass insulation 3.5 Fiberglass insulation 6.25 Jones/Childers, Contemporary College Physics, 3rd ed., 2001 update; p365.