Using Thermal Energy

Transcription

1 Heating Systems Most homes and public buildings contain some type of heating system. In the simplest and oldest heating system, wood or coal is burned in a stove.

2 Heating Systems The thermal energy that is extracted from burning fuel is transferred from the stove to the surrounding air by conduction, convection, and radiation. One disadvantage of this system is that heat transfer from the room in which the stove is located to other rooms in the building can be slow.

3 Forced-Air Systems The most common type of heating system in use today is the forced-air system.

4 Radiator Systems A radiator is a closed metal container that contains hot water or steam. The thermal energy contained in the hot water or steam is transferred to the air surrounding the radiator by conduction. This warm air then moves through the room by convection.

5 Radiator Systems In radiator heating systems, fuel burned in a central furnace heats a tank of water. A system of pipes carries the hot water to radiators in the rooms of the building. After the water cools, it flows through the pipes back to the water tank, and is reheated.

6 Electric Heating Systems An electric heating system has no central furnace. Instead, electrically heated coils placed in floors and in walls heat the surrounding air by conduction. Thermal energy is then distributed through the room by convection.

7 Solar Heating The radiant energy from the Sun can be used to help heat homes and buildings. There are two types of systems that use the Sun s energy for heating passive solar heating systems and active solar heating systems.

8 Passive Solar Heating In passive solar heating systems, materials inside a building absorb radiant energy from the Sun during the day and heat up. At night when the building begins to cool, thermal energy absorbed by these materials helps keep the room warm.

9 Active Solar Heating Active solar heating systems use solar collectors that absorb radiant energy from the Sun. The collectors usually are installed on the roof or south side of a building. Radiant energy from the Sun heats air or water in the solar collectors.

10 Active Solar Heating The black metal plate absorbs radiant energy from the Sun. The absorbed energy heats water in pipes just above the plate.

11 Active Solar Heating A pump circulates the hot water to radiators in rooms of the house. The cooled water then is pumped back to the collector to be reheated.

12 Thermodynamics Thermal energy, heat, and work are related, and the study of the relationship among them is thermodynamics.

13 Heat and Work Increase Thermal Energy You can warm your hands by placing them near a fire. The fire heats your hands by radiation. If you rub your hands and hold them near a fire, the increase in thermal energy of your hands is even greater. Both the work you do and the heat from the fire increase the thermal energy of your hands.

14 Heat and Work Increase Thermal Energy Your hands can be considered as a system. A system is anything you can imagine a boundary around.

15 Heat and Work Increase Thermal Energy The heat to a system is the amount of thermal energy into the system that crosses the boundary. The work done on a system is the work done by something outside the system s boundary.

16 The First Law of Thermodynamics According to the first law of thermodynamics, the increase in thermal energy of a system equals the work done on the system plus the heat to the system.

17 The First Law of Thermodynamics The temperature of a system can be increased by adding heat to the system, doing work on the system, or both. The increase in energy of a system equals the energy added to the system.

18 Isolated and Non-Isolated Systems A system is non-isolated if there are thermal energy transfers across the boundary or if work is done across the boundary. Then energy is added to or taken away from the system. If there are no thermal energy transfers across the boundary and there is no work done across the boundary, then the system is an isolated system.

19 Isolated and Non-Isolated Systems The thermal energy of an isolated system does not change. Because energy cannot be created or destroyed, the total energy stays constant in an isolated system.

20 The Second Law of Thermodynamics Can thermal energy be transferred spontaneously from a cold object to a warm object? This process never happens, but it wouldn t violate the first law of thermodynamics.

21 The Second Law of Thermodynamics The transfer of thermal energy spontaneously from a cool object to a warm object never happens because it violates another law the second law of thermodynamics. One way to state the second law of thermodynamics is that it is impossible for thermal energy to be transferred from a cool object to a warmer object unless work is done.

22 Converting Thermal Energy to Mechanical Energy If you give a book sitting on a table a push, the book will slide and come to a stop. Friction between the book and the table converted the book s mechanical energy to thermal energy. As a result, the book and the table became slightly warmer.

