Temperature, Heat, and Expansion

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1 Thermodynamics (Based on Chapters 21-24) Temperature, Heat, and Expansion (Ch 21) Warmth is the kinetic energy of atoms and molecules. Temperature (21.1) The measure of how hot and cold things are is temperature. We measure temperature in degrees. Almost all matter expands when temperature increases and contracts when it decreases. Thermometers measure this expansion and contraction. The most widely used temperature scale is the Celsius Scale where water freezes at 0 and boils at 100. The Celsius scale is the metric scale for temperature. The most common scale in the U.S. is the Fahrenheit Scale where Water freezes at 32 and boils at 212. If the U.S. ever switched to the metric scale this scale would become obsolete. The scale used in science is the SI scale which is called the Kelvin scale. On this scale 0 is the lowest absolute temperature called Absolute Zero. At absolute zero a substance has no kinetic energy at all. Temperature and Kinetic Energy: Temperature is not a measure of the total kinetic energy, but is a measure of the average kinetic energy. Heat (21.2) Energy will spontaneously transfer from a warmer object to a cooler object (an object with more average kinetic energy to an object with less average kinetic energy). The energy that transfers in this way is called Heat. o Matter does not contain heat, it contains energy heat is the word that describes energy that is in transit between one place and another. When objects in contact with each other are able to transfer heat they are said to be in Thermal Contact Thermal Equilibrium (21.3) When objects in thermal contact with each other reach the same temperature and heat no longer flows between them they are at Thermal Equilibrium. o When you read a thermometer you wait until it has stopped moving up or down, when that happens it is at thermal equilibrium and now has the same temperature as what it was measuring. Measurement of Heat (21.5) We can measure heat transfer by measuring the temperature change when a known quantity and type of mass absorbs heat. The most common unit of heat is the Calorie and it is the amount of heat that raises one gram of water 1 degree Celsius. o A kilocalorie is 1000 calories In standard units heat is measured in Joules. 1 calorie equals J We measure the energy value of food by burning it and looking at how much energy is given off as heat when it is burned. Specific Heat Capacity (21.6) Different substances have different capacities for storing energy. This capacity for storing energy is called Specific Heat Capacity and tells us how quickly something will heat and how quickly it will cool. o The technical definition of Specific Heat Capacity is as follows: The specific heat capacity of any substance is defined as the quantity of heat required to raise the temperature of a unit of mass of the substance by 1 degree. We can think of specific heat capacity as thermal inertia

2 The High Specific Heat Capacity of Water (21.7) Because of its very high Specific Heat Capacity water is used as a cooling agent in cars, it also takes longer to cool off than other substances, which makes it ideal for heating devices like hot water bottles. This is also the reason why cities near the oceans have a much milder climate. Thermal Expansion (21.8) With few exceptions all forms of matter expand when heated and contract when cooled. Gases expand and contract more noticeably than liquids and liquids expand and contract more noticeably than solids. o This expansion and contraction is accounted for in construction with all types of materials (from roads, to dental work) Bimetallic Strips are strips that are made of two different metals. Because of the use of two different metals the strips will expand and contract unevenly. This will cause the strip to bend. These types of strips can be used in devices that are regulated by temperature. o Thermostats are one example of how bimetallic strips are used. When the room is too cold the strip turns one way turning the heat on, or when it becomes too warm it turns the other way turning the air on. Some materials like glass will break when heated or cooled due to the expansion and contraction. Expansion of Water (21.9) Water is one of the exceptions that will expand as it cools to ice instead of contracting, and will contract as it melts to water instead of expanding. Above 4 degrees Celsius the water begins to expand as it heats like other substances, but below this mark it behaves in the opposite way than we might expect it to. o Because of this water is the densest and has contracted the furthest at 4 degrees Celsius. o The reason that water expands as it freezes is because of the polarity of the molecule which causes it to a form an open structure crystal when it is in its solid form. o This strange behavior of water in some ways is what allows life to exist on earth. One example of this is in lakes in the winter time a layer of ice forms on the top of the water allowing organisms inside to remain alive. If ice were more dense than water the ice would form on the bottom of the pond and that environment would not exist for marine life. Because of this and the high specific heat of water, the bottom of lakes is a constant 4 degrees Celsius year round. Heat Transfer (Ch 22) Objects transfer heat from one to another through three processes: Conduction, convection, and radiation. Conduction (22.1) Conduction of heat can happen both within materials and between materials. Metals are the best conductors. Conduction is basically caused by many molecular collisions occurring between atoms or molecules. Good conductors are materials that conduct heat well, and generally all have loose electrons in their outer shells. When we touch something to feel how hot or cold it is, we are really experiencing conduction of heat to or from that object. Objects will feel cooler (or hotter) if they are good conductors than they will if they are poor conductors even though two objects in the same room are probably actually the same temperature. Liquids, gases and porous materials are generally good insulators because they do not conduct well. Cold is just the absence of heat. Insulators cannot prevent heat from getting through, they can just slow the transfer of heat. Convection (22.2)

