Topic #12

. Temperature, Heat and Thermodynamics
1. What is Warmth?
2. Thermal Energy and Temperature
3. Equilibrium and Thermometry
4. Temperature Scales
5. The Kelvin or Absolute Temperature Scale
6. First Law of Thermodynamics
7. The Second Law of Thermodynamics
8. Specific Heat
9. Conservation in Energy Transfer
10. Change of State
Notes should include:
“What is warmth?”, “What is hot?”, “What is cold?” are questions asked since people first became curious about heat. The terms hot and cold are used in everyday speech, but what do they mean? For example, "how hot is nitrogen when it is ten degrees above its boiling point", or "how cold is steel when its ten degrees below its freezing point?" are questions that require a definition of terms. If something is said to be warm or hot, it is always stated with reference to something else. The same is true if something is said to be cool or cold. If the statement does not say or imply what the statement is referring to, you need to ask the question, "Warm or hot (cool or cold), with respect to what?".
In the eighteenth century, it was believed that a warm body had an invisible fluid called the "caloric" added to it, which wasn't there when it was cold. The theory seemed to explain a lot of observations, but didn't explain why friction between two chunks of a material like ice would cause the material to melt. In this latter situation nothing is being added to the material to warm it. By the mid-nineteenth century a new theory was proposed. It is called the “ kinetic molecular theory
”. This theory says that
1.
matter is made up of very tiny particles, 2.
the particles are always in motion, and 3.
in a warm body the particles move faster than in cool bodies. The particles in the warmer body have higher kinetic energies on the average than in the cool body.
The spacing and motion of particles in a substance can be described according to the following model. In a solid, imagine, the particles are all held together by tiny springs or stretchy glue bonds. As the substance warms, the particles vibrate (move around a fixed point) faster and faster because the particles’ kinetic energy increases. Eventually as the warming increases the inter particle attractions are overcome by this kinetic energy
(energy of motion) and the particles begin to slip and slide over and around one another.
At this point the substance is experiencing melting. As the material is warmed even further, the molecules which are still in physical contact begin acquiring enough kinetic energy individually that allows them to overcome any attractive forces acting on them due to the other molecules. The faster ones escape from the group. The substance has begun to change from the liquid state to the gaseous state.
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Thermal Energy and Temperature: Temperature is a measure of the average kinetic energy of the particles making up a substance. Temperature is an intrinsic property, which means its value is not a function of the quantity of material whose temperature is being measured. For example, a glass of water can be at the same temperature as a whole swimming pool full of water.
Equilibrium and Thermometry: A thermometer is a device placed in contact with an object or substance and allowed to come to a state of thermal equilibrium with the object or substance it is touching. The function of Thermometer requires that their design include some property that changes with temperature. For example in the mercury or alcohol thermometer, the property that allows you to measure temperature and temperature change is the thermal expansion of liquids. Other properties may be used such as the thermal expansion of solids and of gases, or the change in electrical resistance in a circuit’s path.
Temperature Scales: The three primary temperature scales are the Fahrenheit, the
Celsius, and the Kelvin. Conversion among the scales is a necessary skill because not all of the scales can be used in every type of calculation involving changes in temperature.
Be sure to review the necessary equations to convert between temperature scales.
Fahrenheit to Celsius: _____ o
C = (5/9) ( _____ o
F – 32)
Celsius to Fahrenheit: _____ o
F = (9/5) _____ o
C + 32
Celsius to Kelvin: _____K = _____ o
C + 273.15
Kelvin to Celsius: _____ o
C = _____K – 273.15
The Kelvin or Absolute Temperature Scale: It was discovered that there is a direct proportion between temperature and volume, when pressure is held constant and temperature is measured in kelvins. Theoretically this means that the temperature of the gas could not go below a certain point, because at that point the gas's volume would have reached zero liters. That point is called absolute zero and is the zero on the kelvin temperature scale. This temperature, found by extrapolation of data on a volume - temperature graph, is 0 kelvin or a - 273.15 oC. This temperature scale is named after a scientist (physicist) by the name of William Thompson. Apparently, he received the title of Lord Kelvin. A relatively famous statement on his part was that if something could not be expressed using mathematics, it was not a true science.