23 Converting Thermal Energy to Mechanical Energy Is it possible to do the reverse, and convert thermal energy completely into mechanical energy? The second law of thermodynamics makes it impossible to build a device that converts thermal energy completely into mechanical energy.

24 Converting Thermal Energy to Mechanical Energy A device that converts thermal energy into mechanical energy is a heat engine. A car s engine converts the chemical energy in gasoline into thermal energy.

25 Converting Thermal Energy to Mechanical Energy The engine then transforms some of the thermal energy into mechanical energy by rotating the car s wheels.

26 Converting Thermal Energy to Mechanical Energy However, only about 25 percent of the thermal energy obtained by burning the gasoline is converted into mechanical energy, and the rest is transferred to the engine s surroundings.

27 Internal Combustion Engines The heat engine in a car is an internal combustion engine in which fuel is burned inside the engine in chambers or cylinders. Each cylinder contains a piston that moves up and down. Each up-and-down movement of the piston is called a stroke. Automobile and diesel engines have four different strokes.

28 Internal Combustion Engines

29 Friction and the Efficiency of Heat Engines Almost three fourths of the thermal energy obtained from an internal combustion engine is not converted into mechanical energy. Friction between moving parts causes some of the automobile s mechanical energy to be converted into thermal energy.

30 Friction and the Efficiency of Heat Engines Even if friction were totally eliminated, a heat engine still could not convert thermal energy completely into mechanical energy. The efficiency of an internal combustion engine depends on the difference in the temperature of the burning gases in the cylinder and the temperature of the air outside the engine.

31 Refrigerators A refrigerator does work as it transfers thermal en\ergy from inside the refrigerator to the warmer room. The energy to do the work comes from the electrical energy the refrigerator obtains from an electrical outlet. C Squared Studios/Getty Images

32 Refrigerators A refrigerator contains a coolant that is pumped through pipes on the inside and outside of the refrigerator. The coolant is a special substance that evaporates at a low temperature.

33 Refrigerators Liquid coolant is pumped through an expansion valve and changes into a gas. When the coolant changes to a gas, it cools. The cold gas is pumped through pipes inside the refrigerator, where it absorbs thermal energy. As a result, the inside of the refrigerator cools.

34 Refrigerators The gas then is pumped to a compressor that does work by compressing the gas. This makes the gas warmer than the temperature of the room. The warm gas is pumped through the condenser coils.

35 Refrigerators The gas is warmer than the room, and thermal energy is transferred from the gas to the room. Some of this heat is the thermal energy that the coolant gas absorbed from the inside of the refrigerator.

36 Refrigerators As thermal energy is transferred from the gas, that gas cools and changes to a liquid. The liquid coolant then is changed back to a gas, and the cycle is repeated.

37 Air Conditioners and Heat Pumps An air conditioner operates like a refrigerator, except that warm air from the room is forced to pass over tubes containing the coolant. The warm air is cooled and is forced back into the room. The thermal energy that is absorbed by the coolant is transferred to the air outdoors.

38 Air Conditioners and Heat Pumps A heat pump is a two-way air conditioner. In warm weather, it operates as an ordinary air conditioner. In cold weather, a heat pump operates like an air conditioner in reverse. McGraw-Hill Companies/Jill Braaten, photographer

39 Air Conditioners and Heat Pumps The coolant gas is cooled and is pumped through pipes outside the house. There, the coolant absorbs thermal energy from the outside air. The coolant is then compressed and pumped back inside the house, where it transfers the thermal energy to the indoors.

40 The Human Coolant Your body uses evaporation to keep its internal temperature constant. When a liquid changes to a gas, energy is absorbed from the liquid s surroundings. As you exercise, your body generates sweat from tiny glands within your skin. As the sweat evaporates, it carries away thermal energy.

41 Efficiency and Thermal Energy Many energy transformations occur around you which convert one form of energy into another form. However, whenever energy transformations occur, some energy is transformed into thermal energy.