3 In conduction the molecules stay where they are while the heat moves. In convection the molecules actually move to heat a new area. One example of this is the water in a furnace, it flows through the furnace heating the radiators. Convection occurs in all fluids (liquids and gases). Winds: o Winds are actually caused by convection. Some parts of the earth absorb the suns heat better and thus heat the air near the surface better and the movement of this pocket of warm air causes the winds. o Whatever direction the winds go during the day, they go the opposite at night because the process reverses and the parts that absorbed the heat better cool off more, thus cooling the air above them more. Why Rising Air Cools: o When warm air rises it expands because there is less atmospheric pressure acting on it. As the air expands it cools off because there are less collisions between the molecules when they are more spaced out. Radiation (22.3) When heat is transferred across great distances, like from the sun to the earth, it is done through radiation. This energy is called radiant Energy. o All objects emit radiant energy, however the objects that are cool emit radiant energy that has a long wavelength, while objects that are hot emit radiant energy that has a short wavelength. Remember visible light has a very short wavelength, and so all things that put out light (fire, light bulbs, the sun, etc) also puts out a significant amount of heat. Absorption of Radiant Energy (22.4) Radiant energy can either be absorbed or reflected. If it is absorbed it will make you warmer, if it is reflected it will make you appear brighter or lighter. o For example if you are wearing a dark shirt on a warm day you will feel much warmer than if you were wearing a white shirt, however in bright sunlight a white shirt appears very, very bright. o When radiant energy enters into something (like the hole in the box in figure 22.13) it will reflect internally and lose energy each time it is reflected. If the hole that it came in is small enough there is very little chance of it getting back out of the container before it loses all of its energy. This is why the hole in the box will appear dark. See the diagrams on page 332 of your book for an illustration. Emission of Radiant Energy (22.5) Just as dark objects absorb radiant energy better, they also emit it better. In fact if you take two objects, one black and one white, that are otherwise identical, and heat them to the same temperature, the black one will cool off faster because it is better able to emit its radiant energy. o Another way to say this is that the darker the object, the faster it will reach thermal equilibrium with its surroundings, either by absorbing radiant energy or emitting it. Newton s Law of Cooling (22.6) Newton s Law of Cooling states that the rate of cooling for any object (whether by conduction, convection, or radiation) is approximately proportional to the temperature difference between the object and its surroundings. o Newton s Law of Cooling also works when heating.