Special Note: “What is the difference between 25 o
C (twenty Five degrees Celsius) and
25 C o
(twenty five Celsius degrees) ?” The answer is that 25 o
C is an actual temperature reading on a thermometer and 25 C o is the difference (based on subtraction) between two temperatures such as in
150 o C – 125 o C = 25 C o .
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First Law of Thermodynamics: If two bodies are brought into contact and they are at different temperatures energy will flow from the hotter body to the cooler body. Heat is the flow of the energy that results from the difference in temperature. Heat, by definition, is the energy transferred between two objects or systems, it is NOT the total energy contained within a single object or system. In some discussions, the heat content of a material is described as the total energy of the material, but because so many things factor into the total energy of a material, the heat content of a material can not be easily determined. The First Law of Thermodynamics says that the total increase in the thermal energy of a system is the sum of the work done on it and the heat added to it. The symbol Q is the variable symbol used to indicate the heat transferred from one object to another , or from one system to another, such as in the amount of heat either gained or lost by an object. Like other forms of energy, heat is measured in joules .
Historically, heat was measured in a unit called the calorie. This is the heat calorie and it so happens that the food Calorie (note the use of the upper case letter C) is equal to one thousand heat calories. The heat calorie is not an SI unit as is the joule, so you are not likely to use the calorie in very many problems. If you should encounter it, just remember that the heat calorie is equal to 4.18 joules of energy.
Producing heat from mechanical energy is not so hard. Think about what friction does to two surfaces that are sliding past one another. For example, many people rub their hands together when it is cold. On the other hand turning heat into mechanical energy can be more complicated. For example, an automobile engine is an example of a heat engine which essentially does exactly that. It turns heat into mechanical energy. Of coarse the pistons themselves turn the crank shaft in the engine due to expanding gas (pressure) in each cylinder, but the source of the expansion is the heat produced by the combustion of the gasoline. The engine cycles over and over again as the hot escaping gas is replaced by a new gasoline and air mixture through the use of a carburetor or a fuel injection system.
The burning (combustion) occurs when the spark plug fires. In terms of the first law the heat produced by the combustion is equal to the mechanical energy produced plus the waste heat left over. Did you ever notice how hot an engine gets when it is running and the job the cooling system has to do to dissipate that waste heat to prevent the engine from over heating and literally seizing up.
While normally heat flows from a warmer body to a colder body, it is possible to move heat the other way. However, since you are moving energy in the direction opposite to its normal flow, much like making water run up hill, you need an external source of energy.
The most common device to accomplish this is a heat pump. Think about refrigeration.
Whether you are thinking about cooling the inside of a refrigerator or the inside of a whole building, you need to be able to move heat out of the cooler interior to the warmer exterior. A fluid to carry the heat and a compressor to change the phase of the fluid so that a lot of heat energy can be transferred quickly are the main two components of such a pump. In addition tubes are necessary for the fluid to flow between the inside and the outside and a blower is required to push or pull air across the tubes outside the space being cooled to dissipate the heat.
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The Second Law of Thermodynamics: Though theoretically, a heat engine could operate without producing waste heat and not violate the first law, it appears that waste heat is a natural part of the operation of a heat engine and cannot be eliminated, though possibly reduced through better engineering designs and materials. In the 1800's a French engineer by the name of Sadi Carnot studied engines that would turn heat into mechanical energy. He observed that the even the best heat engines would generate some waste heat.
His study lead to the definition of the term entropy. Entropy is defined as a quantity that measures the disorder of a system. From this comes the second law of thermodynamics.
