Chapter 12 Thermal Energy

1. Chapter 12 Thermal Energy Thermodynamics is the study of the patterns of energy change. The "thermo" refers to energy, and "dynamics" means patterns of change. Kinetic molecular theory: is based on the assumption that matter is made up of tiny particles that are always in motion. In a hot body, the particles move faster, and thus have greater kinetic energy than particles in a cooler body. Hotness is a property of an object called its temperature.

2. The overall energy of motion of the particles that make up an object is called the thermal energy of that object. To find the thermal energy of a glass of water, you would have to find the average energy of a single water molecule then multiply that by the total number of molecules.  Conduction: is the transfer of kinetic energy when particles collide.

3. Temperature and Energy Temperature : is a measure of the average kinetic energy of all the particles within an object. Thermometer: is a device that measures temperature. A liquid thermometer uses the expansion of liquid alcohol or mercury to indicate changes in temperature. A digital thermometer uses changes in electricity to measure temperature.

5. The thermometer measures temperatures in Fahrenheit, Celsius and Kelvin. Fahrenheit is used mostly in the United States, and most of the rest of the world uses Celsius. Kelvin is used by scientists. Fahrenheit is named after the German physicist Gabriel D. Fahrenheit who developed his scale in 1724. Ice freezes at 32 degrees Fahrenheit (F) and water boils at 212 degrees F. He arbitrarily decided that the difference between the freezing point and boiling point of water should be 180 degrees. The Celsius scale is named after Anders Celsius. The Celsius scale used to be called the "centigrade" scale. Centigrade means "divided into 100 degrees." Anders Celsius developed his scale in 1742. He started with 0 degrees Celsius (?C) as the freezing point of water . At the point where water boils, he marked that at 100 ? C. This scale is much more scientific because the measurement is broken down into an even 100 degrees.

6. Kelvin is named after Lord Kelvin, whose full name is Sir William Thomson, Baron Kelvin of Largs, Lord Kelvin of Scotland. His scale starts at 0 degrees Kelvin, which is called absolute temperature. Lord Kelvin took the idea of temperature one step further with his invention of the Kelvin Scale in 1848. The Kelvin Scale measures the coldest temperature there can be. He said there was no upper limit of how hot things can get, but he said there was a limit as to how cold things can get. Kelvin developed the idea of Absolute Zero. This is at minus 273.15 ?C (or -523.67 F)! At this temperature, everything, including the movement of electrons in an atom, stops completely. As far as scientists know, nothing in the universe can get that cold!

7. The three major temperature scales.

8. The Kelvin scale is used for scientific calculations because there are no negative values. Kelvin Temperature = Celsius temperature + 273 TK = TC + 273 Example Problem: Convert 25 ?C to kelvins. Convert 0K to ?C .

10. Einstein proposed that as atoms approach absolute zero (-273.15�C), the waves expand in inverse proportion to their momentum until they fall into the same quantum state and finally overlap, essentially behaving like a single atom. Heat and Thermal Energy: Heat flows from a warmer body to a colder one. Convection is the transfer of energy by the movement of fluids with different temperatures. Fluid: is any material that can flow. (e.g liquids and gases). As water heated by conduction at the bottom of the pot flows up and the colder water at the top sinks. Radiation is the transfer of energy by electromagnetic waves, which include infrared radiation, visible light, and ultraviolet rays. Radiation does not involve the movement of matter. The sun warms us from over 150 million kilometers via radiation.

11. Specific Heat The specific heat (C) of a material is the amount of energy that must be added to the material to raise the temperature of a unit mass one temperature unit.

12. Using Specific Heat Specific heat can be used to measure changes in energy. Heat Transfer Q = mass(m) � specific heat (C) � Change in temperature (?T) Q = m � C � ?T J g J/g��C �C The symbol ? (delta) means change in ?T : is the change in temperature ?T = Tfinal � Tinitial

14. Practice: A 2.8-g sample of pure metal requires 10.1 J of energy to change its temperature from 21�C to 36 �C. What is this metal? Calorimetry: Measuring Specific Heat A calorimeter is a device used to measure changes in thermal energy.

15. The calorimeter contains a known mass of cold water at a measured temperature. The heat released by the substance is transferred to the cooler water. From the resulting increase in water temperature, the change in thermal energy of the substance is calculated. Example Problem Heat Transfer in a Calorimeter. A calorimeter contains 0.50 kg of water at 15 �C. A 0.040-kg block of zinc at 115 �C is placed in the water. What is the final temperature of the system?

16. 12.2 Change of State and Laws of Thermodynamics The three most common states of matter are solids, liquids, and gases.

18. Heating Curve As heat added to solid, it first raises the temperature of the solid to the melting point. Then added heat goes to overcome the forces holding the particles together in the solid state. Temperature stays at the melting point Heat of Fusion As more heat added it raises the temperature of the liquid to the boiling point. Then added heat goes into boiling the liquid Temperature stays at the boiling point Heat of Vaporization The more heat added raises the temperature of the gas.