4 Change of Phase (Ch 23) Phases are states of matter. There are three phases: Solid, liquid and gas. Evaporation (23.1) Evaporation is the change of phase from liquid to gas that takes place at the surface of a liquid. Temperature is related to average kinetic energy. Molecules in a liquid are constantly colliding, giving and taking kinetic energy from each other. In this process, some molecules at the surface gain enough energy to break free of the liquid and become vapor. o When these molecules leave they take the kinetic energy that they have gained with them, and therefore the main portion of the substance has less energy and is cooler. This means that evaporation is a cooling process. We use evaporation as a cooling process when we sweat, dogs use it when they pant, and many other animals get wet in hot weather in order to take advantage of evaporation s cooling effects. Condensation (23.2) Condensation is the opposite process to evaporation and is the changing of a gas to a liquid. o Condensation happens when vapor molecules collide with colder solid or liquid molecules and loose their energy forcing them to drop back down into the liquid stage. Condensation in the atmosphere: o Air always contains water vapor. At any temp there is a limit to how much vapor the air can contain, when the air reaches that point we say that it is saturated. o When we talk about relative humidity we are making a reference to the amount of humidity the air contains compared to the saturation point, or limit for that temperature. o At saturation there are an equal number of water molecules condensing and evaporating. Fog and Clouds: o Fog is a cloud that has formed near the ground, where water condenses in the air instead of on a surface. Evaporation and Condensation Rates (23.3) When you get out of the shower you feel cool because air is being evaporated off of your skin. If you stay in the shower you don t feel as cool because water vapor is also condensing on your skin. The same thing is true of a dish of water left out on a table. If the air is dry you will see the water level drop because of evaporation taking place, however in a moist climate you will see no change because evaporation and condensation are happening at the same rate. o This state where evaporation and condensation are happening at the same rate is called Equilibrium. If evaporation happens faster than condensation then cooling takes place. If condensation happens faster than evaporation, then warming takes place. Boiling (23.4) When a substance changes phase from liquid to gas underneath the surface of a liquid by forming air bubbles, it is called boiling. o In order for boiling to occur the pressure of the vapor in the bubbles must be at least as high as the pressure of the surrounding water so that they don t get popped.

5 When there is greater atmospheric pressure, then the temperature necessary for boiling to occur increases because it becomes more difficult to overcome the pressure of the surrounding water. When there is low atmospheric pressure, then the temperature necessary for boiling to occur decreases because it becomes easier to overcome the pressure of the surrounding water. A pressure cooker raises the pressure, so that boiling will occur at a higher temperature. It is important to note that boiling is actually a cooling process (just like evaporation), and that you heat the water and that heated water cooks the food, it is not the boiling that cooks it. When you are asked to boil water in a recipe, the recipe is actually calling for 100 degree Celsius water which may or may not be the boiling point where you are. o Because boiling is actually a cooling process, once water starts to boil the temperature stops increasing, and stays at the same temperature because the boiling is cooling it as fast as the stove is heating it. Freezing (23.5) The change is phase from liquid to solid is called freezing. When freezing occurs, the molecules in a liquid slow down and begin to become attracted to each other, until that attraction actually holds them into fixed positions where they are a solid. The freezing point of water is lower when anything is dissolved in it because the extra molecules get in the way of them being attracted to each other and finding their fixed positions. Boiling and Freezing at the Same Time (23.6) You can make a liquid boil and freeze at the same time by putting it in a vacuum container and reducing the pressure until it is so low that both boiling and freezing occur simultaneously. o This is how freeze dried coffee is made. Energy and Change of Phase (23.8) Energy must be put into a substance to change its phase. o The general process of heating is that a solid is heated to its melting point, and then heat is applied with no apparent change in temperature until the entire sample melts. Then heat is applied to raise the liquid to its boiling point, and then heat is applied with no apparent change in temperature until the entire sample has become vapor. Then heat is applied to heat the vapor. This process can be seen graphed on page 348 in figure o The reverse of this process happens for cooling. It is really important to note that the energy involved in changing phase is much greater than the energy involved in simply heating and cooling. For example one gram of water releases 540 calories when it goes from 100 degree Celsius gas to 100 degree Celsius liquid, but only 100 calories when it goes from 100 degree Celsius liquid to 0 degree Celsius liquid. That is a very significant difference. o On page 349 of the book there is an excellent explanation of how this process is used to cool food in a refrigerator o On page 350 the book explains why a moist finger can touch and iron without being harmed and why moist feet can walk across hot coals without being burned all because the change of water from liquid to gas requires so much energy. Thermodynamics (Ch 24) Thermodynamics is the study of heat and its transformation into mechanical energy. Comes from the Greed words thermo meaning heat and dynamics meaning movement. Absolute zero (24.1) There is a low limit of temperature, in other words how cold it can get, we call this absolute zero and it is represented by 0 on the Kelvin scale, and 273 on the Celsius scale.