The second law of thermodynamics says that natural processes tend to go in the direction that maintains or increases the total entropy of the universe. As an example of entropy increasing consider the combustion of a solid piece of wood. After the wood is burned the law of conservation of energy (historically referred to as the law of conservation of matter and energy) tells us that we still have the same total amount of matter and energy we had before the combustion, but now the energy is in a form that is unavailable to us to use again and the matter is in a more random state, having less structure than it did before it was burned. The burned piece of wood has more entropy than the original unburned piece of wood. Entropy increased during the process of burning the wood.
(Environmental Question: "Pollution issues aside for the moment, might this suggest we should consider avoiding using up our combustible fossil fuels too quickly, lest future generations find them in short supply?")
Specific Heat: The specific heat of a substance is the amount of heat necessary to raise a unit mass of the substance one unit of temperature. For example, the specific heat of liquid water is 4180 J / kg K. This means that if you want to raise one kilogram of liquid water one Kelvin (1 K), you would need to add 4180 J to the one kilogram of the water.
Stop and think about this. Would the amount of heat energy necessary to raise the one kilogram of water 2 K be any different than the amount of heat to raise 2 kilograms of water 1 K? Why?
The equation for specific heat is given as Q = m C /\T , where Q is the amount of heat needed to accomplish the described change, m is the mass of the material stated, C is the specific heat of the material experiencing the change, and /\T is the change in temperature the material is experiencing.
Conservation of Heat Energy in an Energy Transfer: When two materials come together each having a different temperature, heat will flow from the warmer body to the cooler body. As a result the warmer body's temperature will decrease while the cooler body's temperature will increase. Eventually the two bodies will reach thermal equilibrium and they will be at the same temperature. The equation used to represent this situation is Q
Lost
+ Q
Gained
= 0. This equation then becomes (m C /\T)
Lost
+ (m C
/\T)
Gained
= 0.
Sometimes there may be a third object or material in the problem. Often when this happens the third factor is a calorimeter cup in which the two objects are brought together for the purpose of heat transfer. The cup will require that a third Q or (m C /\T) appear in
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 the equation. In this case, though the cup will have its own mass and specific heat, it will experience the same temperature change as the material that is first placed in it, assuming they were allowed to reach a state of thermal equilibrium before anything was done to them.
Change of State: In addition to an object experiencing temperature changes, it can also under go phase changes. In the case of melting (or its counterpart freezing) the equation to determine the amount of heat necessary to bring about the phase change is
Q = m H fusion
. In this equation Q is the amount of heat necessary to produce the described phase change, m is the mass of the material under going the change, and H fusion is the heat of fusion, which is the amount of heat necessary to melt a unit mass of the material. For example, frozen water (ice) has a H fusion
of 3.34 x 105 J / kg. In the case of vaporization or boiling (or its counterpart condensing) the equation to determine the amount of heat necessary to bring about the phase change is Q = m H vaporization
. In this equation Q is the amount of heat necessary to produce the described phase change, m is the mass of the material under going the change, and H vaporization
is is the heat of vaporization, which is the amount of heat necessary to vaporize a unit mass of the material. For example, liquid water has a H vaporization
of 2.26 x 106 J / kg.
Vocabulary: Thermodynamics, kinetic molecular theory, thermal energy, temperature, conduction, thermal equilibrium, thermometer, absolute zero, Kelvin and Kelvin temperature scale, heat, convection, radiation, specific heat, calorimeter, melting point, boiling point, heat of fusion, heat of vaporization, first law of thermodynamics, heat engine, entropy, second law of thermodynamics. Also include Fahrenheit and Celsius temperature scales
Skills to be learned:
Temperature conversions among the three common scales
Solve Specific Heat Problems
Solve Heat Energy Conservation Problems
Solve Phase Change Problems
Solve Multiple Step Heating and Cooling Curve (graph) Problems
Assignments:
Textbook: Read / Study / Learn about Temperature, Heat Energy and the Laws of
Thermodynamics
WB Exercise(s): PS# 12-1, 12-2, and 12-3
Activities: TBA
Resources:
This Handout and the Overhead and Board Notes discussed in class
WB Lessons and Problem Sets www.physicsphenomena.com
- “Temperature, Heat and Thermodynamics”
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