19. The amount of energy needed to melt one kilogram of a substance is called the heat of fusion of that substance. The thermal energy needed to vaporize one kilogram of a liquid is called the heat of vaporization. The slope of the line between 0 and 273 k represents the specific heat of the ice. The slope between 273 and 373 represents the specific heat of water. And the slope above 373 k represents the specific heat of steam.

20. The heat Q, required to melt a solid of mass m is given by the equation, Where Hf is the heat of fusion The heat Q, required to vaporize a mass, m, of liquid is given by the equation, Where Hv is the heat of vaporization.

22. Practice Problem How much heat is needed to change 3.00 � 102 g of ice at -30.0 ?C to steam at 130.0 ?C ?

23. The First Law of Thermodynamics 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. Energy can be exchanged between the system of interest and its surroundings. However, the total energy of the system plus the surroundings is constant. That's the First Law of Thermodynamics. The First Law is merely a restatement of the law of conservation of energy, which states that energy is neither created nor destroyed but can be changed into other forms. Thermal energy can be increased either by adding heat or by doing work on a system. If you use a hand pump to inflate a bicycle tire, the air and pump become warm. The mechanical energy in the moving piston is converted into thermal energy of the gas. A toaster converts electric energy into heat to cook bread.

24. The conversion of mechanical energy to thermal energy, as when you rub your hands together, is easy. The reverse process, conversion of thermal to mechanical energy, is more difficult. A device able to convert thermal energy to mechanical energy continuously is called a heat engine.

25. This diagram represents heat at high temperature transformed into mechanical energy and low-temperature waste heat.

26. While part of the input heat energy QH, sometimes known as heat of combustion, is converted into useful work W, the remaining heat is lost to the environment as exhaust heat QL( or Qc). Examples of heat engines are Refrigerator, air conditioners, heat pumps, steam engines, coal and nuclear power plants, the engine in your automobile, and the engines on jet aircraft.

28. Not all the thermal energy from the high-temperature flame is converted into work. The exhaust gases and the engine parts become hot (waste heat). In a car engine, Thermal energy in the flame = Mechanical energy produced + waste heat expelled. The overall change in total energy of the car-air system is zero.

29. Heat pumps

30. Heat removed from the refrigerator contents + the work done by the motor = heat expelled to the outside air at a higher temperature.

31. How Does A Refrigerator Work? Modern refrigerators don't use CFC. Instead they use ammonia gas. Ammonia gas turns into a liquid when it is cooled to -27 degrees Fahrenheit (-6.5 degrees Celsius). A motor and compressor squeezes the ammonia gas. When it is compressed, a gas heats up as it is pressurized. When you pass the compressed gas through the coils on the back or bottom of a modern refrigerator, the hot ammonia gas can loose its heat to the air in the room. As it cools, the ammonia gas can change into ammonia liquid because it is under a high pressure. The ammonia liquid flows through an expansion valve, a tiny small hole that the liquid has to squeeze through. Between the valve and the compressor, there is a low-pressure area because the compressor is pulling the ammonia gas out of that side. When the liquid ammonia hits a low pressure area it boils and changes into a gas. This is called vaporizing. The coils then go through the freezer and regular part of the refrigerator where the colder ammonia is the coil pulls the heat out of the compartments. This makes the inside of the freezer and entire refrigerator cold. The compressor sucks up the cold ammonia gas, and the gas goes back through the same process over and over. How Does the Temperature Stay the Same Inside? A device called a thermocouple (it's basically a thermometer) can sense when the refrigerator in the temperature is as cold as you want it to be. When it reaches that temperature, the device shuts off the electricity to the compressor. So when the cold from inside the refrigerator starts to leak out and the heat leaks in, the thermocouple turns the compressor back on to cool the refrigerator off again. That's why you'll hear your refrigerator compressor motor coming on, running for a little while and then turning itself off.

32. Heat pump Is a refrigerator that can be run in two directions. In summer, heat is removed from the house, and thus cooling the house. The heat is expelled into the warmer air outside. In winter, heat is removed from the cold outside air and transferred into the warmer house.

33. In this cycle, liquid enters the refrigerator in a region of low pressure, where it evaporates to become a gas, absorbing heat in the process. This gas then passes through a pump into a region of high pressure, where it condenses to become a liquid, thereby releasing heat. This cycle thus requires a special liquid which evaporates and condenses within the given operating pressure and temperature regions; these liquids, such as Freon, usually require special care in their handling and disposal.

34. The Second Law of Thermodynamics States that: Natural processes go in a direction that maintains or increases the total entropy, S ( measure of the disorder in a system) of the universe. Heat will not flow spontaneously from a cold object to a hot object. You cannot create a heat engine which extracts heat and converts it all to useful work.

36. A drop of perfume evaporates because states in which molecules of perfume are scattered throughout a large volume of air vastly outnumber states in which the molecules are confined in the tiny volume of a drop. For a system at constant temperature, such as during melting or boiling, the change in entropy ?S is given by ?S=Q/T (in J/K) Where Q is the heat (thermal energy) that flows into or out of the system and T is the absolute temperature. All processes that require energy, for example, biochemical reactions that support life, occur only because the entropy increases as a result of the process.