6 First Law of Thermodynamics (24.2) The word calorie was developed from an old theory that we believed in the eighteenth century in which they believed that heat was an invisible fluid called caloric. The First Law of Thermodynamics is that whenever heat is added into a system, it transforms into an equal amount of some other form of energy. o Remember that a system is a group of atoms, molecules, particles or objects that we are dealing with. It can be as small as a single atom or as large as the universe. The most important step in dealing with systems is defining what exactly is in the system. In equation form the first law says that: Heat Added = Increase in internal energy + external work done by the system This means that if you put heat energy into a system and no external work is done by the system, then all of the heat went into internal energy that increased the temperature. Most systems you will see that adding heat will both increase the temperature and do external work. Another application of the first law is to say that if no energy is put into a system from an external source, then the energy of any work done needs to equal the internal energy change of the system. Adiabatic Process (24.3) An adiabatic process happens when you compress or expand a gas so that no heat enters or leaves the system. (The word adiabatic is actually Greek for impassible) o To make a process happen adiabatically it has to either be done very quickly so that there is not time for heat to enter or leave the system, or it has to be done in an insulated environment where heat cannot enter or leave easily. Common example: cylinders of an automobile. The piston moves very quickly and so heat cannot leave the combustion chamber. In some cases the compression is so high that the heat raises enough to not need a spark plug. o When the piston is compressing the gas it is adiabatically heating it. When the gas expands as the piston goes back down the energy released does adiabatic work externally. Adiabatic form of the first law Change in air temperature ~ pressure change (this is because of the first law there is no heat coming in from the outside and so these two have to be about equal) Adiabatic processes also happen with air blobs, which cool off greatly as they rise. Extreme example of this is the Chinook that comes down out of the Rockies onto the plains. A Chinook is a breeze that is cool at the top of the mountain but as it comes down the mountain warms up greatly, allowing some areas to have much milder winters than their neighbors. Second Law of Thermodynamics (24.4) The Second Law of Thermodynamics says that heat will never of itself flow from a cold object to a hot object. Heat engines and the second law (24.5) A heat engine converts internal energy into work (it cannot do so with 100 percent efficiency but it can do it). In heat engines energy flows out of a high-energy reservoir into a low energy reservoir. There are 3 things all heat engines will do: 1. Absorb heat from a reservoir of higher temperature, increasing its internal energy. 2. Convert some of this energy into mechanical work 3. Expel the remaining energy as heat to some lower-temperature reservoir, usually called a sink The second law as applied to heat engines would be that when work is done by a heat engine running between two temperatures, T hot and T cold, only some of the input heat as T hot can be converted to work, and the rest is expelled as heat to T cold.

7 When the expelled heat in the last step of the heat engine is undesirable it is called thermal pollution (for example on a hot day the heat from a dryer vent is not good, but in the middle of winter you are grateful for it. The efficiency of heat engines was looked at by Sadi Carnot who wrote an equation for the ideal efficiency of a heat engine called the Carnot Efficiency, which says o Ideal efficiency = T hot - T cold / T hot o All temperatures in this equation are in K. See the chapter for some examples of heat engines, especially page 362 where it shows a simplified drawing of a steam turbine Order Tends to Disorder (24.6) There is a second part to the first law of thermodynamics and that is that in addition to matter neither being created or destroyed, whenever it is transformed some of it is lost to waste energy that is disorganized and unusable. The second law can also be restated in terms of order and disorder to say that natural systems tend to proceed toward a state of greater disorder. Entropy (24.7) Entropy is the measure of the amount of disorder in a system. Whenever a system is left to run freely the entropy of that system will increase. It is also important to note that the first law of thermodynamics has never been shown to have any exceptions but due to the random nature of entropy and molecular activity, it is possible that the second law could have exceptions. The example that the book gives is that a barrel of pennies dropped on the ground could randomly all turn up heads, but it is not likely that that would happen.